Reactions of the Alcohols

This lesson covers: 

1. Combustion of alcohols
2. Oxidation of alcohols
3. Structures of aldehydes, ketones and carboxylic acids
4. Substitution of alcohols to produce halogenoalkanes
5. Dehydration of alcohols to produce alkenes
6. Esterification of alcohols
7. Reaction with sodium

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 acidified K₂Cr₂O₇ or KMnO₄ solutions

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⁺).

Alcohols can also be oxidised using acidified potassium permanganate(VII) (KMnO₄), which changes colour from purple to colourless as the reaction proceeds. This colour change is due to the reduction of permanganate(VII) ions (MnO₄^-) to manganese(II) ions (Mn²⁺).

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.

Converting alcohols to halogenoalkanes

Alcohols can undergo substitution reactions to form halogenoalkanes using various reagents:

1. Reaction with hydrogen halides (HX) or sodium halides and concentrated acids:

• Alcohols can react directly with hydrogen halides, such as HCl or HBr, or with sodium halides (e.g., NaCl or KBr) in the presence of concentrated sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄).
• When using sodium halides and concentrated acids, the acid generates the hydrogen halide in situ, which then reacts with the alcohol.
• These reactions replace the -OH group with a halogen atom, producing a halogenoalkane and water. Examples:

C₂H₅OH + HCl ➔ C₂H₅Cl + H₂O

C₂H₅OH + NaCl + H₂SO₄ ➔ C₂H₅Cl + NaHSO₄ + H₂O

1. Reaction with phosphorus chlorides:

• Alcohols can react with phosphorus halides, such as PCl₃ or PCl₅, to form halogenoalkanes.
• These reactions may require heating and produce acidic byproducts. Examples:

C₂H₅OH + PCl5 ➔ C₂H₅Cl + HCl + POCl₃

3C₂H₅OH + PCl3 ➔ 3C₂H₅Cl + H₃PO₃ (requires heating)

1. Reaction with thionyl chloride (SOCl₂):

• Alcohols can be converted to chloroalkanes using thionyl chloride.
• This reaction produces gaseous byproducts (HCl and SO₂), which escape from the reaction mixture. Example:

C₂H₅OH + SOCl₂ ➔ C₂H₅Cl + HCl + SO₂

Dehydrating alcohols to form alkenes

Dehydration of alcohols, an elimination reaction facilitated by a heated catalyst such as Al₂O₃ or concentrated H₂SO₄, results in the formation of alkenes through the elimination of water:

alcohol ➔ alkene + water

For instance, dehydrating ethanol with concentrated sulfuric acid produces ethene:

CH₃CH₂OH ➔ CH₂=CH₂ + H₂O

Esterification of alcohols

Esterification is a condensation reaction that involves reacting alcohols with carboxylic acids in the presence of a concentrated H₂SO₄ catalyst to form esters:

alcohol + carboxylic acid ⇌ ester + water

An example is the reaction of ethanol with ethanoic acid to produce ethyl ethanoate:

C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O

Reaction of alcohols with sodium

Alcohols react with sodium metal to form sodium alkoxides and hydrogen gas:

alcohol + sodium ➔ sodium alkoxide + hydrogen

For example, ethanol reacts with sodium to form sodium ethoxide and hydrogen gas:

C₂H₅OH + Na ➔ C₂H₅ONa + 1⁄2H₂