Alcohols whose molecules contain two hydroxyl groups are called diatomic or glycols. The general formula of diatomic alcohols is C n H 2n (OH) 2. Dihydric alcohols form a homologous series, which can be easily written using the homologous series of saturated hydrocarbons, replacing two hydrogen atoms in their molecule with hydroxyl groups.
The first and most important representative of dihydric alcohols is ethylene glycol HOCH 2 -CH 2 OH (Bp = 197 o C). Antifreeze is made from it.
Glycols are stable, in the molecules of which the hydroxyl groups are located near different carbon atoms. If two hydroxyl groups are located near one carbon atom, then such dihydric alcohols are unstable, easily decompose, splitting off water due to hydroxyl groups and turning into aldehydes or ketones:
ketone
NOMENCLATURE
Depending on the relative position of the hydroxyl groups, α-glycols are distinguished (their hydroxyl groups are located near neighboring carbon atoms, which stand side by side, in position 1,2), β-glycols (their OH groups are located in position 1,3), γ-glycols (OH groups in position 1.4), δ-glycols (OH groups in position 1.5), etc.
For example: α-glycol - CH 2 OH-CHOH-CH 2 -CH 3
β-glycol - CH 2 OH-CH 2 -CHOH-CH 3
γ-glycol - CH 2 OH-CH 2 -CH 2 -CH 2 OH
According to rational nomenclature, the name of α-glycols is formed from the name of the corresponding ethylene hydrocarbon, to which the word glycol is added. For example, ethylene glycol, propylene glycol, etc.
According to the systematic nomenclature, the names of glycols are formed from the name of a saturated hydrocarbon, to which the suffix -diol is added, indicating the numbers of carbon atoms. Near which are hydroxyl groups. For example, ethylene glycol CH 2 -OH-CH 2 OH according to the IUPAC nomenclature is ethanediol-1,2, and propylene glycol CH 3 -CHOH-CH 2 OH is propanediol-1,2.
isomerism
The isomerism of dihydric alcohols depends on the structure of the carbon chain:
positions of hydroxyl groups in the alcohol molecule, for example, propanediol-1,2 and propanediol-1,3.
PRODUCTION METHODS
Glycols can be obtained by the following methods:
1. Hydrolysis of dihalogen derivatives of saturated hydrocarbons:
2.Hydrolysis of haloalcohols:
3. Oxidation of ethylene hydrocarbons with potassium permanganate or performic acid:
4. Hydration of α-oxides:
5.Bimolecular reduction of carbonyl compounds:
CHEMICAL PROPERTIES
The chemical properties of glycols are similar to those of monohydric alcohols and are determined by the presence of two hydroxyl groups in their molecules. Moreover, one or both hydroxyl groups can take part in the reactions. However, due to the mutual influence of one hydroxyl group on another (especially in α-glycols), the acid-base properties of glycols differ somewhat from those of monohydric alcohols. Due to the fact that the hydroxyl exhibits a negative inductive effect, one hydroxyl group pulls the electron density away from the other in the same way as the halogen atom does in the molecules of substituted monohydric alcohols. As a result of this influence, the acidic properties of dihydric alcohols increase compared to monohydric alcohols:
H-O CH 2 CH 2 OH
Therefore, glycols, unlike monohydric alcohols, easily react not only with alkali metals, but also with alkalis and even with hydroxides of heavy metals. With alkali metals, alkalis, glycols form complete and incomplete alcoholates (glycolates):
With the hydroxides of some heavy metals, such as copper hydroxide, glycols form complex glycolates. At the same time, Cu (OH) 2, which is insoluble in water, easily dissolves in glycol:
Copper in this complex forms two covalent bonds with oxygen atoms and two coordination bonds. The reaction is qualitative for dihydric alcohols.
