Alcohols

Alcohols

General formula: CnH2n+1OH

Examples: CH3OH - methanol, C2H5OH - ethanol.

Isomerism

For example: C3H7OH

Copyright S-cool

Stereoisomers of butan-2-ol exist (i.e. two optical isomers) form due to the presence of an asymmetrical carbon atom - one carbon which has attached to it four different groups.

Primary and secondary but not tertiary alcohols can be easily oxidised. Most alcohols also undergo elimination (dehydration) and substitution reactions.

Combustion

All alcohols can be oxidised by combustion, to carbon dioxide and water.

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H20(l)

Substitution to give haloalkanes

C2H5OH(l) + HBr(aq) → C2H5Br(aq) + H2O(l)

Mechanism:

Alcohols

In the above mechanism, the lone pair on the oxygen atom of the ethanol accepts a proton from the HBr. In doing this, a covalent bond is formed between the oxygen and the proton.

The intermediate formed in the reaction then loses a molecule of water and the Br- then attacks to carbonium ion to form the product.

Reaction with sodium

2R-OH + 2Na → 2RO-Na+ + H2 (sodium alkoxide + hydrogen)

Oxidation to carbonyl compounds and carboxylic acid.

The usual oxidising agent is acid. K2Cr2O7 which turns from orange to green (Cr3+). The reactions involve the loss of the hydroxyl hydrogen together with a hydrogen atom from the adjacent alkyl group.

With primary alcohols, the initial aldehyde product may be oxidised further to a carboxylic acid, particularly if heat or excess oxidising agent is used, but ketones cannot be readily oxidised (involves breaking C-C bond).

Tertiary alcohols have no hydrogen atom on the alkyl group adjacent to the hydroxyl group and they cannot be easily oxidised.

Primary alcohols → Aldehydes → Carboxylic acid

Secondary alcohols → Ketones

Tertiary alcohols resist oxidation

Dehydration of alcohols to form alkenes

This dehydration is achieved by passing ethanol vapour over hot alumina. In the lab, conc. H2SO4 is used.

C2H5OH(g) → C2H4(g) + H2O(g)

C2H5OH(l) + H+(aq) C2H4(g) + H20(l) + H+(aq)

Formation of esters

Alcohols react with acids to form esters. These esters can be regarded as being formed by replacing the H atom in the OH group of an organic acid by an alkyl group, and this can be achieved by direct reaction between alcohol and acid.

Alcohols

These reactions are both slow and reversible.

S-cool exclusive!!