NEET Chemistry Notes Alcohols,Phenols and ethers – Alcohols and Phenols
Alcohols and Phenols
Alcohols and Phenols
Alcohols and phenols are formed when a hydrogen atom from a hydrocarbon, aliphatic and aromatic respectively, is replaced by — OH group. The substitution of a hydrogen atom from a hydrocarbon by an alkoxy or aryloxy group (R — O / Ar — O) gives ethers.
The hydroxy derivatives of aliphatic hydrocarbons are called alcohols. They are obtained by replacing one or more hydrogen atoms of a hydrocarbon by the — OH group.
The important methods used to prepare alcohols are as follows:
- By Acid Catalysed Hydration
It takes place in the presence of dilute H2S04.
In this reaction, intermediate carbocation is formed and rearrange, therefore — OH gets attached at maximum degree of carbon.
- By Oxymercuration-Demercuration Reaction
Intermediate carbocation is not formed and alcohol is formed according to Markownikoffs rule.
- By Hydroboration Oxidation Reaction
Intermediate carbocation is not formed and alcohol is formed according to anti-Markownikoffs rule.
- By Reduction of Carbonyl Compounds
Aldehydes on reduction give primary alcohols and ketones give secondary alcohols in the presence of weak reducing agent (NaBH2).
- By Reduction of Acids and Esters
Carboxylic acids and esters on reduction, in the presence of strong reducing agent (LiAlH), give primary alcohols.
- From Grignard Reagent
Grignard reagent [R’ Mg X) on reaction with aldehydes or ketones followed by hydrolysis gives alcohols.
The nature of alcohol formed depends upon the aldehyde or ketone taken, e.g. if the reacting aldehyde is formaldehyde, primary alcohol —CH2OH is obtained while other aldehydes give secondary alcohols (— CHOH) with Grignard reagent. Ketones give tertiary alcohols with Grignard reagent.
Physical Properties of Alcohols
The detailed description of physical properties of alcohols, i.e. their boiling points, solubility are given below:
- Boiling Points
Alcohols have higher boiling point than haloalkanes of comparable molecular mass because alcohols have intermolecular hydrogen bonding. As the number of carbon atoms increases, boiling point increases.
The boiling point decreases with increase of branching in carbon chain
Alcohols are soluble in water due to ability to form hydrogen bonds with water. As the number of carbon atoms increases, solubility decreases.