Derivatives of hydrocarbons with one or more hydrogen atoms in the molecule, substituted by the –OH group (hydroxyl group or hydroxy group), are alcohols. Chemical properties are determined by a hydrocarbon radical and a hydroxyl group. Alcohols form a separate homologous series, in it each subsequent representative differs from the previous term by a homological difference corresponding to = CH2. All substances of this class can be represented by the formula: R – OH. For monatomic limit compounds, the general chemical formula has the form CnH2n + 1OH. According to the international nomenclature, names can be formed from a hydrocarbon with the addition of the –ol ending (methanol, ethanol, propanol and so on).
This is a very diverse and extensive class of chemical compounds. Depending on the number of –OH groups in the molecule, it is subdivided into mono-, 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, both acidic (decrease with increasing molecular weight and branching of the hydrocarbon chain) and basic properties (increase with increasing molecular weight and branching of the molecule) properties can be weakly manifested.
The chemical properties of alcohols depend on the type and spatial arrangement of atoms: molecules come with isomerism of the chain and isomerism of position. Depending on the maximum number of single bonds of a carbon atom (bonded to an oxy group) with other carbon atoms (with 1, 2 or 3) primary (normal), secondary or tertiary alcohols are distinguished. In primary alcohols, the hydroxyl group is 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 basic reactions that characterize the chemical properties of alcohols:
- When interacting with alkali metals or their hydroxides (deprotonation reaction), alkoxides are formed (the hydrogen atom is replaced by a metal atom), depending on the hydrocarbon radical, methylates, ethylates, propylates and so on, for example, sodium propylate: 2CH3CH2OH + 2Na → 2CH3CH2ONa + H2 ↑
- When interacting with concentrated hydrohalic acids, halogenated hydrocarbons are formed : HBr + CH3CH2OH ↔ CH3CH2Br + H2O. This reaction is reversible. As a result, nucleophilic substitution of a hydroxyl group by a halogen ion occurs.
- 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, potassium permanganate , etc.), primary alcohols are converted to aldehydes: C2H5OH → CH3COH + H2O, and secondary alcohols to ketones: CH3— (CHOH) —CH3 → CH3— (CHO) —CH3 + H2O.
- The dehydration reaction proceeds when heated in the presence of water-removing substances (zinc chloride, sulfuric acid and so on). As a result, alkenes are formed: C2H5OH → CH2 = CH2 + H2O.
- The esterification reaction also proceeds when heated in the presence of water-taking compounds, but, unlike the previous reaction, at a lower temperature and with the formation of ethers: 2C2H5OH → C2H5 – O – C2H5O. With sulfuric acid, the reaction occurs in two stages. First, sulfuric acid ester is formed: C2H5OH + H2SO4 → C2H5O — SO2OH + H2O, then when heated to 140 ° C and in excess of alcohol, diethyl ether (often called sulfuric) is formed: C2H5OH + C2H5O — SO2OH → C2H5 — O — C2H5O + H2SO4 .
The 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) that mixes with H2O and also lower alcohols in any ratio. Ethylene glycol, like its higher homologs, enter into all the reactions characteristic of monohydric alcohols. Glycerin CH2OH – CHOH – CH2OH (boiling point 290 ° ) is the simplest representative of 3 atomic alcohols. This is a thick, sweet, tasting liquid that is heavier than water, but mixes with it in any ratio. It is soluble in alcohol. For glycerol and its homologs, all reactions of monohydric alcohols are also characteristic.
The chemical properties of alcohols determine their application. They are used as fuel (bioethanol or biobutanol and others), as solvents in various industries; as raw materials for the production of surfactants and detergents; for the synthesis of polymeric materials. Some representatives of this class of organic compounds are widely used as lubricants or hydraulic fluids, as well as for the manufacture of medicines and biologically active substances.