Topic 17: Carbonyl Compounds

Question 1

What is the aldehyde functional group?

Answer 1

At the end of the end of the chain:
R-C(=O)H

Question 1

What are carbonyl compounds?

Answer 1

Aldehydes and ketones

Question 2

What is the ketone functional group?

Answer 2

In the middle of the chain:
R - C(=O) - R

Question 3

What are the intermolecular forces on a carbonyl?

Answer 3

There is a dipole formed as the oxygen is more electronegative than carbon, so the oxygen has a partial -ve charge and carbon a partial +ve charge

This means they have:
–> london forces
–> permanent dipole interactions
–> there are no polar bonds that result in a +ve H for H-bonding –> lower boiling points and melting points than corresponding alcohol

Question 4

Can carbonyls form hydrogen bonds?

Answer 4

Yes - the lone pairs on their oxygen means they can hydrogen bond to water
—> thus smaller ketones and aldehydes will dissolve in water

Question 5

Why cant a large carbonyl compound bond with water?

Answer 5

• strength of the potential hydrogen bonding of the carbonyl with water must be higher than the combined strength of the intermolecular forces of the carbonyl and the hydrogen bonding of water

Question 6

What is the reaction between aldehydes and acidified potassium dichromate?

Answer 6

Aldehydes can be oxidised to form carboxylic acids

Aldehyde + [O] –> Carboxylic acid
[O] = K2Cr2O7

Question 7

What is the reaction between ketones and acidified potassium dichromate?

Answer 7

There is no reaction: This is because ketones do not have a readily available hydrogen atom, like aldehydes do

Question 8

What is the positive test for an aldehyde when reacted with K2Cr2O7?

Answer 8

The aldehyde would be oxidised, and you would see an orange to green colour change

–> If a ketone is present there would be no colour change

Question 9

What happens when an aldehyde is reacted with tollens reagent?

Answer 9

If gently warmed with Tollens’ reagent, an aldehyde will become oxidised

Question 10

What is a positive test for an aldehyde with Tollens reagent?

Answer 10

A silver mirror is formed:
As the aldehyde is oxidised, it causes the Ag+ ions to become reduced to solid metallic silver, Ag

Question 11

What happens if a ketone is warmed with Tollens reagent?

Answer 11

There is no reaction, as ketones cannot be oxidised

Question 12

What happens when an aldehyde is reacted with Fehling’s / Benedict’s solution?

Answer 12

The aldehyde is oxidised, and the Cu2+ ions in the Fehling’s solution are reduced to Cu+ ions which forms a brick-red precipitate

Question 13

What is a positive test for an aldehyde in Fehling’s solution?

Answer 13

Colour change from Blue to brick-red as the aldehyde is oxidised

Question 14

What are aldehydes reduced to?

Answer 14

Primary alcohols

Question 15

What are ketones reduced to?

Answer 15

Secondary alcohols

Question 16

How are carbonyls reduced?

Answer 16

In LiAlH4, in dry ether
The LiAlH4 provides H- nucleophile which reduced the carbonyl group

Question 17

What is the general equation for the reduction of an aldehyde?

Answer 17

C(H)=O + 2[H] –> CRH2OH

Question 18

What is the general equation for the reduction of a ketone?

Answer 18

C(H)=O + 2[H] –> CR2HOH

Question 19

What type of reaction is it that carbonyls undergo?

Answer 19

Nucleophilic substitution
2 step process

Question 20

What happens if a carbonyl is reacted with HCN

Answer 20

1. the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate
2. 2, the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H+ (either from HCN, water or dilute acid) to form 2-hydroxynitrile compounds,

Question 21

What conditions are required for the reaction between a carbonyl and HCN

Answer 21

The presence of KCN

Question 22

Why is the reaction with HCN important?

Answer 22

It increases chain length
Produces hydroxynitriles

Question 23

What is 2,4DNPH?

Answer 23

a reagent which detects the presence of carbonyl compounds

Question 24

How does the reaction with 2,4 DNPH work?

Answer 24

The carbonyl group of aldehydes and ketones undergoes a condensation reaction with 2,4-dinitrophenylhydrazine

Question 25

What is a positive test for a carbonyl with 2,4 DNPH?

Answer 25

a deep-orange precipitate forms (it can be purified by recrystallisation)

Question 26

How can you determine whether the orange precipitate from the 2,4 DNPH is from a ketone or an aldehyde?

Answer 26

Measure the melting point to determine whether it is an aldehyde or ketone

Question 27

What does the iodoform test identify?

Answer 27

Methyl ketones
CH3CO- group e.g. ethanal

Question 28

What is the positive result for the iodoform test?

Answer 28

formation of a yellow precipitate

Question 29

What happens during the iodoform test?

Answer 29

1. The reagent is heated with an alkaline solution of iodine
2. This reaction involves a halogenation and hydrolysis step
3. In the halogenation step, all three H-atoms in the -CH3 (methyl) group are replaced with iodine atoms, forming a -CI3 group
4. The intermediate compound is hydrolysed by an alkaline solution to form a sodium salt (RCO2 - Na+) and a yellow precipitate of CHI3

Question 30

What other compound will react with iodoform and give a positive test?

Answer 30

• All methyl secondary alcohols (such as hexan-2-ol) and the primary alcohol ethanol
• Iodine in sodium hydroxide reacts to form iodate(I) ions which will oxidise the secondary alcohols to ketones