According to Lewis, the acidic and basic properties of organic compounds are measured by the ability to accept or donate an electron pair, followed by the formation of a bond. An atom that accepts an electron pair is an electron acceptor, and a compound containing such an atom should be classified as an acid. An atom that provides an electron pair is an electron donor, and a compound containing such an atom is a base.
For example: Lewis acids include BF 3 , ZnCl 2 , AlCl 3 , FeCl 3 , FeBr 3 , TiCl 4 , SnCl , SbCl 5 , metal cations, sulfuric anhydride - SO 3 , carbocation. Lewis bases include amines RNH 2 , R 2 NH, R 3 N, ROH alcohols, ROR ethers, RSH thiols, RSR thioethers, anions, compounds containing π-bonds (including aromatic and heterocyclic compounds.
5.3. The concept of hard and soft acids and bases (the principle of HICA, the Pearson principle)
The general approach to dividing acids and bases into hard and soft can be characterized as follows.
Hard acids- Lewis acids, in which acceptor atoms are small in size, have a large positive charge, high electronegativity and low polarizability. The molecular orbital of hard acids, to which donor electrons pass, has a low energy level.
Soft acids - Lewis acids containing acceptor atoms of large size with a small positive charge, with low electronegativity and high polarizability. The molecular orbital of soft acids, which accepts donor electrons, has a high energy level.
Rigid bases- donor particles, in which donor atoms have high electronegativity and low polarizability. Valence electrons are held firmly, the product is oxidized with difficulty. The orbital whose pair of electrons is transferred to the acceptor has a low energy level. Donor atoms in hard bases can be oxygen, nitrogen, fluorine, chlorine.
Soft grounds- donor particles, in which donor atoms have low electronegativity and high polarizability, they are easily oxidized; valence electrons are held weakly. An orbital whose pair of electrons is transferred to an acceptor has a high energy level. Donor atoms in soft bases are carbon, sulfur, and iodine atoms. Table 4
According to Pearson's principle of hard and soft acids and bases (HMCA), Lewis acids are divided into hard and soft. Hard acids- acceptor atoms with small size, large positive charge, high electronegativity and low polarizability.
Soft acids- acceptor atoms of large size with a small positive charge, with a small electronegativity and high polarizability.
The essence of LCMO is that hard acids react with hard bases and soft acids react with soft bases. For example: when sodium ethoxide interacts with isopropyl iodide, ethoxide - C 2 H 5 O ion - as a hard base will react with a hard acid, which is a proton in - position. The elimination reaction will be predominant. 
Acidity and basicity are the most important concepts that determine many fundamental physicochemical properties and biological activity of organic compounds. There are several concepts of acids and bases in organic chemistry. The protolithic theory of Brønsted-Lowry (1923) is generally accepted. Almost simultaneously, G. Lewis proposed a more general concept of acids and bases, on the basis of which R. Pearson (1963) later developed the principle of hard and soft acids and bases.
Acidity and basicity according to Bronsted-Lowry. In accordance with the Bronsted-Lowry theory, the acidity and basicity of compounds are associated with the transfer of the H + proton.
acids- substances capable of donating a proton (run donors); grounds - substances capable of accepting a proton (proton acceptors). An acid and a base form a conjugate acid-base pair. Acidic properties are manifested in the presence of a base, basic - in the presence of an acid.
In general, acid-base interaction is described by the equation
In principle, most organic compounds can be considered as potential acids, since they contain hydrogen atoms associated with different elements (O, S, N, C). An element and its associated hydrogen atom are called acid center . Organic acids are respectively classified according to their acid center as OH-, SH-, NH- and CH-acids. Acids can be not only neutral molecules, but also positively charged ions, as well as dipolar ions. Organic bases for the formation of a covalent bond with the acid proton must either have a lone pair of electrons at the heteroatom (neutral molecules) or be anions. In general, bases that have a heteroatom in molecules are called n-bases . There is another group of bases - π bases , in which the center of basicity is the electrons of a localized π-bond or a π-electron cloud of a conjugated system. π-bases do not form covalent bonds with the proton, but short-lived π-complexes.
The acidity and basicity of substances according to Brönsted-Lowry is characterized quantitatively. Applying the law of mass action, one can express the acidic properties of acid A-H in terms of equilibrium constant K p , presented above the reaction of reversible acid-base interaction:
Obviously, the equilibrium constant of the acid ionization reaction has a constant value only for a given system, and each base has its own scale of acidity constants. The most important case is the ionization of acids in an aqueous solution (water plays the role of a base):
Since water is present in large excess, its concentration remains almost constant, equal to 55.5 mol/l. This value is included in the equilibrium constant and a characteristic is obtained, called acidity constant K a :
The more K,the stronger the acid . However, even such a relatively strong acid by the standards of organic compounds as acetic acid has K a = 1.75 10 -5 . For most organic compounds, K a have even smaller values. Therefore, to assess the strength of organic acids, it is much more convenient to use the values R K a representing the negative logarithm of the acidity constants: RK a = -lg K a. Wherein the lower pK a ,the stronger the acid . Acids with pKa > 7 do not change the color of neutral indicator paper; acids with pKa >10 do not taste sour.
The basicity of compounds in an aqueous solution can be characterized by the pK b value, which is related to pK a through the ionic product of water: RK b \u003d 14 - pK a. However, at present, to characterize the basicity, the pKa value of the conjugate base B of the acid BH + is more often used, denoted as рK BH + . This approach allows one and the same scale to be used to characterize the ionization of both acids and bases. In this case the more pK BH + , the stronger the base .
Weak acids and bases in biological systems. Most biologically active organic compounds, in particular medicinal substances, are weak acids or bases. The degree of ionization of such compounds in a particular medium is important for the manifestation of biological action. Many medicinal substances are known, the therapeutic activity of which is determined by the proportion of non-ionized molecules present, although there are other examples when, on the contrary, the ionized part of the substance causes a biological effect due to interaction with the cationic or anionic centers of the receptors. Differences in the degree of ionization ensure the selectivity of action, and this is due to such factors as, for example, penetration through membranes into the blood plasma or cell, adsorption on enzyme surfaces, possible ionization of receptor centers depending on pH, etc.
