Revision: Alcohols, Phenols and Ethers Chemistry Science (English Medium) Class 12 CBSE

Organic compounds containing one or more hydroxyl (–OH) groups attached to a saturated carbon atom are called alcohols.

Organic compounds containing –OH group directly attached to an aromatic ring are called phenols.

Compounds formed by replacing the hydrogen of hydroxyl group of alcohol or phenol by an alkyl or aryl group are called ethers.

Alcohols containing one hydroxyl group are called monohydric alcohols.

Alcohols containing two hydroxyl groups are called dihydric alcohols.

Alcohols containing three hydroxyl groups are called trihydric alcohols.

An alcohol in which the –OH group is attached to a primary carbon atom is called primary alcohol.

An alcohol in which the –OH group is attached to a secondary carbon atom is called secondary alcohol.

An alcohol in which the –OH group is attached to a tertiary carbon atom is called tertiary alcohol.

The reaction in which phenol reacts with carbon dioxide in presence of sodium hydroxide is called Kolbe’s reaction.

The reaction in which phenol reacts with chloroform and sodium hydroxide to introduce –CHO group at ortho position is called Reimer–Tiemann reaction.

The preparation of ethers by reacting alkyl halide with sodium alkoxide is called Williamson synthesis.

The reaction in which alcohol reacts with carboxylic acid to form ester is called esterification reaction.

Statement:
Ethers undergo cleavage of C–O bond in presence of concentrated HI or HBr to form alkyl halides.

Mechanism:

Step 1: Protonation of ether oxygen
Step 2: Nucleophilic attack by halide ion

• Primary ether → SN2 mechanism

• Tertiary ether → SN1 mechanism

Order of reactivity of HX:
HI > HBr > HCl

Statement:
In addition of HX or water to an unsymmetrical alkene, hydrogen attaches to the carbon atom having greater number of hydrogen atoms.

Example:
Propene + H₂O → Propan-2-ol (major)

Reason:
Formation of more stable carbocation.

Statement:
In hydroboration–oxidation, boron attaches to less substituted carbon and finally OH group appears at less substituted carbon.

Result:
Anti-Markovnikov product.

Characteristic:
Occurs without carbocation rearrangement.

Step 1: Nucleophilic addition to carbonyl carbon.
Step 2: Hydrolysis of adduct.

Products:

• With formaldehyde → Primary alcohol

• With aldehyde → Secondary alcohol

• With ketone → Tertiary alcohol

Statement:
Alcohols and phenols form hydrogen bonds due to presence of –OH group.

Effect:

• Higher boiling points

• Greater solubility in water

Statement:
Alcohols are weak acids due to polar O–H bond and can donate a proton.

Order of acidity:
Primary > Secondary > Tertiary

Reason:
Electron donating alkyl groups decrease polarity of O–H bond.

Alcohols are weaker acids than water.

Statement:
Phenols are more acidic than alcohols due to resonance stabilisation of phenoxide ion.

Reason:

• Negative charge delocalised in phenoxide ion

• sp² hybridised carbon increases O–H polarity

Electron withdrawing groups (–NO₂) increase acidity.

Electron donating groups decrease acidity.

Step 1: Protonation of alcohol
Step 2: Formation of carbocation (slow step)
Step 3: Elimination of proton to form alkene

Order:
Tertiary > Secondary > Primary

Due to stability of carbocation.

Statement:
Ethers are prepared by reacting alkyl halide with sodium alkoxide.

Mechanism:
SN2 reaction.

Best for:
Primary alkyl halides.

Limitation:
Tertiary halides undergo elimination.