Glycols can form full and partial ethers and esters. So, when an incomplete alkali metal glycolate reacts with alkyl halides, incomplete ethers are obtained, and from a complete glycolate, a complete ether is obtained:
Methyl and ethyl cellosolves are used as a solvent in the production of varnishes, smokeless powder (pyroxylin), acetate silk, etc.
With organic and mineral acids, dihydric alcohols form two series of esters:
Ethylene glycol mononitrate Ethylene glycol dinitrate
Ethylene glycol dinitrate is a strong explosive used in place of nitroglycerin.
The oxidation of glycols is carried out stepwise, with the participation of one or both hydroxyl groups simultaneously with the formation of the following products:
Dihydric alcohols enter into a dehydration reaction. Moreover, α-, β- and γ-glycols, depending on the reaction conditions, split off water in different ways. The cleavage of water from glycols can be carried out intra- and intermolecularly. For example:
Intramolecular elimination of water:
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Intermolecular elimination of water.
In 1906, A. E. Favorsky, distilling ethylene glycol with sulfuric acid, obtained a cyclic dioxane ether:
Dioxane is a liquid that boils at 101 o C, mixes with water in any ratio, is used as a solvent and as an intermediate in some syntheses.
With the intermolecular elimination of water from glycols, oxyethers (alcohol ethers) can be formed, such as, for example, diethylene glycol:
diethylene glycol
Diethylene glycol is also obtained by the interaction of ethylene glycol with ethylene oxide:
Diethylene glycol - a liquid with a boiling point of 245.5 ° C; used as a solvent, for filling hydraulic devices, as well as in the textile industry.
Diethylene glycol dimethyl ether (diglyme) H 3 C-O-CH 2 -CH 2 -O-CH 2 -CH 2 -O-CH 3 has found wide application as a good solvent.
Ethylene glycol, when heated with ethylene oxide in the presence of catalysts, forms viscous liquids - polyethylene glycols:Polyethylene glycol
Polyglycols are used as components of various synthetic detergents.
Ethylene glycol polyesters with dibasic acids have been widely used in the production of synthetic fibers, such as lavsan (the name "lavsan" is formed from the initial letters of the following words - laboratory of macromolecular compounds of the Academy of Sciences):
With methanol, terephthalic acid forms dimethyl ether (dimethyl terephthalate, Tbp. = 140 o C), which is further converted into ethylene glycol terephthalate by transesterification. During the polycondensation of ethylene glycol terephthalate, polyethylene terephthalate with a molecular weight of 15,000-20,000 is formed. Lavsan fiber does not wrinkle, resistant to different weather conditions.
They have the general formula C n H 2n (OH) 2 . The simplest glycol is ethylene glycol HO-CH 2 -CH 2 -OH.
Nomenclature
The names of glycols are formed from the names of the corresponding hydrocarbons with the suffixes -diol or -glycol:
H O - C H 2 - C H 2 - O H (\displaystyle (\mathsf (HO(\text(-))CH_(2)(\text(-))CH_(2)(\text(-))OH)))- 1,2-ethanediol, ethylene glycol
H O - C H 2 - C H 2 - C H 2 - O H (\displaystyle (\mathsf (HO(\text(-))CH_(2)(\text(-))CH_(2)(\text(-))CH_ (2)(\text(-))OH)))- 1,3-propanediol, 1,3-propylene glycol
Physical and chemical properties
Lower glycols are colorless transparent liquids with a sweetish taste. Anhydrous glycols are hygroscopic. Due to the presence of two polar OH groups in glycol molecules, they have high viscosity, density, melting and boiling points.
Lower glycols are highly soluble in water and organic solvents (alcohols, ketones, acids and amines). At the same time, glycols themselves are good solvents for many substances, with the exception of aromatic and higher saturated hydrocarbons.
Glycols have all the properties of alcohols (form alcoholates, ethers and esters), while the hydroxyl groups react independently of each other to form a mixture of products.
With aldehydes and ketones, glycols form 1,3-dioxolanes and 1,3-dioxanes.