The degree of ionization of organic acids and bases in a solution is determined by the values of two parameters: the pH of the solution and pK a of the acid (or pK BH + base). If the values of pK a (or pK BH +) of the substance and the pH of the solution are known, then the degree of ionization can be calculated as follows:
The degree of ionization is important for the processes of penetration of substances through various membranes in the body, for example, during the absorption (absorption) of drugs from the gastrointestinal tract. The membranes of the epithelium of the digestive tract can be considered as a lipid bilayer in which protein molecules are embedded. The hydrophobic regions of membrane proteins are immersed in the inner cavity of the membrane, while the ionized regions face the aqueous phase inside and out. According to the classical theory, membranes of this type prevent the passage of ions, since, firstly, ions due to hydration have a relatively large size and, secondly, if the charge of the ion and the charge of the protein surface to which it approaches are similar in sign, then repulsion, and if they are opposite, then the ion is adsorbed on the membrane surface. Only those ions for which there are specific transport systems or carriers penetrate through natural membranes. Neutral lipid-soluble molecules penetrate membranes and the faster, the higher their lipophilic properties. Thus, the absorption of non-ionized molecules of medicinal substances occurs in the gastrointestinal tract.
Acid drugs will be better absorbed from the stomach (pH 1-3), and absorption of base drugs will occur only after they pass from the stomach to the intestines (the contents of the small intestine have a pH of 7-8). Within one hour, almost 60% of acetylsalicylic acid and only 6% of aniline from the administered dose are absorbed from the stomach of rats. Already 56% of the administered dose of aniline is absorbed in the intestines of rats. It is noteworthy that such a weak base as caffeine (pK В H + 0.8) is absorbed in the same time to a much greater extent (36%), since even in the strongly acidic environment of the stomach, caffeine is largely in a non-ionized state.
The effectiveness of the action of drugs is determined by the ability of their penetration to the receptor. For substances capable of ionization, biological activity can be determined by the proportion of non-ionized molecules or, conversely, by the ionized part of the substance. There are numerous examples of both options. So, both phenol and acetic acid stop the growth of various molds; their biological effect is due to non-ionized molecules, and therefore acetic acid is most effective at pH below 4, and for phenol at any pH below 9, since in these pH ranges both phenol and acetic acid are in a non-ionized state. Also, only non-ionized theophylline, unlike its anion, stimulates the activity of the turtle's heart. On the example of a number of sulfanilamide preparations, on the contrary, it was found that their antibacterial activity is due to anions. The pKa value of sulfonamides, which is optimal for the manifestation of activity, is in the range of 6-8. Non-ionized molecules enter the cell through the membrane, but at physiological pH values, ions are again formed until an equal degree of ionization is established on both sides of the membrane:
The antibacterial activity of sulfonamides is proportional to the degree of ionization, but also depends on the lipophilicity of the molecules.
And one more example, when the biological activity is due to the ionized form of the substance: the antibacterial (bacteriostatic) effect of aminoacridines is manifested only in the cationic form of these compounds and increases with an increase in the degree of their cationic ionization. The change in the degree of ionization depending on the pH of the medium is widely used to isolate drugs from biological fluids (blood, urine) for the purpose of their subsequent analysis, for example, when conducting pharmacokinetic studies.
Lewis acids and bases. According to Lewis theory, the acid-base properties of compounds are determined by their ability to accept or donate a pair of electrons to form a new bond. Lewis acids - electron pair acceptors. Lewis foundations electron pair donors.
Brönsted bases and Lewis bases are donors of a pair of electrons - either unshared or located in the p-orbital, i.e. the concepts are identical in both theories. Lewis acidity has a new and broader meaning. An acid is any particle with a vacant orbital that can add a pair of electrons to its electron shell. According to Bronsted, an acid is a proton donor, and according to Lewis, the proton H + itself is an acid, since it has a vacant orbital.
Lewis acids are halides of elements of the second and third groups of the periodic system (BF 3 , A1C1 3 , FeCl 3 , FeBr 3 , ZnCl 2 and others). Lewis acids also include halides of other elements with vacant orbitals - SnX 4 , SbX 5 , AsX 5 and even sulfur oxide (VI) SO 3 . Boron and aluminum halides have six electrons in the outer shell and are capable of accepting a pair of electrons to form a covalent bond. Tin tetrachloride, for example, has 8 electrons in its outer shell, but as an element with vacant orbitals, it is able to accept a couple more electrons. Lewis acids also include metal cations (Na +, Mg 2+, Ag +), carbocations R 3 C +, nitroyl cation NO 2 +, etc. Lewis acids participate in heterolytic reactions as electrophilic reagents. The following are some examples of interactions between Lewis acids and bases:
Many common organic reactions are referred to as acid-base interactions within the framework of the Lewis theory. However, in this theory it is much more difficult to quantify acidity and basicity, and such an assessment can only be relative. For this, the interaction energies of various compounds are determined under strictly defined conditions (solvent, temperature) with the same standard, which is, respectively, a Lewis acid or a Lewis base. Therefore, much less quantitative measurements have been made for Lewis acids and bases than for Bronsted acids and bases.
Hard and soft acids and bases. The development of the Lewis theory led to the creation of the principle of hard and soft acids and bases (the principle of HICA, the Pearson principle). According to Pearson's principle, acids and bases are divided into hard and soft.
Hard acids are Lewis acids in which acceptor atoms are small in size, have a large positive charge, high electronegativity and low polarizability. Soft Lewis acids contain large acceptor atoms with little positive charge, low electronegativity, and high polarizability.
The lowest free molecular orbital (LUMO), which is involved in binding to the electron pair donor orbital, has a low energy in hard acids. The hardest acid is the proton. LUMO of soft acids has a high energy. Soft acids contain easily polarizable vacant orbitals. The positive charge of an atom - an acceptor of a pair of electrons is small due to delocalization or is completely absent (for example, an iodine molecule is a soft acid).
