Organic chemistry. Alcohols

Which contain one or more hydroxyl groups. Depending on the number of OH groups, these are divided into monohydric alcohols, trihydric alcohols, etc. Most often, these complex substances are considered as derivatives of hydrocarbons, the molecules of which have undergone changes, because one or more hydrogen atoms have been replaced by a hydroxyl group.

The simplest representatives of this class are monohydric alcohols, general formula which looks like this: R-OH or

Cn+H 2n+1OH.

1. Alcohols containing up to 15 carbon atoms are liquids, 15 or more are solids.
2. Solubility in water depends on molecular weight The higher it is, the less soluble alcohol is in water. Thus, lower alcohols (up to propanol) are mixed with water in any proportions, while higher alcohols are practically insoluble in it.
3. The boiling point also increases with increasing atomic mass, for example, t bp. CH3OH = 65 °C, and boiling point. C2H5OH =78 °C.
4. The higher the boiling point, the lower the volatility, i.e. the substance does not evaporate well.

These physical properties of saturated alcohols with one hydroxyl group can be explained by the occurrence of intermolecular hydrogen bonds between individual molecules of the compound itself or the alcohol and water.

Monohydric alcohols are capable of entering into the following chemical reactions:

Having examined the chemical properties of alcohols, we can conclude that monohydric alcohols are amphoteric compounds, because they can react with alkali metals, exhibiting weak and with hydrogen halides, exhibiting basic properties. All chemical reactions occur with a discontinuity O-N connections or S-O.

Thus, saturated monohydric alcohols are complex compounds with one OH group that do not have free valences after formation S-S connections and exhibiting weak properties of both acids and bases. Due to their physical and chemical properties, they are widely used in organic synthesis, in the production of solvents, fuel additives, as well as in Food Industry, medicine, cosmetology (ethanol).

DEFINITION

Saturated monohydric alcohols can be considered as derivatives of hydrocarbons of the methane series, in the molecules of which one hydrogen atom is replaced by a hydroxyl group.

So, saturated monohydric alcohols consist of a hydrocarbon radical and the -OH functional group. In the names of alcohols, the hydroxyl group is designated by the suffix -ol.

The general formula of saturated monohydric alcohols is C n H 2 n +1 OH or R-OH or C n H 2 n +2 O. The molecular formula of an alcohol does not reflect the structure of the molecule, since two completely different substances can correspond to the same gross formula, for example, the molecular formula C 2 H 5 OH is common to both ethyl alcohol and acetone (dimethyl ketone):

CH 3 -CH 2 -OH (ethanol);

CH 3 -O-CH 3 (acetone).

Just like the hydrocarbons of the methane series, saturated monohydric alcohols form a homologous series of methanol.

Let's compose this series of homologues and consider the patterns of changes in the physical properties of compounds of this series depending on the increase in the hydrocarbon radical (Table 1).

Homologous series (incomplete) of saturated monohydric alcohols

Table 1. Homologous series (incomplete) of saturated monohydric alcohols.

Saturated monohydric alcohols are lighter than water, since their density less than one. Lower alcohols are miscible with water in all respects; as the hydrocarbon radical increases, this ability decreases. Most alcohols are highly soluble in organic solvents. Alcohols have higher boiling and melting points than the corresponding hydrocarbons or halogen derivatives, which is due to the possibility of their formation of intermolecular bonds.

The most important representatives of saturated monohydric alcohols are methanol (CH 3 OH) and ethanol (C 2 H 5 OH).

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

DEFINITION

Alcohols– compounds containing one or more hydroxyl groups –OH associated with a hydrocarbon radical.

The general formula of the homologous series of saturated monohydric alcohols is C n H 2 n +1 OH. The names of alcohols contain the suffix – ol.

Depending on the number of hydroxyl groups, alcohols are divided into one- (CH 3 OH - methanol, C 2 H 5 OH - ethanol), two- (CH 2 (OH)-CH 2 -OH - ethylene glycol) and triatomic (CH 2 (OH )-CH(OH)-CH 2 -OH - glycerol). Depending on which carbon atom the hydroxyl group is located at, primary (R-CH 2 -OH), secondary (R 2 CH-OH) and tertiary alcohols (R 3 C-OH) are distinguished.