Getting and using
Glycols are synthesized in several main ways:
- hydrolysis of the corresponding dichloroalkanes
- oxidation of alkenes with potassium permanganate:
- hydration of oxiranes (epoxides)
Glycols serve as solvents and plasticizers. Ethylene glycol and propylene glycol are used as antifreeze and hydraulic fluids. Due to the high boiling point (for example, 285 ° C for triethylene glycol), glycols have found use as a brake fluid. Glycols are used to obtain various ethers, polyurethanes, etc.
Definition and nomenclature of dihydric alcohols
Organic compounds containing two hydroxyl groups ($-OH-$) are called dihydric alcohols or diols.
The general formula of dihydric alcohols is $CnH_(2n)(OH)_2$.
When designating dihydric alcohols, according to the IUPAC nomenclature, the prefix di- is added to the ending -ol, that is, the dihydric alcohol has the ending "diol". The numbers indicate which carbon atoms the hydroxyl groups are attached to, for example:
Picture 1.
1,2-propanediol trans-1,2-cyclohexanediol 1-cyclohexyl-1,4-pentadiol
In the systematic nomenclature, there is a differentiation between 1,2-, 1,3-, 1,4-, etc. diols.
If the compound contains hydroxyl groups at adjacent (vicial) carbon atoms, then dihydric alcohols are called glycols.
The names of glycols reflect the way they are obtained by hydroxylation of alkenes, for example:
Figure 2.
The existence of stable dihydric alcohols is possible, starting with ethane, which corresponds to one diol - ethylene glycol. For propane, the existence of two alcohols is possible: 1,2- and 1,3- propanediols.
Of the alcohols corresponding to normal butane, the following compounds may exist:
- both hydroxo groups are nearby - one in the $CH_3$ group, the other in the $CH_2$ group;
- both hydroxyls are located in neighboring $CH_2$ groups;
- hydroxo groups are adjacent to non-neighboring carbon atoms, in $CH_3$ and $CH_2$ groups;
- both hydroxyls are located in the $CH_3$ groups.
Isobutane corresponds to the following diols:
- hydroxo groups are nearby - in groups $CH_3$ and $CH$;
- both hydroxyls are located in the $CH_3$ groups:
Figure 3
Dihydric alcohols can be classified based on which alcohol groups are included in the composition of their particles:
- Dual Primary Glycols. Ethylene glycol contains two primary alcohol groups.
- Bi-secondary glycols. They contain two secondary alcohol groups.
- Bi-tertiary glycols. They contain three secondary alcohol groups.
- Mixed glycols: primary - secondary, primary - tertiary, secondary - tertiary.
For example: isopentane corresponds to secondary-tertiary glycol
Figure 4
Hexane (tetramethyl-ethane) corresponds to a two-tertiary glycol:
Figure 5
If in a dihydric alcohol both hydroxyls are located at neighboring carbon atoms, then these are $\alpha$-glycols. $\beta$-glycols appear when the hydroxo groups are separated by one carbon atom. Diols of the $\gamma$-series have hydroxyls located through two carbon atoms. At a greater distance from each other of hydroxyls, diols of the $\delta$- and $\varepsilon$-series appear.
Geminal diols
In the free state, only such diols can exist that originated from hydrocarbons as a result of the replacement by hydroxyl groups of two hydrogen atoms located at two different carbon atoms. When hydroxo groups replace two hydrogen atoms at the same carbon atom, unstable compounds arise - geminal diols or gem diols.
Geminal diols are dihydric alcohols containing both hydroxyl groups on the same carbon atom. These are unstable compounds, they easily decompose with the elimination of water and the formation of a carbonyl compound:
Figure 6
The equilibrium is shifted towards the formation of a ketone, which is why geminal diols are also called hydrates of aldehydes or ketones.