Rigid bases are donor particles with high electronegativity, low polarizability, and are difficult to oxidize. Soft bases, on the other hand, are donor particles with low electronegativity, high polarizability, and rather easily oxidized. The term "rigid base" emphasizes that the electron pair donor compound holds its electrons firmly. For rigid bases, the highest occupied molecular orbital (HOMO), which is involved in binding to the electron pair acceptor orbital, has a low energy (located close to the atomic nucleus). The donor atoms in hard bases are nitrogen, oxygen, fluorine, and chlorine. Soft bases weakly retain their valence electrons, the HOMO of the donor has a high energy. Donors of a pair of electrons are atoms of carbon, sulfur, phosphorus, iodine.
It should be noted that the concepts of "hard" and "soft" acids and bases are not equivalent to the concepts of "strong" and "weak" acids and bases. These are two independent characteristics of acids and bases. Principle of GIMC is used for a qualitative description of the efficiency of the acid-base interaction: (!) hard acids are better coordinated with hard bases, soft acids - with soft bases. Pearson's concept is based on the fact that the interaction between orbitals with similar energies is more efficient than between orbitals with different energies.
The operation of the GICL principle can be illustrated by the following example. When haloalkanes interact with nucleophiles (which are also bases), competitive reactions can occur - nucleophilic substitution or elimination. The nucleophilic substitution reaction is carried out by the interaction of the nucleophile with the carbon atom bonded to the halogen. In the elimination reaction, a proton is also split off from a neighboring carbon atom under the influence of a base.
When 1,2-dichloroethane interacts with a hard base (methoxide ion), due to the attack of the reagent on a hard acid - a proton, an elimination reaction occurs predominantly. A soft base - thiophene oxide ion - preferentially reacts with a softer acid - a carbon atom, resulting in the formation of a nucleophilic substitution reaction product:
According to Lewis, the acidic and basic properties of organic compounds are measured by the ability to accept or donate an electron pair, followed by the formation of a bond. An atom that accepts an electron pair is an electron acceptor, and a compound containing such an atom should be classified as an acid. An atom that provides an electron pair is an electron donor, and a compound containing such an atom is a base.
Compared with the Bronsted proton theory, the Lewis theory is more general and covers a wider range of compounds. Taking into account the energy characteristics of the orbitals involved in acid-base interactions, a Lewis acid is a molecule with a low-energy free molecular orbital, and a Lewis base is a molecule that provides a high-energy filled molecular orbital for intermolecular interaction. Specifically, Lewis acids can be an atom, molecule or cation: proton, halides of elements of the second and third groups of the Periodic system, transition metal halides - BF3, ZnCl2, AlCl3, FeCl3, FeBr3, TiCl4, SnCl4, SbCl5, metal cations, sulfuric anhydride - SO3, carbocation. Lewis bases include amines (RNH2, R2NH, R3N), ROH alcohols, ROR ethers, RSH thiols, RSR thioethers, anions, compounds containing p-bonds (including aromatic and heterocyclic compounds), especially if their donor ability is enhanced electron donor substituents.
Now let's try to compare two approaches (Brönsted and Lewis) to the definition of acids and bases. As can be seen from the definitions, Lewis bases are identical to Bronsted bases: both are donors of a pair of electrons. The only difference is where this electron pair is spent. Bronsted bases provide it to bond with the proton and are therefore a special case of Lewis bases, which provide an electron pair to any particle with a vacant orbital. More significant differences are noted in the interpretation of acids. The Brönsted theory covers only protic acids, while Lewis acids are any compounds with a free orbital. Protic acids are considered in the Lewis theory not as acids, but as products of neutralization of a proton by bases. For example, sulfuric acid is the product of acid neutralization with H+ base, hydrochloric acid is the product of H+ neutralization with Cl- base.
When acids and Lewis bases interact, donor-acceptor (acid-base) complexes of a very different nature are formed. The following are examples of such interactions.
Organic chemistry is rich in examples of such interactions, in which a covalent bond is formed as a result of the interaction of a particle with a filled orbital with a particle with a vacant orbital. These processes can be considered as Lewis acid-base reactions. The wider coverage of specific objects, characteristic of the Lewis theory, more significant differences in the nature of compounds lead to the fact that the series of relative strengths of Lewis acids and bases is not as universal as for Bronsted acids and bases. For Lewis acids, it is impossible to compile a table with strict quantitative characteristics of acidity, as is done for Bronsted acids (see Table 1). For them, there is only a qualitative approximate sequence of acidity. So, for Lewis acids of the type of metal halides, the acidity decreases in the series: BX3 > AlX3 > FeX3 > SbX5 > > SnX4 > ZnX2.
Summarizing the above, we note that at present there are two theories in assessing the acid-base properties of organic compounds. Is it possible to say that one of them has significant advantages over the other. There can be no single answer to such a question. Yes, Lewis's theory is more general and covers a wider range of specific objects. The Bronsted-Lowry theory is characterized by a more rigorous consideration of the quantitative characteristics of acidity and basicity. Preference for one or the other theory can only be given taking into account the specific content of the issue under discussion. If processes involving hydrogen-containing substances are discussed, in which proton transfer reactions play an important role and hydrogen bonds have a significant influence, it seems that in these cases preference should be given to the Brønsted-Lowry theory. An important advantage of the Lewis theory is that any organic compound can be represented as an acid-base complex. When discussing heterolytic reactions in which Lewis acids participate as electrophilic reagents and Lewis bases as nucleophiles, preference should be given to the Lewis theory. Chemists have learned to skillfully use the merits of each of these theories.
Representation of reaction mechanisms. Homo- and heterolytic bond breaking. The idea of intermediate particles: radicals, carbocations, carbanions. Classification of reagents: radicals, nucleophiles, electrophiles.
The reaction mechanism is a detailed description of the process of converting reactants into products, including the fullest possible description of the composition, structure, geometry, energy and other St. TV intermediates, transition states and products.
Homolytic bond breaking - a break when each atom leaves one electron. It is characteristic of the exchange mechanism for the formation of a covalent bond.
Heterolytic bond rupture - a rupture when positively and negatively charged particles are formed as a result, tk. both electrons from a common electron pair remain at one of the atoms. It is characteristic of the donor-acceptor mechanism for the formation of a covalent bond.
