State the structural difference between an aldehyde and a ketone.

In an aldehyde the carbonyl carbon carries at least one hydrogen atom and lies at the end of the chain, whereas in a ketone it carries two carbon groups.

Which of the two is oxidised by acidified dichromate?

Only the aldehyde is oxidised; a ketone is not.

State the reagents and conditions for oxidising an aldehyde to a carboxylic acid.

Potassium dichromate(VI) acidified with dilute sulfuric acid, heated under reflux.

Give the formula of the product when ethanal is oxidised.

Ethanoic acid, CH₃COOH, is formed.

Give the formula of the product when propanal is oxidised.

Propanoic acid, CH₃CH₂COOH, is formed.

OCR A-Level Chemistry B H433

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Elements of Life

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Colour by Design

Oxidising an aldehyde and the three carbonyl tests

3 things to know
Components 01, 02 and 03

Overview

Recall questions on the dichromate oxidation and its colour change, why ketones resist it, and the Tollens' and Fehling's results for each.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What do ethanal and propanal give on oxidation, and what colour change accompanies it?

Process

Why are ketones not oxidised by acidified dichromate, what is Tollens' reagent, and what does an aldehyde do to it?

Numbers

What does ethanal give with Tollens', which species is reduced, and what does a ketone do with Tollens' and Fehling's?

Questions from this subtopic

Each one with the model answer underneath.

1
Which two homologous series contain the carbonyl group?
Model answer
Aldehydes and ketones.
2
State the structural difference between an aldehyde and a ketone.
Model answer
In an aldehyde the carbonyl carbon carries at least one hydrogen atom and lies at the end of the chain, whereas in a ketone it carries two carbon groups.
3
Which of the two is oxidised by acidified dichromate?
Model answer
Only the aldehyde is oxidised; a ketone is not.
4
State the reagents and conditions for oxidising an aldehyde to a carboxylic acid.
Model answer
Potassium dichromate(VI) acidified with dilute sulfuric acid, heated under reflux.
5
Give the formula of the product when ethanal is oxidised.
Model answer
Ethanoic acid, CH₃COOH, is formed.
6
Give the formula of the product when propanal is oxidised.
Model answer
Propanoic acid, CH₃CH₂COOH, is formed.

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