The simplest representative of geminal diols is methylene glycol. This compound is relatively stable in aqueous solutions. However, attempts to isolate it lead to the appearance of a dehydration product - formaldehyde:
$HO-CH_2-OH \leftrightarrow H_2C=O + H_2O$
For example: A dihydric alcohol corresponding to ethane cannot exist in a free state if both hydroxyl groups are on the same carbon atom. Water is immediately released and acetaldehyde is formed:
Figure 7
Two dihydric alcohols corresponding to propane are also not capable of independent existence, since they will release water due to hydroxyls located at one carbon atom. In this case, propionaldehyde will be formed in one case, acetone in the other:
Figure 8
A small number of gem-diols may exist in a non-dissolved state. These are compounds that contain strong electron-withdrawing substituents, such as chloral hydrate and hexaphotracetone hydrate.
Figure 9
Physical properties of glycols
Glycols have the following physical properties:
- lower glycols are colorless transparent liquids with a sweetish taste;
- high boiling and melting point (tbp of ethylene glycol 197$^\circ$С);
- high density and viscosity;
- good solubility in water, ethyl alcohol;
- poor solubility in non-polar solvents (eg ethers and hydrocarbons).
General pattern: with an increase in the molecular weight of dihydric alcohols, the boiling point increases. As a result, the solubility in water decreases. Lower alcohols are miscible with water in any ratio. Higher diols have greater solubility in ether and less in water.
For many substances, dihydric alcohols act as good solvents (with the exception of aromatic and higher saturated hydrocarbons).
Glycols. Hydroxyl groups in glycols are found at different carbon atoms. Glycols with two hydroxyls on one carbon atom are unstable. They split off water to form aldehydes or ketones.
Isomerism of glycols is determined by the mutual arrangement of hydroxyl groups and the isomerism of the carbon skeleton. Depending on the mutual arrangement of OH– groups, there are α-, β-, γ-, δ-, … glycols. Depending on the nature of the carbon atoms bearing hydroxyls, glycols can be primary-secondary, primary-tertiary, two-primary, two-secondary, etc.
Names of glycols can be given in two ways. According to the IUPAC nomenclature, the suffix is added to the name of the main carbon chain –diol and indicate the numbers of carbon atoms of the longest carbon chain bearing hydroxyl groups. Titles α- glycols can be derived from the name of the corresponding ethylene carbon with the addition of the word glycol. The classification and names of glycols are given below using butanediols as an example:
Ways to get. In principle, glycols can be obtained by all conventional synthetic methods for the preparation of alcohols.
The following reactions are examples.
– Hydrolysis of dihalogen derivatives of saturated hydrocarbons and halohydrins:
– Hydration α -oxides in an acidic environment:
– Oxidation of olefins potassium permanganate in a dilute aqueous slightly alkaline solution (Wagner reaction) or hydrogen peroxide in the presence of catalysts (CrO 3):
physical properties. Lower glycols are highly soluble in water. Their density is higher than that of monohydric alcohols. Accordingly, the boiling point is also higher due to the significant association of molecules: for example, ethylene glycol boils at a temperature of 197.2 ° C; propylene glycol - at a temperature of 189 ° C and butanediol-1,4 - at a temperature of 230 ° C.
Chemical properties. Everything said earlier about the properties of the corresponding monohydric alcohols is also applicable to glycols. It should be remembered that either one hydroxyl or both can enter into the reaction. – Oxidation of two-primary glycols gives aldehydes:
– When oxidized α- glycols with iodic acid there is a break in the bond between the carbon atoms bearing hydroxyls, and the formation of the corresponding aldehydes or ketones:
The method is of great importance for establishing the structure α- glycols.
-Results intramolecular elimination of water glycols to a large extent depend on the type of glycol.