Carbocation - a particle, in a cat. a positive charge is concentrated on the carbon atom, the carbon atom has a vacant p-orbital. Carbocation - a strong Lewis acid, has electrophilic activity. Chemical saints:
· Interaction with nucleophiles.
· Ability to β-elimination - elimination of a proton with the formation of a multiple bond.
· Rearrangement into a more stable carbocation - isomerization of the primary into a more stable secondary or tertiary carbocation.
A carbanion is an anion that has an even number of electrons with a free electron pair on a tetravalent carbon atom. Carbanions include both anions with a negative charge localized on the carbon atom and anions with a delocalized negative charge, in which at least one of the canonical structures has a charge localized on the carbon atom. Chem. saints:
Interaction with electrophiles.
· Oxidation to radicals.
Free radicals - particles (usually unstable), sod-e one or several. unpaired electrons to the outer electron. shell. A radical can form as a result of the loss of one electron by a non-radical molecule or upon receipt of one electron by a non-radical molecule.
Acids and bases (Brönsted, Lewis)
Protolytic (proton) theory of acids and bases by Bronsted - Lowry (1923). According to this theory, acids are molecules or ions that can be proton donors in a given reaction, and bases are molecules or ions that accept protons (acceptors). Acids and bases are collectively known as protoliths.
The essence of the acid-base interaction I yavl. transfer of a proton from an acid to a base. In this case, the acid, having transferred a proton to the base, itself becomes a base, because. can again attach a proton, and the base, forming a protonated particle, becomes an acid. Thus, in any acid-base interaction, two pairs of acids and bases, called conjugated by Brönsted, are involved.
Lewis electronic theory. In the theory of Lewis (1923), on the basis of electronic representations, the concept of acid and base was further expanded. Lewis acid - a molecule or ion that has vacant electron orbitals, as a result of which they are able to accept electron pairs. These are, for example, H ions - protons, metal ions (Ag +, Fe3 +), oxides of some non-metals (for example, SO3, SiO2), a number of salts (AlCl3), as well as such things as BF3, Al2O3. Lewis acids that do not contain hydrogen ions are called aprotic. Protic acids are considered as a special case of the class of acids. A Lewis base is a molecule or ion capable of being an electron pair donor: all anions, ammonia and amines, water, alcohols, halogens.
Ways to use alkanes
Alkanes are widely used in many areas of human activity. None of us can imagine life without natural gas, the basis of the cat. is methane. It is also used to produce carbon black (soot), cat. used in the production of tires, printing ink. Alkane compounds are used as refrigerants in home refrigerators. Acetylene, cat. obtained from methane, used for welding and cutting metals. Among the compounds of alkanes, halogen derivatives can be distinguished, such as chloroform, carbon tetrachloride, which are among the best solvents. Alkanes can be used as motor fuel (methane, propane, butane), cat. little environmental pollution. Vaseline oil (a mixture of liquid hydrocarbons with up to 15 carbon atoms) is a clear, odorless and tasteless liquid used in medicine, perfumery and cosmetics. Vaseline (a mixture of liquid and solid saturated hydrocarbons with up to 25 carbon atoms) is used to prepare ointments used in medicine. Paraffin (a mixture of solid alkanes C19-C35) - a white solid mass, odorless and tasteless (melt = 50-70 ° C) - used for making candles, impregnating matches and packaging paper, for thermal procedures in medicine, etc. .
Sp hybridization
Occurs when mixing one s- and one p-orbitals. Two equivalent sp-atomic orbitals are formed, located linearly at an angle of 180 degrees and directed in different directions from the nucleus of the central atom. The two remaining non-hybrid p-orbitals are located in mutually perpendicular planes and participate in the formation of π-bonds, or are occupied by lone pairs of electrons.
Sp2 hybridization
Occurs when mixing one s- and two p-orbitals. Three hybrid orbitals are formed with axes located in the same plane and directed to the vertices of the triangle at an angle of 120 degrees. The non-hybrid p-atomic orbital is perpendicular to the plane and, as a rule, participates in the formation of π-bonds
26. Alkynes. Reduction of a triple bond to a double bond: catalytic hydrogenation and reduction with sodium in liquid ammonia, use in synthesis ( Z)- and ( E) - alkenes.
triple bond represents one s-bond C-C and two p-bonds. When moving from a double to a triple bond, the average p-bond energy decreases. This means that the triple bond is less stable than the double bond. Acetylene itself is an unstable compound and is capable of spontaneous explosive decomposition into elements. The acetylene molecule has a linear structure, which is due to the sp state of carbon atoms. The triple bond in alkynes is characterized by a higher polarizability than in alkenes R Cº C = 5.96; RC=C=4.17.
Catalytic hydrogenation method , along with other important processes in organic chemistry, is currently in wide use. The introduction of hydrogenation into technology was a stimulus for the wide development of fuel upgrading processes, syntheses from carbon oxides, and numerous reduction reactions. The hydrogenation of alkynes occurs under approximately the same conditions and in the presence of the same catalysts as the hydrogenation of alkenes. The first step in the hydrogenation of acetylene to ethylene is more exothermic than the second step, where ethylene is converted to ethane:
It follows from these data that the hydrogenation of alkynes can, in principle, be stopped at the stage of alkene formation. However, with most catalysts, alkynes hydrogenate directly to alkanes:
Reduction of alkynes with sodium or lithium in liquid ammonia or in amines gives trance-alkenes:
Alkenes (olefins, ethylene hydrocarbons) - acyclic unsaturated hydrocarbons containing one double bond between carbon atoms, forming a homologous series with the general formula C n H 2n. The carbon atoms in the double bond are in a state of sp² hybridization, and have a bond angle of 120 °C. The simplest alkene is ethylene (C 2 H 4). According to the nomenclature, the names of alkenes are formed from the names of the corresponding alkanes by replacing the suffix "-an" with "-en"; the position of the double bond is indicated by an Arabic numeral. E-isomers are those geometric isomers in which the senior substituents on the carbon atoms of the double bond are on opposite sides of the double bond. Z-isomers are those geometric isomers in which the senior substituents on the carbon atoms of the double bond are on the same side of the double bond (from the German word "zusamen" - together). The designation E- and Z- are placed before the name of the compound according to the IUPAC nomenclature and enclosed in brackets (designation cis- and trance- not enclosed in parentheses). For example:
Arena nomenclature
The simplest aromatic hydrocarbon of the composition C6HbC6Hb has a trivial name benzene. All other hydrocarbons of this series can be named as substituted derivatives of benzene, or have their own trivial names. At the same time, according to the tradition established in the Russian language, almost all trivial names of benzene homologues also have the ending -ol. For example: C6H5CH3C6H5CH3 - methylbenzene, or toluene; C6H4 (CH3) 2C6H4 (CH3) 2 - dimethylbenzene, or xylene; C6H5CH(CH3)2C6H5CH(CH3)2 - isopropylbenzene, or cymene. As an exception, C6H3(CH3)3C6H3(CH3)3 or 1, 3, 5-trimethylbenzene is called mesitylene. According to IUPAC rules, all names of aromatic hydrocarbons are characterized by the ending -en. Accordingly: benzene, toluene, xylene, cymene, styrene, etc.