Saturated monohydric alcohols are characterized by isomerism of the carbon skeleton (starting from butanol), as well as isomerism of the position of the hydroxyl group (starting from propanol) and interclass isomerism with ethers.

CH 3 -CH 2 -CH 2 -CH 2 -OH (butanol – 1)

CH 3 -CH (CH 3) - CH 2 -OH (2-methylpropanol - 1)

CH 3 -CH (OH) -CH 2 -CH 3 (butanol - 2)

CH 3 -CH 2 -O-CH 2 -CH 3 (diethyl ether)

Chemical properties of alcohols

1. Reactions that occur with the rupture of the O-H bond:

— the acidic properties of alcohols are very weakly expressed. Alcohols react with alkali metals

2C 2 H 5 OH + 2K → 2C 2 H 5 OK + H 2

but do not react with alkalis. In the presence of water, alcoholates are completely hydrolyzed:

C 2 H 5 OK + H 2 O → C 2 H 5 OH + KOH

This means that alcohols are weaker acids than water.

- education esters under the influence of mineral and organic acids:

CH 3 -CO-OH + H-OCH 3 ↔ CH 3 COOCH 3 + H 2 O

- oxidation of alcohols under the action of potassium dichromate or permanganate to carbonyl compounds. Primary alcohols are oxidized to aldehydes, which in turn can be oxidized to carboxylic acids.

R-CH 2 -OH + [O] → R-CH = O + [O] → R-COOH

Secondary alcohols are oxidized to ketones:

R-CH(OH)-R’ + [O] → R-C(R’) = O

Tertiary alcohols are more resistant to oxidation.

2. Reaction with breaking of the C-O bond.

- intramolecular dehydration with the formation of alkenes (occurs with strong heating of alcohols with water-removing substances (concentrated sulfuric acid)):

CH 3 -CH 2 -CH 2 -OH → CH 3 -CH = CH 2 + H 2 O

— intermolecular dehydration of alcohols with the formation of ethers (occurs when alcohols are slightly heated with water-removing substances (concentrated sulfuric acid)):

2C 2 H 5 OH → C 2 H 5 -O-C 2 H 5 + H 2 O

— weak basic properties of alcohols manifest themselves in reversible reactions with hydrogen halides:

C 2 H 5 OH + HBr → C 2 H 5 Br + H 2 O

Physical properties of alcohols

Lower alcohols (up to C 15) are liquids, higher alcohols are solids. Methanol and ethanol are mixed with water in any ratio. As the molecular weight increases, the solubility of alcohols in alcohol decreases. Alcohols have high boiling and melting points due to the formation of hydrogen bonds.

Preparation of alcohols

The production of alcohols is possible using a biotechnological (fermentation) method from wood or sugar.

Laboratory methods for producing alcohols include:

- hydration of alkenes (the reaction occurs when heated and in the presence of concentrated sulfuric acid)

CH 2 = CH 2 + H 2 O → CH 3 OH

— hydrolysis of alkyl halides under the influence of aqueous solutions of alkalis

CH 3 Br + NaOH → CH 3 OH + NaBr

CH 3 Br + H 2 O → CH 3 OH + HBr

— reduction of carbonyl compounds

CH 3 -CH-O + 2[H] → CH 3 – CH 2 -OH

Examples of problem solving

EXAMPLE 1

The content of the article

ALCOHOLS(alcohols) - a class of organic compounds containing one or more C–OH groups, with the hydroxyl group OH bonded to an aliphatic carbon atom (compounds in which the carbon atom in the C–OH group is part of the aromatic ring are called phenols)

The classification of alcohols is varied and depends on which structural feature is taken as a basis.

1. Depending on the number of hydroxyl groups in the molecule, alcohols are divided into:

a) monoatomic (contain one hydroxyl OH group), for example, methanol CH 3 OH, ethanol C 2 H 5 OH, propanol C 3 H 7 OH

b) polyatomic (two or more hydroxyl groups), for example, ethylene glycol

HO–CH 2 –CH 2 –OH, glycerol HO–CH 2 –CH(OH)–CH 2 –OH, pentaerythritol C(CH 2 OH) 4.

