4.4 - Carbonyls (aldehydes and ketones)

Question 1

What is hydrogen cyanide?

Answer 1

HCN is a colourless and extremely poisonous gas, however society produces millions of tonnes of it a year.

Question 2

What is HCN industrially important for?

Answer 2

• production of NaCN for gold mining
• organic synthesis
• production of polymers such as nylon

Question 3

How does cyanide react with a ketone?

Answer 3

Propanal + HCN = 2 -hydroxybutanenitrile
HCN reacts with the double bond in the carbonyl group

Question 4

Why can’t we use concentrated HCN and what can we use instead?

Answer 4

As HCN is so poisonous, we often use NaCN or KCN dissolved in dilute acid
- this provides the H+ and CN_ ions which we need but not combined into a dangerous chemical

Question 5

What is used to test for the presence of carbonyls?

Answer 5

2,4 - dinitrophenylhydrazine (2,4DNPH)
Treating a ketone or aldehyde with 2,4 - DNPH produces an orange or yellow precipitate

Question 6

What is fehlings solution?

Answer 6

Made from complex ions containing Cu2+ and 2,3 - dihydroxybutanedioc acid
Ketones don’t react
It is bright blue, when warmed with aldehyde a red precipitate of copper oxide is produced

Question 7

What is tollens reagent?

Answer 7

Solution of ammoniacal silver nitrate
A positive test also occurs with an aldehyde producing a silver mirror
Again ketone does not react

Question 8

What is tri-iodomethane?

Answer 8

As well as methyl alcohols, methyl ketones react with iodine
The reaction involves the addition of iodine and sodium hydoxide leading to the production of a pale yellow precipitate

Question 9

Why is ethanal the only compound to give a positive result for tri-iodomethane?

Answer 9

Ethanal is the only aldehyde to give a positive result with iodine as it has the methyl ketone group