Aldehydes and ketones

Question 1

What is a carbonyl group?

Answer 1

C=O

Question 2

What is the difference between an aldehyde and a ketone?

Answer 2

aldehyde:

carbony group is at the end of the hydrocarbon chain

RCHO

-al

ketone:

carbonyl group is in the body of the chain

RCOR

-one

Question 3

What do the tests for aldehydes and ketones?

Answer 3

gentle oxidation

ketones are not changed by gentle oxidation

aldehydes are oxidised to carboxylic acid ( -RCHO + [O] → -RCOOH), same as the ssecond stage of the oxidation stage of a primary alcohol

Question 4

What is Tollens’ test?

Answer 4

gentle oxidising agent

solution of silver nitrate in aqueous ammonia

oxidises aldehydes but has no affect on ketones

Question 5

What are the results of a positive Tollens’ test?

Answer 5

contains colourless silver(I) complex ions, containing Ag⁺, which are reduced to metallic silver, Ag, as the aldehyde is oxidised

on warming an aldehyde with Tollens’ reagent, a deposit of metallic silver is formed on the inside of the test tube

this forms a silver mirro - this was once used commercially for making mirrors

Question 6

What is Fehling’s reagent?

Answer 6

gentle oxidising agent

contains blue copper(II) complex ions

will oxidise aldehydes but not ketones

Question 7

What is the reaction that occurs in Fehling’s test?

Answer 7

during the oxidation, the blue solution gradually changes to a brick red precipitate of copper(I) oxide

Cu²⁺ + e^- → Cu⁺

on warming an aldehyde with blue Fehling’s solution a brick red precipitate gradually forms