In practice, to form the name of two- or more substituted single core arenas the following options are more commonly used:
1. The name is based on the trivial name of the arena (toluene, styrene, etc.), Russian letters (o-, m-, p-) or Latin letters (o-, m-, p-) are used to indicate the location of the side chains , which means the ortho, meta, or para positions of the benzene ring. Alkyl radicals or functional groups are named according to the IUPAC nomenclature: methyl-, ethyl-, isopropyl-, amino-, hydroxo-, nitro-, etc. Often such rules are used to form the names of aromatic compounds of other classes - aminobenzenes, phenols, etc., containing various substituents.
2. Less commonly used are names based on the word "benzene", and the location of substituent radicals is indicated by numbers. When naming more complex benzene derivatives, as in the case of alicyclic compounds, one is chosen from the possible orders in which the sum of the digits of the substituent numbers is the smallest. In this case, there are no generally accepted rules for the numbering order of the atoms of the benzene ring. According to the Geneva nomenclature, number 1 is assigned to that substituent atom to which the substituent atom with the smallest atomic weight is directly connected (for example, if there are -Cl and -OH in the nucleus, number 1 gets the atom carrying -OH, but in the presence of -NO2NO2 and -OH - atom bearing -NO2, in substituted derivatives of benzene homologues, the beginning of numbering is determined by the simplest side chain. multi-core arenas The IUPAC nomenclature rules establish a list of names that underlie the nomenclature of fused polynuclear carbocyclic systems, the rules for orienting their formulas, and the atomic numbering order. The nomenclature uses trivial names (naphthalene, phenanthrene, anthracene) indicating the location of substituents. For example, for naphthalene derivatives, both rules described above for single-core arenes can also be used:
The nature of the bonds in the benzene molecule The benzene molecule contains a system of conjugated bonds. All six carbon atoms of the cyclic C6H6 benzene molecule lie in the same plane. σ-bonds act between carbon atoms in the plane of the ring; the same bonds exist for each carbon atom with hydrogen atoms. Each carbon atom spends three electrons to make these bonds. Clouds of the fourth valence electrons of carbon atoms, having the shape of eights, are located perpendicular to the plane of the benzene molecule. Each such cloud overlaps equally with the electron clouds of neighboring carbon atoms. In the benzene molecule, not three separate π-bonds are formed, but a single π-electron system of six electrons, common to all carbon atoms. The bonds between the carbon atoms in the benzene molecule are exactly the same. All bonds between carbon atoms in benzene are equivalent, which determines the characteristic properties of the benzene nucleus. This is most accurately reflected by the structural formula of benzene in the form of a regular hexagon with a circle inside. (The circle symbolizes the equivalence of bonds between carbon atoms.) However, the Kekule formula is often used, indicating double bonds.
Acid-base properties of alcohols. Metal alcoholates, their basic and nucleophilic properties. Nucleophilic substitution reactions involving alcohols. Examples of biologically important nucleophilic substitution reactions involving phosphate esters.
alcohols are weak OH-acids according to Brönsted and hard acids according to Pearson. Alcohols are similar in acidity to water. The acidic properties of alcohols are determined by the ability to protonize the hydrogen atom of the hydroxyl group. The latter is determined not only by the difference in electronegativity between the oxygen (3.5) and hydrogen (2.1) atoms, but also by the nature of the radical. Methanol (pK a = 15.5) is a slightly stronger acid than water (pK a = 15.7), but most alcohols are weaker acids than water. The reason for this is steric hindrances that prevent the solvation of the resulting alkoxide anion in branched alcohols. Solvation stabilizes the alkoxide anion and hence enhances the acidic properties. Reactions involving a nucleophilic center. The high electronegativity of the oxygen atom (3.5 on the Pauling scale), which is the main center, allows us to consider alcohols as weak Brönsted n-bases and hard bases according to Pearson. Alcohols can form oxonium salts only with strong protonic acids and hard acids according to Pearson (boron fluoride , zinc chloride, etc. Thus, alcohols have weak acidic and weak basic properties, i.e. are amphiprotic compounds. At a sufficiently high temperature and in the absence of a good nucleophile, protonated alcohols are capable of reaction, i.e. to the dehydration reaction. Being rigid bases, due to the low polarizability and high electronegativity of the oxygen atom, alcohols are weak nucleophiles. Bronsted acids protonate the oxygen atom of the hydroxyl group.
