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 or Benedict's solution:

1. Both Fehling's and Benedict's solutions are alkaline and contain blue aqueous Cu²⁺ ions.
2. When warmed, aldehydes reduce the blue Cu²⁺ ions in either solution to form a brick-red Cu₂O precipitate.
3. Ketones do not react with either solution - the mixture remains blue with no colour change observed.

Converting alcohols to halogenoalkanes

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

Synthesis of chloroalkanes using PCl5 or HCl

Chloroalkanes (RCl) can be prepared from alcohols by reacting them with phosphorus pentachloride (PCl₅) or concentrated hydrochloric acid (HCl), depending on the type of alcohol.

1. Reaction with PCl₅ - When alcohols react with phosphorus pentachloride, chloroakanes, hydrochloric acid, and phosphoryl chloride (POCl₃) are produced. The overall reaction is:

ROH + PCl₅ ➔ RCl + HCl + POCl₃

For example:

CH₃OH + PCl₅ ➔ CH₃Cl + HCl + POCl₃

1. Reaction with HCl - Tertiary chloroalkanes can be synthesised from tertiary alcohols by reacting them with concentrated hydrochloric acid. The overall reaction is:

ROH + HCl ➔ RCl + H₂O

For example:

(CH₃)₃COH + HCl ➔ (CH₃)₃CCl + H₂O

Synthesis of bromoalkanes using KBr and H2SO4

Bromoalkanes (RBr) can be synthesised from alcohols by reacting them with hydrogen bromide (HBr).

In this reaction, HBr is usually generated in situ within the reaction mixture by mixing potassium bromide (KBr) with 50% concentrated sulfuric acid (H₂SO₄).

The overall reaction is:

ROH + HBr ➔ RBr + H₂O

For example:

CH₃OH + HBr ➔ CH₃Br + H₂O

Synthesis of iodoalkanes using red phosphorus and iodine

Iodoalkanes (RI) can be prepared from alcohols by reacting them with phosphorus triiodide (PI₃).

PI₃ is typically generated in situ within the reaction mixture by refluxing the alcohol with red phosphorus and iodine.

The overall reaction is:

3ROH + PI₃ ➔ 3RI + H₃PO₃

For example:

3CH₃OH + PI₃ ➔ 3CH₃I + H₃PO₃

Dehydrating alcohols to form alkenes

Dehydration of alcohols, an elimination reaction facilitated by a concentrated phosphoric acid catalyst (H₃PO₄) results in the formation of alkenes through the elimination of water:

alcohol ➔ alkene + water

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

CH₃CH₂OH ➔ CH₂=CH₂ + H₂O

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: