6.1.2 Carbonyl Compounds

Question 1

What is the carbonyl functional group

Answer 1

C=O

Question 2

How is an aldehyde different from a ketone

Answer 2

An aldeHyde has the carbonyl on the side (terminal carbon) and the carbon with the double bond to O also has a H attached.
In a ketone the C with the double bond to the O has no H attached (on its own).

Question 3

What are the main types of carbonyl containing compounds

Answer 3

aldehydes and ketones

Question 4

Explain the solubility of carbonyls

Answer 4

The carbonyl group is soluble because a lone pair on O can form a hydrogen bond with an H in a water molecule

Question 5

What type of intermolecular forces exist in carbonyl compounds?

Answer 5

permanent dipole-permanent dipole interactions due to the d+ carbon and d- oxygen.

Question 6

How can an aldehyde be produced via oxidation?

Answer 6

Heat a primary alcohol with acidified potassium dichromate and distil off the product.

Question 7

How can a ketone be produced via oxidation?

Answer 7

Heat a secondary alcohol under reflux with acidified potassium dichromate

Question 8

Write an equation for the oxidation of ethanol to a carbonyl

Answer 8

CH3CH2OH + [O] —> CH3CHO + H2O

Question 9

Write an equation for the oxidation of ethanal

Answer 9

CH3CHO + [O} –> CH3COOH

Question 10

Write an equation for the oxidation of propan-2-ol to a carbonyl

Answer 10

CH3CH(OH)CH3 + [O] –> CH3COCH3 + H20

Question 11

What observation is made when oxidising an alcohol with acidified potassium dichromate and why is this?

Answer 11

orange –> green

The Cr ion in dichromate changes colour as it is reduced from Cr +6 to Cr +3

Question 12

What conditions are needed for the reduction of carbonyls and carboxylic acids?

Answer 12

Use of sodium borohydride (NaBH4) reducing agent and dilute acid

Question 13

How does NaBH4 work?

Answer 13

It provides H- nucleophile which attacks the d+ carbon atom.

Question 14

Write an equation for the reduction of ethanal

Answer 14

CH3CHO + 2{H} –> CH3CH2OH

Question 15

What type of reaction is reduction with NaBH4?

Answer 15

nucleophilic addition
nucleophile because the hydride ion (H-) is an electron pair donor
Addition because two smaller molecules join to form one larger molecule.

Question 16

Give the mechanism for the reaction of ethanal with HCN

Answer 16

See notes

Question 17

What reagents are needed for the reaction of a carbonyl with HCN and why

Answer 17

NaCN and acid

HCN is toxic so NaCN and H+ are needed to generate the HCN in situ

Question 18

Write an equation for the reaction of ethanal with HCN. Name the product

Answer 18

CH3CHO + HCN –> CH3CH(OH)CN
(NaCN/H2SO4 written over arrow)
Product = 2-hydroxypropanenitrile

Question 19

What type of reaction is the reaction of a carbonyl with HCN?

Answer 19

nucleophilic addition
nucleophile because the hydride ion (H-) is an electron pair donor
Addition because two smaller molecules join to form one larger molecule.

Question 20

Why are reactions of carbonyl compounds with HCN useful to organic chemists?

Answer 20

They extend the length of the carbon chain.

Question 21

How can you identify an organic compound with optical isomers

Answer 21

If a carbon atom in the compound has 4 different groups attached to it.

Question 22

How can you test for a carbonyl group

Answer 22

Add 2,4-dinitrophenylhydrazine (2,4-DNP) (Brady’s reagent). If a carbonyl group is present then a yellow-orange precipitate will be seen

Question 23

How can you distinguish an aldehyde and ketone chemically

Answer 23

Warm the compound with Tollen’s reagent. If a silver mirror forms, the compound is an aldehyde. If there is no visible change, then the compound is a ketone

Question 24

What is Tollen’s reagent?

Answer 24

Ammoniacal silver

Question 25

How can you distinguish an aldehyde and ketone using a physical method?

Answer 25

Take the precipitate formed with Brady’s reagent and filter and purify it by recrystallisation. Test the melting point and compare with a database.

Question 26

How does Tollen’s reagent work?

Answer 26

The silver ions in Tollen’s reagent are reduced
Ag+ + e –> Ag
This works with an aldehyde because it can be oxidised. A ketone cannot be oxidised so it can’t act as a reducing agent.