Theory
1. From alkenes:
(i) By acid catalysed hydration: Alkenes react with water in the presence of acid as catalyst to form alcohols. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule.
The mechanism of the reaction involves the following three steps:
(ii) By hydroboration–oxidation: Diborane (BH3)2 reacts with alkenes to give trialkyl boranes as addition product. This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide.
•The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp2 carbon carrying greater number of hydrogen atoms.
•The alcohol so formed looks as if it has been formed by the addition of water to the alkene in a way opposite to the Markovnikov’s rule.
•In this reaction, alcohol is obtained in excellent yield.
2. From carbonyl compounds:
(i) By reduction of aldehydes and ketones:
•Aldehydes and ketones are reduced to the corresponding alcohols by addition of hydrogen in the presence of catalysts (catalytic hydrogenation).
•Catalyst: Finely divided metal such as platinum, palladium or nickel.
•Reducing agent: Sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4).
Aldehydes yield primary alcohols whereas ketones give secondary alcohols.
(ii) By reduction of carboxylic acids and esters:
Carboxylic acids are reduced to primary alcohols in excellent yields by lithium aluminium hydride, a strong reducing agent.
LiAlH4 is an expensive reagent, and therefore, used for preparing special chemicals only. Commercially, acids are reduced to alcohols by converting them to the esters, followed by their reduction using hydrogen in the presence of catalyst (catalytic hydrogenation).
3. From Grignard reagents
Alcohols are produced by the reaction of Grignard reagents with aldehydes and ketones.
The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an adduct. Hydrolysis of the adduct yields an alcohol.
The reaction produces a primary alcohol with methanal, a secondary alcohol with other aldehydes and tertiary alcohol with ketones.
Physical properties of alcohols/phenols:
- Mainly due to the hydroxyl group
- Nature of alkyl and aryl groups simply modify these properties
Boiling Points:
•increase with increase in the number of carbon atoms (increase in van der Waals forces).
•boiling points decrease with increase of branching in carbon chain (because of decrease in van der Waals forces with decrease in surface area)
• –OH group in alcohols and phenols is involved in intermolecular hydrogen bonding
Boiling points of alcohols and phenols are higher in comparison to other classes of compounds, namely hydrocarbons, ethers, haloalkanes and haloarenes of comparable molecular masses.
Solubility
•Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules as shown.
•The solubility decreases with increase in size of alkyl/aryl (hydrophobic) groups.
•Several of the lower molecular mass alcohols are miscible with water in all proportions.