42. Intra- and intermolecular dehydration of alcohols .: Dehydration of alcohols can be carried out in two directions: intramolecular and intermolecular. The direction of dehydration depends on the nature of the alcohol and the reaction conditions. During intramolecular dehydration of alcohol, an unsaturated ethylene hydrocarbon is formed, and as a result of intermolecular dehydration, an ether is formed. So, when alcohols are heated with such water-removing substances as concentrated H 2 SO 4, H 3 PO 4, anhydrous oxalic acid, aluminum oxide, etc., unsaturated ethylene compounds are formed. The reactivity of alcohols to dehydration, that is, to the formation of these, ethylene compounds, changes in this order: tertiary alcohols > secondary alcohols > primary alcohols. Some tertiary alcohols dehydrate so easily that they can be distilled only if even laboratory air, which contains trace amounts of acid vapor, is prevented from entering them. .Dehydration of alcohols in the presence of concentrated H 2 SO 4 depending on the temperature, the ratio of the volumes of alcohol and acid can be carried out with the formation of different products. For example, ethyl alcohol at 105 ° C forms an acid ester with sulfuric acid - ethyl sulfuric acid (reaction 1). With an excess of alcohol and a higher temperature (130–140 o C), intermolecular dehydration occurs, the main product of which is diethyl ether (ether; reaction 2). At temperatures above 160 ° C, ethyl sulfuric acid decomposes to form ethylene (reaction 3):
43. Oxidation of primary and secondary alcohols. Alcohols at 300-400 o C and in the presence of copper and other catalysts are oxidized by atmospheric oxygen. Oxidizing agents such as KMnO 4 , chromium mixture, oxidize alcohols already at room temperature. Depending on whether the alcohol is primary, secondary or tertiary, different products are formed during oxidation.
Primary alcohols, when oxidized, give aldehydes with the same number of carbon atoms as in the original alcohol molecule. Aldehydes under these conditions can be oxidized to carboxylic acids. To avoid further oxidation, aldehydes must be quickly removed from the reaction mixture.
Primary alcohols can also be oxidized to aldehydes with finely divided copper. Heated to 280-300 o C. Under these conditions, two hydrogen atoms are split off from an alcohol molecule and a carbon-oxygen double bond (>C=O) appears in the molecule of organic matter, which is formed in this case. This transformation of alcohols is called dehydrogenation:
Secondary alcohols during oxidation, as well as during dehydrogenation, turn into ketones:
Tertiary alcohols are oxidized rather difficultly with the simultaneous breaking of the carbon chain of their molecules and the formation of a mixture of carboxylic acids and ketones. Such oxidation of these alcohols is due to the fact that under the conditions of the oxidation reaction they are dehydrated and converted into ethylene hydrocarbons, which, in the presence of a strong oxidizing agent, are oxidized with rupture of the molecule at the site of the double C=C bond
Sulfonation
Sulfonation of phenol is carried out by heating with concentrated sulfuric acid. The reaction temperature decisively determines the structure of the resulting hydroxybenzenesulfonic acids. ortho-Isomer, the rate of formation of which is higher than pair-isomer, is the dominant product if the reaction temperature does not exceed 100 °C. It is called the kinetic product. In contrast, at higher temperatures the main product is pair-isomer, the rate of formation of which is lower, but it has a high thermodynamic stability The reaction of electrophilic aromatic sulfonation is also reversible when heated ortho-oxybenzenesulfonic acids, with sulfuric acid above 100 ° C, get pair-isomer is a product of thermodynamic control of the reaction. Alkylation.In contrast to the alkylation of phenol at the hydroxy group, which occurs in an alkaline medium, the introduction of alkyl substituents into the aromatic ring of phenol proceeds under the action of haloalkanes, alcohols or alkenes in the presence of catalysts - mineral acids or Lewis acids (Friedel-Crafts reaction). Picric acid. The presence of three nitro groups in the nucleus sharply increases the acidity of the phenol group. Picric acid is, unlike phenol, already a fairly strong acid. The presence of three nitro groups makes picric acid explosive, it is used to prepare melinite. To obtain mononitrophenols, it is necessary to use dilute nitric acid and carry out the reaction at low temperatures: A mixture of o- and p-nitrophenols is obtained with a predominance of the o-isomer. This mixture is easily separated due to the fact that only the o-isomer is volatile with water vapor. The high volatility of o-nitrophenol is explained by the formation of an intramolecular hydrogen bond, while in the case of p-nitrophenol an intermolecular hydrogen bond occurs.
47. Carboxylation of alkali metal phenolates. Salicylic acid. Phenolic acids are obtained by reacting alkali metal phenolates with carbon (IV) oxide. Salicylic (o-hydroxybenzoic) acid is one of the most important phenolic acids. It is used to obtain drugs (sodium salicylate, acetylsalicylic acid, phenyl salicylate, methyl salicylate), in the synthesis of dyes, in the production of aromatic substances (esters), to obtain coumarin, etc. Stages of production of salicylic acid: 1) obtaining anhydrous sodium phenolate:
2) carboxylation of sodium phenolate carbon (IV) oxide:
Phenol as a by-product is distilled off; 3) decomposition of raw sodium salicylate:
Sparingly soluble salicylic acid precipitates;
4) separation and purification of salicylic acid. The resulting technical salicylic acid contains up to 99% pure product. Salicylic acid intended for the preparation of medicinal substances must be purified by sublimation.
48. Oxidation of phenols. The increased electron density in the phenol core makes it sensitive to the effects of oxidizing agents. Depending on the nature of the oxidizing agent and the reaction conditions, various phenol oxidation products are formed. 1) When phenol is oxidized with hydrogen peroxide in the presence of
In the absence of an iron catalyst, ortho-benzoquinone is obtained through the intermediate formation of pyrocatechol:
2) Strong oxidizing agents such as chromium mixture (K 2 Cr 2 O 7 + H 2 SO 4), bromates (KBrO 3 , H 2 SO 4) oxidize phenol to para-benzoquinone through the intermediate formation of hydroquinone:
3) With a more vigorous action of oxidizing agents, the benzene ring is destroyed. Due to the tendency to oxidize, phenols can be colored when stored in air.
