Complete combustion of alcohols

Alcohols combust completely when burned in an excess of oxygen, breaking all C-C and C-H bonds. This results in the production of carbon dioxide and water, along with the release of heat energy.

For example, the complete combustion of ethanol is represented by the equation:

C₂H₅OH_(l) + 3O_2(g) ➔ 2CO_2(g) + 3H₂O_(g)

Oxidation of alcohols using potassium dichromate(VI) solution

Oxidation of alcohols can be performed using a potassium dichromate(VI) solution (K₂Cr₂O₇), acidified with dilute sulfuric acid. The solution changes colour from orange to green as the reaction proceeds due to the reduction of dichromate(VI) ions (Cr₂O₇^2-) to chromium(III) ions (Cr³⁺).

The extent of oxidation depends on the alcohol's structure:

• Primary alcohols are oxidised to aldehydes and then to carboxylic acids.
• Secondary alcohols are oxidised to ketones only.
• Tertiary alcohols do not oxidise under these conditions.

Controlling oxidation of primary alcohols

The oxidation of primary alcohols can proceed in two stages, initially forming an aldehyde and subsequently a carboxylic acid:

Here, [O] represents an oxidising agent.

To isolate the aldehyde, gently heat the alcohol in a distillation apparatus with a limited amount of potassium dichromate(VI) and distil the aldehyde as it forms to prevent further oxidation. To obtain the carboxylic acid, heat the alcohol with an excess of dichromate(VI) under reflux conditions.

Oxidising secondary alcohols

Secondary alcohols, such as propan-2-ol, can be converted into ketones by refluxing with acidified dichromate(VI).

This process does not allow for further oxidation of the ketone:

Structure of aldehydes, ketones and carboxylic acids

Aldehydes and ketones are characterised by the presence of a carbonyl functional group (C=O), but differ in their structure:

• Aldehydes have a hydrogen atom and an alkyl group attached to the carbonyl carbon.
• Ketones have two alkyl groups attached to the carbonyl carbon.

Carboxylic acids feature a carboxyl functional group (COOH) attached to an alkyl group.

Distinguishing aldehydes and ketones

Aldehydes and ketones can be differentiated using Fehling's solution and Tollens’ reagent:

1. Aldehydes reduce Fehling's solution from blue to brick-red Cu₂O when warmed, whereas ketones do not cause any colour change.
2. Aldehydes reduce Tollens’ reagent, resulting in a silver mirror on the glassware, while ketones do not react.

Dehydrating alcohols to form alkenes

Dehydration of alcohols, an elimination reaction facilitated by a heated acid catalyst such as concentrated H₂SO₄, results in the formation of alkenes through the elimination of water:

alcohol ➔ alkene + water

For instance, dehydrating ethanol in the presence of a concentrated sulfuric acid catalyst produces ethene:

CH₃CH₂OH ➔ CH₂=CH₂ + H₂O

This elimination mechanism involves three steps:

1. The protonation of the alcohol oxygen.
2. The formation of a carbocation intermediate via the loss of water.
3. The formation of an alkene via the loss of a proton.

When more complex alcohols undergo dehydration, elimination can occur from different positions along the carbon chain. This results in a mixture of alkene positional isomers as products.

For example, the dehydration of butan-2-ol gives a mixture of but-1-ene, E-but-2-ene and Z-but-2-ene: