Aldehydes and Ketones

Question 1

Why do aldehydes and ketones have a higher boiling point than alkanes of similar formula mass?

Answer 1

Because the presence of the polar carbonyl group allows permanent dipole-permanent dipole reactions to be set-up between their molecules. Ref to van der vaals in alkanes.

Question 2

Why do aldehydes and ketones have a lower boiling point than alcohols?

Answer 2

Because they only have permanent dipole - permanent dipole attractions which are weaker than the hydrogen bonds between the alcohol molecules.

Question 3

Can aldehydes and ketones dissolve in water?

Answer 3

They are generally insoluble in water but lower members can form hydrogen bonds with water so can dissolve to some extent

Question 4

What is used to reduce aldehydes and ketones back to alcohols?

Answer 4

Lithium aluminium hydride dissolved in ether.

Question 5

How do aldehydes react with hydrogen cyanide?

Answer 5

They react by nucleophilic addition to form cyanohydrins.

Question 6

Why isn’t hydrogen cyanide used directly?

Answer 6

Very poisonous gas

Question 7

What is used as the reagent instead of hydrogen cyanide?

Answer 7

Acidified potassium cyanide.

Question 8

What is the action of the nucleophile cyanide on the aldehyde?

Answer 8

It attacks the carbon atom of the carbonyl group since it has a slight positive charge.

Question 9

What is the structure of a cyanohydrin?

Answer 9

CN
:
R—C—OH

Question 10

How can a cyanohydrin be hydrolysed to a hydrocarboxylic acid?

Answer 10

Refluxing them with dilute hydrochloric acid.

Question 11

Draw the structure of a hydroxycarboxylic acid

Answer 11

COOH
:
R—C—OH

Question 12

What is formed during a nucleophilic addition-elimination reaction with hydrazine?

Answer 12

A hydrazone and water

Question 13

Which is more reactive when under nucleophilic attack?

Answer 13

Aldehydes

Question 14

Why are ketones less reactive than aldehydes?

Answer 14

Ketones have two alkyl groups next to the slightly positive carbon atom of the carbonyl group. Alkyl groups are electron donating meaning the partial positive charge of this carbon atom smaller in a ketone making it less susceptible to an attack by a nucleophile. The two ally groups of a keno tend to hinder the nucleophiles access to the carbon and this further reduces its activity.

Question 15

What is the oxidation and reduction between aldehydes and fehlings solution?

Answer 15

Copper(II) ions are reduced to copper (I) ions.

The aldehyde is oxidised to a carboxylic acid.

Question 16

What is the reaction between an aldehyde and tollen’s reagent?

Answer 16

The aldehyde is oxidised and the silver (I) ions are reduced to silver atoms

Question 17

What is Brady’s reagent?

Answer 17

A solution of 2,4-dinitrophenylhydrazine

Question 18

What happens when an aldehyde or ketone reacts with Brady’s reagent?

Answer 18

An orange yellow precipitate is formed, indicating the presence of the carbonyl group. A nucleophilic addition- elimination reaction takes place.The precipitate that is formed is called a 2,4-dinitrophenylhydrazone.

Question 19

How can an unknown ketone or aldehyde become identified?

Answer 19

By preparing its 2,4-dinitrophenylhydrazone derivative. The melting point of the derivative is determined and compared with the melting points of the 2,4-dinitrophenylhydrazones of known compounds.