49. Quinones and their biological role. Quinones are six-membered cyclic diketones with two double bonds. Of these, paraquinone, obtained by the oxidation of hydroquinone or aniline, is of the greatest practical importance. Paraquinone is the starting product in the synthesis of hydroquinone. The arrangement of double bonds characteristic of quinone determines the color of a number of compounds. Naphthoquinones are naphthalene derivatives containing a quinoid nucleus. The most important is 1,4-naphthoquinone, which can be obtained by the oxidation of naphthalene. In a number of its properties, 1,4-naphthoquinone is similar to p-benzoquinone. It crystallizes in the form of yellow needles, is volatile, and has a sharp, irritating odor. The core of 1,4-naphthoquinone is the basis of vitamin K, or antihemorrhagic vitamin (which prevents hemorrhages). Vitamin K is 2-methyl-3-wick-1,4-naphthoquinone. Vitamin K is found in green herbs, leaves, and vegetables. Represent. a yellow oil, insoluble. in water; distilled in high vacuum. Some quinone derivatives play an important role in the intermediate processes of biological oxidation. Anthraquinones are anthracene derivatives containing a quinoid nucleus. Anthraquinone can be easily obtained by oxidizing anthracene with nitric acid or a chromium mixture. In this case, two keto groups are formed in the molecule, and the middle ring acquires the structure of a quinone. Anthraquinone is a yellow crystalline substance, unlike conventional quinones, it is quite resistant to a number of chemical influences, in particular to oxidation. Anthrahydroquinone is an intermediate in the reduction of anthraquinone to anthracene. Anthrahydroquinone in free form is brown crystals. Having two phenolic hydroxyls, anthrahydroquinone dissolves in alkalis; the resulting phenolate-type substance has a bright red color. Anthraquinone is able to be brominated, nitrated and sulfonated. Alizarin is a 1,2-dioxianthraquinone. Emodins. In medical practice, preparations (tinctures, decoctions, etc.) from aloe, rhubarb, buckthorn, senna leaves, etc. are often used as laxatives. The active substances of these plants, as it turned out, are anthraquinone derivatives, namely, substituted di- and trihydroxy-anthraquinones, contained in plants partly in free form, partly in the form of esters and glycosides. These derivatives of di- and trihydroxyanthraquinones are often combined into the group of emodins. An example of emodins is franguloemodin, which is 3-methyl-1,6,8-trihydroxyanthraquinone. Franguloemodin is found in buckthorn
50. The idea of phenolic antioxidants. Phenolic compounds in nature. Vitamin E. Flavonoids. Antioxidants (AO) or antioxidants are usually called compounds of various chemical nature, capable of inhibiting or eliminating free-radical oxidation of organic substances by molecular oxygen. For many years, antioxidants have been widely used to extend the service life and improve the performance of polymeric and fuel and lubricant materials, to prevent oxidative deterioration of food products, fat-soluble vitamins, feed and cosmetics. The use of AO in these areas gives a huge economic effect and allows you to save significant raw materials. Among synthetic AOs, alkylated phenols are widely used, which is due to both the relative simplicity of their production and a set of valuable properties: high efficiency, low toxicity, universality of action, and the ability to change their properties over a wide range by varying substituents. Phenolic antioxidants (PAO) are understood to mean any compounds of the Ar(OH)n type, in which one or more hydroxyl groups are connected to an aromatic nucleus, and the AO molecule may contain several Ar(OH)n fragments. Phenolic compounds are capable of influencing many physiological processes in the human body. For example, in the composition of herbal preparations, these substances (such as coumarin, the properties of which are still poorly understood, rutin, flavonoids) stimulate the activity of the adrenal cortex, due to which the adrenal glands begin to more actively secrete hormones of the glucocorticoid group (a type of hormone secreted by the adrenal glands). They have a wide variety of biological properties. For example, phenolic compounds of the leaves of bearberry, pear, lingonberry act as antiseptics.
Phenolic acids are derivatives of aromatic hydrocarbons, in the molecule of which the H atoms of the benzene ring are replaced by carbo (-COOH) or hydroxyl groups (-OH). Phenolic compounds have long been used in medicine, they are used in the treatment of neurosis and coronary insufficiency. Phenolic compounds have diuretic, sedative, choleretic and hemostatic effects. Flavonoids, bioflavonoids are phenols, these are yellow-red plant pigments. There are many of them in food and medicinal plants. Bioflavonoids strengthen capillaries, act as oncoprotectors, participate in the removal of heavy metal salts and radionuclides from the body. The group of bioflavonoids includes special substances that also exhibit P-vitamin activity and other properties of bioflavonoids, which are called anthocyanins. According to the chemical structure, anthocyanins are flavone glycosides. Tannins are polymeric phenolic compounds. In medicine, they are used as astringent, anti-inflammatory gastrointestinal agents. The most famous tannin is tannin. It should not be taken orally: it will cause indigestion. Catechins (they are also referred to as bioflavonoids) are derivatives of flavonols and anthocyanins. Coumarins are aromatic substances with the smell of fresh hay. Coumarins are anticoagulants, that is, they prevent rapid blood clotting, such as the coumarin derivative dicumarol. It is an antivitamin K and is used for the prevention and treatment of thrombosis and thrombophlebitis.
51. Ethers. Nomenclature, classification. Breakdown by acids. Ethers- organic substances in which the molecules contain hydrocarbon radicals connected by an oxygen atom. This can be written as follows: R - O - R ', where R and R "are the same or different radicals. Ethers are considered as derivatives of alcohols. These compounds have compound names. In this case, the name of the radicals is used (in order of increasing molecular weight) and , in fact, the word "ether" (CH3OCH3 dimethyl ether, C2H5OCH3 methyl ethyl ether, and so on)
Ether nomenclature According to the trivial nomenclature, ethers are named after the radicals associated with the oxygen atom, adding the word "ether".
According to the IUPAC nomenclature, ethers are considered as alkoxyalkanes. The word root is determined by the longest alkyl group.
Ethers are among the slightly reactive substances and are stable with respect to many reagents, but they are sensitive to oxygen and easily form explosive hydroperoxides, which cause explosions if handled carelessly.
1 . Acid cleavage of ethers
Ethers are split when heated to 120-150 about with concentr. aqueous 48% HBr or HI. Under equally harsh conditions, ethers of phenols are cleaved.
However, esters containing a tertiary alkyl group break down very easily.
Acid cleavage of ethers should be considered as a nucleophilic substitution reaction at a saturated carbon atom. Depending on the nature of the alkyl groups bound to oxygen, either S N 1 or S N 2 mechanisms are realized. If the ester contains primary or secondary alkyl groups, an S N 2 mechanism occurs in which the bromide or iodide ion attacks the protonated form of the ether at the less substituted carbon atom. In this case, the cleavage is highly regioselective and, as a rule, only one of the two possible alcohols (secondary) and a primary alkyl halide are formed.