Dehydration of α-glycols proceeds with the formation of aldehydes or ketones, γ-glycols due to the atoms of hydroxyl groups, water is split off with the formation of heterocyclic compounds - tetrahydrofuran or its homologues:
The first reaction proceeds through the formation of a carbonium ion, followed by the displacement of a hydrogen atom with its electron pair:
At vapor phase dehydration over Al 2 O 3 α- two-tertiary glycols, called pinacons, diene hydrocarbons are obtained:
– Intermolecular dehydration leads to the formation of hydroxyethers or cyclic ethers:
The boiling point of diethylene glycol is 245.5 °C. It is used as a solvent for filling hydraulic brake systems, in finishing and dyeing fabrics.
Among the cyclic ethers, dioxane is the most widely used solvent. It was obtained for the first time by A.E. Favor heating of ethylene glycol with sulfuric acid:
ethylene glycol- it is a viscous colorless liquid, sweetish in taste, t kip \u003d 197.2 ° C. On an industrial scale, it is obtained from ethylene in three ways.
When mixed with water, ethylene glycol greatly lowers its freezing point. For example, 60% aqueous glycol solution freezes at -49 °C and is successfully used as an antifreeze. The high hygroscopicity of ethylene glycol is used to prepare printing inks. A large amount of ethylene glycol is used to obtain film-forming materials, varnishes, paints, synthetic fibers (for example, lavsan - polyethylene terephthalate), dioxane, diethylene glycol and other products.
Polyhydric alcohols
Polyhydric alcohols - alcohols having several hydroxyl groups OH.
Polyhydric alcohols with a small number of carbon atoms are viscous liquids, higher alcohols are solids. Polyhydric alcohols can be obtained by the same synthetic methods as saturated polyhydric alcohols. Obtaining alcohols
1. Obtaining ethyl alcohol (or wine alcohol) by fermenting carbohydrates:
C2H12O6 => C2H5-OH + CO2
The essence of fermentation lies in the fact that one of the simplest sugars - glucose, obtained in technology from starch, under the influence of yeast fungi, breaks down into ethyl alcohol and carbon dioxide. It has been established that the fermentation process is caused not by the microorganisms themselves, but by the substances they secrete - zymases. To obtain ethyl alcohol, vegetable raw materials rich in starch are usually used: potato tubers, bread grains, rice grains, etc.
2. Hydration of ethylene in the presence of sulfuric or phosphoric acid
CH2=CH2 + KOH => C2H5-OH
3. In the reaction of haloalkanes with alkali:
4. In the oxidation reaction of alkenes
5. Hydrolysis of fats: in this reaction, the well-known alcohol is obtained - glycerin
Properties of alcohols
1) Combustion: Like most organic substances, alcohols burn to form carbon dioxide and water:
C2H5-OH + 3O2 -->2CO2 + 3H2O
When they burn, a lot of heat is released, which is often used in laboratories. Lower alcohols burn with an almost colorless flame, while higher alcohols have a yellowish flame due to incomplete combustion of carbon.
2) Reaction with alkali metals
C2H5-OH + 2Na --> 2C2H5-ONa + H2
In this reaction, hydrogen is released and sodium alcoholate is formed. Alcoholates are similar to salts of a very weak acid, and they are also easily hydrolyzed. Alcoholates are extremely unstable and, under the action of water, decompose into alcohol and alkali.
3) Reaction with hydrogen halide C2H5-OH + HBr --> CH3-CH2-Br + H2O
This reaction produces a haloalkane (bromoethane and water). Such a chemical reaction of alcohols is due not only to the hydrogen atom in the hydroxyl group, but also to the entire hydroxyl group! But this reaction is reversible: for it to proceed, a water-removing agent, such as sulfuric acid, must be used.
4) Intramolecular dehydration (in the presence of H2SO4 catalyst)
The removal of a hydrogen atom from alcohol can occur in its own. This reaction is an intermolecular dehydration reaction. For example, like this:
During the reaction, an ether and water are formed.
5) reaction with carboxylic acids:
If a carboxylic acid, such as acetic acid, is added to alcohol, an ether is formed. But esters are less stable than ethers. If the reaction of formation of a simple ether is almost irreversible, then the formation of a complex ester is a reversible process. Esters easily undergo hydrolysis, decomposing into alcohol and carboxylic acid.
