15.3 - Reactions of Alcohols

Question 1

Isomers of butan-2-ol dehydration

Answer 1

Positional isomers:

Question 2

Three steps of the elimination mechanism

Answer 2

Question 3

Dehydration of ethanol

Answer 3

CH3CH2OH➔ CH2=CH2+ H2O

Question 4

General dehydration formula for alcohols

Answer 4

alcohol➔ alkene + water

Question 5

Distinguishing between ketones and aldehydes

Answer 5

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

Question 6

Structure of aldehydes, ketones and carboxylic acids

Answer 6

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.

Question 7

Oxidation of secondary alcohols?

Answer 7

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:

Question 8

How do you control the oxidation of primary alcohols?

Answer 8

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.

Question 9

What does oxidation depend upon? (3):

Answer 9

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

Question 10

Oxidation of alcohols (oxidising agent, and what is the overall observed reaction):

Answer 10

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 (Cr2O72-) to chromium(III) ions (Cr3+).

Question 11

Complete combustion of alcohols (example of ethanol)

Answer 11

C2H5OH(l)+ 3O2(g)➔ 2CO2(g)+ 3H2O(g)