Compounds in which one carbon atom has two hydroxyl groups are in most cases unstable and easily turn into aldehydes, eliminating water: RCH(OH) 2 ® RCH=O + H 2 O

2. Based on the type of carbon atom to which the OH group is bonded, alcohols are divided into:

a) primary, in which the OH group is bonded to the primary carbon atom. A carbon atom (highlighted in red) that is bonded to just one carbon atom is called primary. Examples of primary alcohols - ethanol CH 3 - C H 2 –OH, propanol CH 3 –CH 2 – C H2–OH.

b) secondary, in which the OH group is bonded to a secondary carbon atom. A secondary carbon atom (highlighted in blue) is bonded to two carbon atoms at the same time, for example, secondary propanol, secondary butanol (Fig. 1).

Rice. 1. STRUCTURE OF SECONDARY ALCOHOLS

c) tertiary, in which the OH group is bonded to the tertiary carbon atom. Tertiary carbon atom (highlighted green) is bonded simultaneously to three neighboring carbon atoms, for example, tertiary butanol and pentanol (Fig. 2).

Rice. 2. STRUCTURE OF TERTIARY ALCOHOLS

According to the type of carbon atom, the alcohol group attached to it is also called primary, secondary or tertiary.

U polyhydric alcohols containing two or more OH groups, both primary and secondary HO groups can be present simultaneously, for example, in glycerol or xylitol (Fig. 3).

Rice. 3. COMBINATION OF PRIMARY AND SECONDARY OH-GROUPS IN THE STRUCTURE OF POLYATOMIC ALCOHOLS.

3. According to the structure of organic groups connected by an OH group, alcohols are divided into saturated (methanol, ethanol, propanol), unsaturated, for example, allyl alcohol CH 2 =CH–CH 2 –OH, aromatic (for example, benzyl alcohol C 6 H 5 CH 2 OH) containing an aromatic group in the R group.

Unsaturated alcohols in which the OH group is “adjacent” to the double bond, i.e. bonded to a carbon atom simultaneously involved in the formation of a double bond (for example, vinyl alcohol CH 2 =CH–OH), are extremely unstable and immediately isomerize ( cm ISOMERIZATION) to aldehydes or ketones:

CH 2 =CH–OH ® CH 3 –CH=O

Nomenclature of alcohols.

For common alcohols with a simple structure, a simplified nomenclature is used: the name of the organic group is converted into an adjective (using the suffix and ending “ new") and add the word "alcohol":

In the case where the structure of an organic group is more complex, rules common to all organic chemistry are used. Names compiled according to such rules are called systematic. In accordance with these rules, the hydrocarbon chain is numbered from the end to which the OH group is located closest. Next, this numbering is used to indicate the position of various substituents along the main chain; at the end of the name, the suffix “ol” and a number indicating the position of the OH group are added (Fig. 4):

Rice. 4. SYSTEMATIC NAMES OF ALCOHOLS. Functional (OH) and substituent (CH 3) groups, as well as their corresponding digital indices, are highlighted in different colors.

The systematic names of the simplest alcohols follow the same rules: methanol, ethanol, butanol. For some alcohols, trivial (simplified) names that have developed historically have been preserved: propargyl alcohol HCє C–CH 2 –OH, glycerin HO–CH 2 –CH(OH)–CH 2 –OH, pentaerythritol C(CH 2 OH) 4, phenethyl alcohol C 6 H 5 –CH 2 –CH 2 –OH.

Physical properties of alcohols.

Alcohols are soluble in most organic solvents; the first three simplest representatives - methanol, ethanol and propanol, as well as tertiary butanol (H 3 C) 3 СОН - are mixed with water in any ratio. With an increase in the number of C atoms in the organic group, a hydrophobic (water-repellent) effect begins to take effect, solubility in water becomes limited, and when R contains more than 9 carbon atoms, it practically disappears.

Due to the presence of OH groups, hydrogen bonds arise between alcohol molecules.

Rice. 5. HYDROGEN BONDS IN ALCOHOLS(shown in dotted line)

As a result, all alcohols have more heat boiling point than that of the corresponding hydrocarbons, for example, T. bp. ethanol +78° C, and T. boil. ethane –88.63° C; T. kip. butanol and butane, respectively, +117.4° C and –0.5° C.