6) Oxidation of alcohols. At ordinary temperatures, alcohols are not oxidized by atmospheric oxygen, but when heated in the presence of catalysts, oxidation occurs. An example is copper oxide (CuO), potassium permanganate (KMnO4), chromium mixture. Under the action of oxidizing agents, various products are obtained and depend on the structure of the initial alcohol. So, primary alcohols turn into aldehydes (reaction A), secondary ones - into ketones (reaction B), and tertiary alcohols are resistant to oxidizing agents.
- a) for primary alcohols
- b) for secondary alcohols
- c) tertiary alcohols are not oxidized by copper oxide!
As for polyhydric alcohols, they have a sweetish taste, but some of them are poisonous. The properties of polyhydric alcohols are similar to monohydric alcohols, with the difference being that the reaction does not proceed one by one to the hydroxyl group, but several at once.
One of the main differences is that polyhydric alcohols easily react with copper hydroxide. This produces a clear solution of a bright blue-violet color. It is this reaction that can detect the presence of a polyhydric alcohol in any solution.
Interact with nitric acid:
Ethylene glycol is a typical representative of polyhydric alcohols. Its chemical formula is CH2OH - CH2OH. - dihydric alcohol. It is a sweet liquid that can dissolve perfectly in water in any proportion. Both one hydroxyl group (-OH) and two at the same time can participate in chemical reactions. Ethylene glycol - its solutions - are widely used as an anti-icing agent (antifreeze). The ethylene glycol solution freezes at a temperature of -340C, which can replace water during the cold season, for example, for cooling cars.
With all the benefits of ethylene glycol, it must be taken into account that this is a very strong poison!
Individual representatives
methanol(methyl, wood alcohol) - a colorless liquid with a slight alcohol odor. A large amount of it is used in the production of formaldehyde, formic acid, methyl and dimethylaniline, methylamines and many dyes, pharmaceuticals, and fragrances. Methanol is a good solvent, so it is widely used in the paint and varnish industry, as well as in the oil industry for the purification of gasoline from mercaptans, for the isolation of toluene by azeotropic distillation.
ethanol(ethyl, wine alcohol) - a colorless liquid with a characteristic alcohol odor. Ethyl alcohol is used in large quantities in the production of divinyl (processed into synthetic rubbers), diethyl ether, chloroform, chloral, high purity ethylene, ethyl acetate and other esters used as solvents for varnishes and fragrances (fruit essences). As a solvent, ethyl alcohol is widely used in the production of pharmaceutical, fragrance, coloring and other substances. Ethanol is a good antiseptic.
propyl and isopropyl alcohols. These alcohols, as well as their esters, are used as solvents. In some cases, they replace ethyl alcohol. Isopropyl alcohol is used to make acetone.
Butyl alcohol and its esters are used in large quantities as solvents for varnishes and resins.
Isobutyl alcohol used to obtain isobutylene, isobutyric aldehyde, isobutyric acid, and also as a solvent.
Primary amyl and isoamyl alcohols make up the bulk of fusel oil (by-products in the production of ethyl alcohol from potatoes or cereals). Amyl alcohols and their esters are good solvents. Isoamyl acetate (pear essence) is used in the manufacture of soft drinks and some confectionery.
Lecture number 15.Polyhydric alcohols
polyhydric alcohols. Classification. Isomerism. Nomenclature. Dihydric alcohols (glycols). trihydric alcohols. Glycerol. Synthesis from fats and propylene. The use of glycol and glycerin in industry.
Two hydroxyl groups cannot be on the same carbon atom; such compounds easily lose water, turning into aldehydes or ketones:
This property is common to all gem-diols. Sustainability gem-diols increases in the presence of electron-withdrawing substituents. An example of sustainable gem-diol is a chloral hydrate.
