Redox reactions, and testing for carbonyl compounds

Question 1

What are aldehydes oxidised to?

Answer 1

carboxylic acids

Question 2

What is the oxidising agent commonly used?

Answer 2

acidified (with dilute sulfuric acid) potassium dichromate (VI)

K2Cr2O7/H+

Question 3

Can ketones be oxidiesed?

Answer 3

they cannot be oxidised easily to carboxylic acids because, unlike aldehydes, as C-C bond must be broken

although, stronger oxidising agents break the hydrocarbon chain of the ketone molecule resulting in a shorter chain molecule, carbon dioxide, and water

Question 4

What is the basis for the two tests to distinguish between aldehydes and ketones?

Answer 4

weak oxidising agents can oxidise aldehydes but not ketones

Question 5

What is Fehlin’s solution?

Answer 5

mixture of two solutions

Fehling’s A which contains the Cu²⁺ ion and is therefore coloured blue

Fehling’s B which contains an alkali and a complexing agent

Question 6

How is Fehling’s test carried out? What are the indication of a positive result

Answer 6

• when an aldehyde is warmed with Fehling’s solution, a brick red precipitate of copper(I) oxide is produced as the copper(II) oxidises the aldehyde to a carboxylic acid, and is itself reduced to copper(1)
• ketones give no reaction

Question 7

What is Tollen’s reagent? How is it formed?

Answer 7

contains the complex ion [Ag(NH₃)₂]⁺

this is formeed when aqueous ammonia is added to an aqueous solution of silver nitrate

Question 8

How is the silver mirror test carried out?

Answer 8

• when an aldehyde is warmed with Tollens’ reagent, metallic silver is formed.
• Aldehydes are oxidised to carboxylic acids by Tollens’ reagent.
• The Ag+ is reduced to metallic silver
• A silver mirror will be formed on the inside of the test tube

Ketones give no reaction

Question 9

What are the equations for the reactions that occur in the silver mirror test?

Answer 9

RCHO + [O] → RCOOH - aldehyde is oxidised

[Ag(NH₃)₂]⁺ + e^- → Ag + 2NH₃ - silver is reduced

Question 10

Can carbonyl compounds be reduced?

Answer 10

yes reducing agents will reduce both aldehydes and ketones to alcohols

Question 11

What is an example of a reducing agent? How does it work?

Answer 11

sodium tetrahydriodoborate(III) (NaBH4) in aqueous solution

this generate the nucleopule :H-, the hydride ion

This reduced the C=O but not C=C as it is repelled by the high electron density in the C-C, but attracted to the partially positive carbon in the C=O bond

Question 12

Draw the mechanism for reducing an aldehyde

Answer 12

Question 13

What is the equation for the reduction of an aldehyde?

Answer 13

RCOH + 2[H] → RCH₂OH