Chemical properties of alcohols.

Alcohols have a variety of transformations. The reactions of alcohols have some general patterns: reactivity primary monohydric alcohols are higher than secondary ones, in turn, secondary alcohols are chemically more active than tertiary ones. For dihydric alcohols, in the case when OH groups are located at neighboring carbon atoms, increased (compared to monohydric alcohols) reactivity is observed due to the mutual influence of these groups. For alcohols, reactions are possible that involve the breaking of both C–O and O–H bonds.

1. Reactions occurring at the O–H bond.

When interacting with active metals (Na, K, Mg, Al), alcohols exhibit the properties of weak acids and form salts called alcoholates or alkoxides:

2CH 3 OH + 2Na ® 2CH 3 OK + H 2

Alcoholates are chemically unstable and, when exposed to water, hydrolyze to form alcohol and metal hydroxide:

C 2 H 5 OK + H 2 O ® C 2 H 5 OH + KOH

This reaction shows that alcohols are weaker acids compared to water (a strong acid displaces a weak one); in addition, when interacting with alkali solutions, alcohols do not form alcoholates. However, in polyhydric alcohols (in the case when OH groups are attached to neighboring C atoms), the acidity of the alcohol groups is much higher, and they can form alcoholates not only when interacting with metals, but also with alkalis:

HO–CH 2 –CH 2 –OH + 2NaOH ® NaO–CH 2 –CH 2 –ONa + 2H 2 O

When HO groups in polyhydric alcohols are attached to non-adjacent C atoms, the properties of alcohols are close to monoatomic ones, since the mutual influence of HO groups does not appear.

When interacting with mineral or organic acids, alcohols form esters - compounds containing the R-O-A fragment (A is the acid residue). The formation of esters also occurs during the interaction of alcohols with anhydrides and acid chlorides of carboxylic acids (Fig. 6).

Under the action of oxidizing agents (K 2 Cr 2 O 7, KMnO 4), primary alcohols form aldehydes, and secondary alcohols form ketones (Fig. 7)

Rice. 7. FORMATION OF ALDEHYDES AND KETONES DURING THE OXIDATION OF ALCOHOLS

The reduction of alcohols leads to the formation of hydrocarbons containing the same number of C atoms as the molecule of the original alcohol (Fig. 8).

Rice. 8. BUTANOL RESTORATION

2. Reactions occurring at the C–O bond.

In the presence of catalysts or strong mineral acids, dehydration of alcohols (elimination of water) occurs, and the reaction can proceed in two directions:

a) intermolecular dehydration involving two alcohol molecules, in which the C–O bonds of one of the molecules are broken, resulting in the formation of ethers—compounds containing the R–O–R fragment (Fig. 9A).

b) intramolecular dehydration produces alkenes - hydrocarbons with a double bond. Often both processes—the formation of an ether and an alkene—occur in parallel (Fig. 9B).

In the case of secondary alcohols, during the formation of an alkene, two reaction directions are possible (Fig. 9B), the predominant direction is in which, during the condensation process, hydrogen is split off from the least hydrogenated carbon atom (marked by number 3), i.e. surrounded by fewer hydrogen atoms (compared to atom 1). Shown in Fig. 10 reactions are used to produce alkenes and ethers.

The cleavage of the C–O bond in alcohols also occurs when the OH group is replaced by a halogen or amino group (Fig. 10).

Rice. 10. REPLACEMENT OF OH-GROUP IN ALCOHOLS WITH HALOGEN OR AMINO GROUP

The reactions shown in Fig. 10 is used for the production of halocarbons and amines.

Preparation of alcohols.

Some of the reactions shown above (Fig. 6,9,10) are reversible and, when conditions change, can proceed in the opposite direction, leading to the production of alcohols, for example, during the hydrolysis of esters and halocarbons (Fig. 11A and B, respectively), as well as by hydration alkenes - by adding water (Fig. 11B).

Rice. eleven. OBTAINING ALCOHOLS BY HYDROLYSIS AND HYDRATION OF ORGANIC COMPOUNDS

The hydrolysis reaction of alkenes (Fig. 11, Scheme B) is the basis industrial production lower alcohols containing up to 4 C atoms.

