26.2 Identifying Aldehydes & Ketones

Question 1

What is the test for the presence of a carbonyl group?

Answer 1

Brady’s reagent/2,4-DNP (dinitrophenylhydrazine)

positive result = forms yellow/orange precipitate

Question 2

How can you distinguish between an aldehyde & a ketone?

Answer 2

Tollen’s reagent

forms silver mirror in presence of aldehyde

(acts as oxidising agent - uses property that aldehydes can be oxidised whereas ketones cannot)

Question 3

What is Tollens reagent made from?

Answer 3

solution of silver nitrate in aqueous ammonia

Question 4

How can you differentiate between an ester & an aldehyde?

Answer 4

Add Brady’s reagent,

esters are too stable to react

Question 5

Why do ketones not react with Tollen’s reagent?

Answer 5

Tollen’s reagent is an oxidising agent

forms silver mirror when reduced.

Ketones cannot be oxidised, therefore no change occurs with Tollen’s.