Chloride and fluoride ions in water are strongly solvated by hydrogen bonds and have insufficient nucleophilicity for the acid cleavage of ethers by the S N 2 mechanism.
Ethers with tertiary alkyl, benzyl, or allyl groups react by the S N 1 mechanism to form a carbocation as an intermediate. These reactions proceed under mild conditions, and trifluoroacetic acid can be used as an acid agent.
Preparatively, BCl 3 or BBr 3 are much more convenient ester cleavage reagents. In these cases, the splitting takes place already at -20 about C. This is especially necessary in the presence of other functional groups or when isomerization of the carbon skeleton is possible.
Formation of hydroperoxides, their detection and decomposition.
Hydroperoxides are the first molecular products of hydrocarbon oxidation. The chain link during their formation has the form:
The interaction of a peroxide radical with a hydrocarbon determines the structure of the resulting hydroperoxide and subsequent oxidation products. In this case, the order of change in the reactivity of hydrogen atoms, which is usual for radical reactions, is observed, determined by the relative stability of the intermediate radical. As a result, the -position of the side chain with respect to the aromatic nucleus becomes the predominant site of attack of the molecule during the oxidation of alkanes, and the alkyl position for olefins. In addition, for hydrocarbons of all classes, a well-known sequence is valid in changing the ability to replace hydrogen atoms located at different carbon atoms (tertiary secondary primary).
Hydroperoxides are among the rather unstable compounds that are converted into other products during oxidation. Hydroperoxides, when decomposed under the action of elevated temperature or oxidation catalysts, give alcohols and carbonyl compounds. This decomposition may have a molecular mechanism, however, in the developed oxidation process, the products are formed mainly in a chain way. When obtaining alcohols, the chain link is as follows:
Ketones are formed from secondary hydroperoxides via the radical-hydroperoxide step:
Tertiary hydroperoxides during chain transformation give, in addition to alcohol with the same number of carbon atoms, also alcohol and ketone with a smaller number of carbon atoms due to the destruction of the carbon-carbon bond:
Features of the properties of arylamines. Reactions of electrophilic substitution in the benzene nucleus of arylamines and their derivatives. Diazotization reactions, aryldiazonium salts. Reactions of aryldiazonium salts with and without nitrogen evolution.
The arylaminos are characterized by reactions involving the nitrogen atom and reactions involving the carbon atoms of the aromatic nucleus. Basicity. Aromatic amines have a basic character. However, they are weaker than fatty amines and even weaker than ammonia. The decrease in basicity is due to the conjugation of the lone pair of electrons of the nitrogen atom with the n:-electron system of the aromatic nucleus. Aniline does not form salts with H2CO2 Substituents in the benzene ring have a significant effect on the basicity of arylamines. Electron-donating substituents increase basicity, while electron-withdrawing substituents decrease it. When passing from primary to tertiary, the basicity of aromatic amines decreases.
Electrophilic substitution in the benzene ring.
In electrophilic substitution reactions in the benzene ring, a hydrogen atom is replaced by an electrophilic reagent while maintaining the aromatic nature of the starting compound.
Arenediazonium salts are formed by the interaction of primary aromatic amines with nitrous acid. In the industry, arendiazonium salts are widely used to obtain a variety of azo dyes of all colors and shades. For this reason, diazotization is one of the most important and best studied reactions in organic chemistry.
Diazotization of primary aromatic amines is described by the following overall equation:
ArNH 2 + NaNO 2 + 2 HClArN + \u003d N Cl - + NaCl + 2 H 2 O
Reactions with the release of nitrogen. When acidic solutions of diazonium salts are boiled, nitrogen is released and phenols are obtained. Conversion of diazonium salts without nitrogen release. The reactions of this group make the transition from diazo compounds to azo compounds (derivatives of azobenzene) possible. Organic substances of this class form the basis of one of the sections of the industry that produces synthetic dyes from products extracted from coal tar. All azo dyes are obtained by the so-called coupling reaction of diazonium salts.
59. Carbonyl compounds. Classification, nomenclature and isomerism of carbonyl compounds. Organic compounds in the molecule of which there is a carbonyl group\u003e C \u003d O are called carbonyl compounds, or oxo compounds. Carbonyl compounds are divided into two large groups - aldehydes and ketones.
The strength of Lewis acids and bases is determined by the equilibrium constant K of the formation of the neutralization product AB:
In Brönsted acid-base reactions, bases always coordinate with a proton. If a given molecule or anion is coordinated to one of the millions of possible Lewis acids, then this molecule or ion is already considered a Lewis base. Thus, Brönsted basicity can be defined as a special case of Lewis basicity in which the bases form a bond with a proton.
Lewis acids can be divided into classes depending on which atom of the acid the base coordinates to. For example, BF3 can be considered a boron acid (B-acid) because the reactive base forms a bond with boron. When the tert-butyl cation coordinates with the chloride ion to give tert-butyl chloride, it can be considered a C-acid. The nitronium ion (NO2+) acts as an N-acid in most reactions, and so on.
The strength of the base B, determined by the equilibrium constant in equation (3.2), naturally depends on the nature of the acid A. This is due to the fact that the interaction energy depends on the relative position of the HOMO levels of the base and LUMO of the acid (Sec. 2.3.4, Ch. 2) , and the position of these levels is related to the electronegativity of the given element. So, the base strength in the reaction with C-acids (carbocations) is called carbon basicity, in the reaction with BF3 - boron basicity, etc. It is to be expected that the basicity, for example, on carbon, for a given base will not be constant for all C-acids; K in equation (3.2) should, perhaps to a small extent, but still change with a change in the carbocation with which the base coordinates. Therefore, it is also necessary to distinguish between basicity in CH3+, C6H5+, (C6H5)3C+, CH3-C+=O, etc.
Like acids, Lewis (and Bronsted) bases can be classified as C- (eg, CN-), N-, or O-bases.
For organic chemistry, the most important are C-acids, i.e. carbocations, and C-bases, i.e. carbanions. These generally unstable species are formed as intermediates in many reactions and will be discussed in detail in this and subsequent chapters.
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