Ethanol is also formed during the so-called alcoholic fermentation of sugars, for example, glucose C 6 H 12 O 6. The process occurs in the presence of yeast and leads to the formation of ethanol and CO 2:

C 6 H 12 O 6 ® 2C 2 H 5 OH + 2CO 2

Fermentation can produce no more than a 15% aqueous solution of alcohol, since at a higher concentration of alcohol the yeast fungi die. Higher concentration alcohol solutions are obtained by distillation.

Methanol is produced industrially by the reduction of carbon monoxide at 400° C under a pressure of 20–30 MPa in the presence of a catalyst consisting of copper, chromium, and aluminum oxides:

CO + 2 H 2 ® H 3 COH

If instead of hydrolysis of alkenes (Fig. 11) oxidation is carried out, then dihydric alcohols are formed (Fig. 12)

Rice. 12. PREPARATION OF DIOHOMIC ALCOHOLS

Use of alcohols.

The ability of alcohols to participate in various chemical reactions allows them to be used for the production of all kinds of organic compounds: aldehydes, ketones, carboxylic acids, ethers and esters used as organic solvents in the production of polymers, dyes and drugs.

Methanol CH 3 OH is used as a solvent, as well as in the production of formaldehyde, used to obtain phenol-formaldehyde resins, in Lately Methanol is considered as a promising motor fuel. Large volumes of methanol are used in production and transportation natural gas. Methanol is the most toxic compound among all alcohols, lethal dose when taken orally – 100 ml.

Ethanol C 2 H 5 OH is the starting compound for the production of acetaldehyde, acetic acid, as well as for the production of esters of carboxylic acids used as solvents. In addition, ethanol is the main component of all alcoholic beverages; it is widely used in medicine as a disinfectant.

Butanol is used as a solvent for fats and resins; in addition, it serves as a raw material for the production of fragrant substances (butyl acetate, butyl salicylate, etc.). In shampoos it is used as a component that increases the transparency of solutions.

Benzyl alcohol C 6 H 5 –CH 2 –OH in the free state (and in the form of esters) is contained in essential oils jasmine and hyacinth. It has antiseptic (disinfecting) properties; in cosmetics it is used as a preservative for creams, lotions, dental elixirs, and in perfumery as a fragrant substance.

Phenethyl alcohol C 6 H 5 –CH 2 –CH 2 –OH has a rose scent, is found in rose oil, and is used in perfumery.

Ethylene glycol HOCH 2 –CH 2 OH is used in the production of plastics and as an antifreeze (an additive that reduces the freezing point of aqueous solutions), in addition, in the manufacture of textile and printing inks.

Diethylene glycol HOCH 2 –CH 2 OCH 2 –CH 2 OH is used to fill hydraulic brake devices, as well as in the textile industry for finishing and dyeing fabrics.

Glycerol HOCH 2 –CH(OH)–CH 2 OH is used to produce polyester glyphthalic resins; in addition, it is a component of many cosmetic preparations. Nitroglycerin (Fig. 6) is the main component of dynamite, used in mining and railway construction as explosive.

Pentaerythritol (HOCH 2) 4 C is used to produce polyesters (pentaphthalic resins), as a hardener for synthetic resins, as a plasticizer for polyvinyl chloride, and also in the production of the explosive tetranitropentaerythritol.

Polyhydric alcohols xylitol СОН2–(СНН)3–CH2ОН and sorbitol СОН2– (СНН)4–СН2ОН have a sweet taste; they are used instead of sugar in production confectionery for diabetics and people suffering from obesity. Sorbitol is found in rowan and cherry berries.

Mikhail Levitsky

Hydrocarbon derivatives with one or more hydrogen atoms in the molecule replaced by an -OH group (hydroxyl group or hydroxy group) are alcohols. Chemical properties determined by a hydrocarbon radical and a hydroxyl group. Alcohols form a separate group in which each subsequent representative differs from the previous member by a homological difference corresponding to =CH2. All substances in this class can be represented by the formula: R-OH. For monoatomic saturated compounds, the general chemical formula is CnH2n+1OH. According to international nomenclature, names can be derived from hydrocarbons with the addition of the ending -ol (methanol, ethanol, propanol, and so on).

This is a very diverse and broad class. chemical compounds. Depending on the number of -OH groups in the molecule, it is divided into one-, two-, triatomic and so on - polyatomic compounds. The chemical properties of alcohols also depend on the content of hydroxy groups in the molecule. These substances are neutral and do not dissociate into ions in water, such as strong acids or strong bases. However, they can weakly exhibit both acidic (they decrease with increasing molecular weight and branching of the hydrocarbon chain in the series of alcohols) and basic (increasing with increasing molecular weight and branching of the molecule) properties.

The chemical properties of alcohols depend on the type and spatial arrangement of atoms: molecules come with chain isomerism and positional isomerism. Depending on the maximum number of single bonds of a carbon atom (linked to the hydroxy group) with other carbon atoms (with 1, 2 or 3), primary (normal), secondary or tertiary alcohols are distinguished. Primary alcohols have a hydroxyl group attached to the primary carbon atom. In secondary and tertiary - to secondary and tertiary, respectively. Starting with propanol, isomers appear that differ in the position of the hydroxyl group: propyl alcohol C3H7-OH and isopropyl alcohol CH3-(CHOH)-CH3.

It is necessary to name several main reactions that characterize the chemical properties of alcohols:

1. When reacting with or their hydroxides (deprotonation reaction), alcoholates are formed (the hydrogen atom is replaced by a metal atom), depending on the hydrocarbon radical, methylates, ethylates, propylates and so on are obtained, for example, sodium propoxide: 2CH3CH2OH + 2Na → 2CH3CH2ONa + H2.
2. When interacting with concentrated hydrohalic acids, HBr + CH3CH2OH ↔ CH3CH2Br + H2O are formed. This reaction is reversible. As a result, nucleophilic substitution of the hydroxyl group with a halogen ion occurs.
3. Alcohols can be oxidized to carbon dioxide, to aldehydes, or to ketones. Alcohols burn in the presence of oxygen: 3O2 + C2H5OH →2CO2 + 3H2O. Under the influence of a strong oxidizing agent (chromic acid, etc.), primary alcohols are converted into aldehydes: C2H5OH → CH3COH + H2O, and secondary alcohols are converted into ketones: CH3—(CHOH)—CH3 → CH3—(CHO)—CH3 + H2O.
4. The dehydration reaction occurs when heated in the presence of water-removing substances (sulfuric acid, etc.). As a result, alkenes are formed: C2H5OH → CH2=CH2 + H2O.
5. The esterification reaction also occurs when heated in the presence of water-subtracting compounds, but, unlike the previous reaction, at a lower temperature and with the formation of 2C2H5OH → C2H5-O-C2H5O. With sulfuric acid the reaction occurs in two stages. First, an ester of sulfuric acid is formed: C2H5OH + H2SO4 → C2H5O—SO2OH + H2O, then when heated to 140 ° C and in excess of alcohol, diethyl (often called sulfuric) ether is formed: C2H5OH + C2H5O—SO2OH → C2H5—O—C2H5O + H2SO4 .

Chemical properties of polyhydric alcohols, by analogy with their physical properties, depend on the type of hydrocarbon radical forming the molecule and, of course, the number of hydroxyl groups in it. For example, ethylene glycol CH3OH-CH3OH (boiling point 197 °C), which is a 2-atomic alcohol, is a colorless liquid (has a sweetish taste), which mixes with H2O, as well as lower alcohols in any ratio. Ethylene glycol, like its higher homologues, enter into all reactions characteristic of monohydric alcohols. Glycerol CH2OH—CHOH—CH2OH (boiling point 290 °C) is the simplest representative of 3-hydroxy alcohols. This is a thick, sweet-tasting liquid that cannot be mixed with it in any proportion. Dissolves in alcohol. Glycerol and its homologues are also characterized by all reactions of monohydric alcohols.

The chemical properties of alcohols determine the areas of their use. They are used as fuel (bioethanol or biobutanol and others), as solvents in various industries; as a raw material for the production of surfactants and detergents; for synthesis polymer materials. Some members of this class of organic compounds are widely used as lubricants or hydraulic fluids, as well as in the manufacture of medicines and biologically active substances.