Organic 8: Aldehydes and Ketones

Question 1

Why is the boiling point of an aldehyde/ketone higher than an alkane with comparable molecular mass?

Answer 1

Stronger intermolecular forces (permanent dipole-dipole)
Takes more energy to break

Question 2

Why are aldehydes and ketones so reactive?

Answer 2

Due to the presence of strongly polar C=O bond

Question 3

Why is the boiling point of an aldehyde/ketone lower than an alcohol with comparable molecular mass?

Answer 3

Because alcohols have stronger intermolecular forces (hydrogen bonds)
Takes more energy to break

Question 4

Why are short-chain aldehydes/ketones soluble in water, but long-chains aren’t?

Answer 4

C=O bond is polar so can dissolve in water by forming hydrogen bonds
In long chains, a larger portion of the molecule is non-polar, so cannot dissolve in water

Question 5

Arrange the following compounds in order of melting point from highest to lowest: propanone, butane, propan-1-ol

Answer 5

Highest: Propan-1-ol
Propanone
Lowest: Butane

Question 6

What state are short chain aldehydes/ketones at room temperature?

Answer 6

Liquids
(Methanal is gas)

Question 7

What state are long chain aldehydes/ketones at room temperature?

Answer 7

Solids

Question 8

Why is the C=O bond strongly polar?

Answer 8

Large difference in electronegativity between carbon and oxygen

Question 9

Describe the general mechanism for a nucleophilic addition reaction with an aldehyde or a ketone.

Answer 9

Reagent: Sodium or Potassium Cyanide followed by dilute hydrochloric acid

1) Nucleophile is added to the carbon on the C=O bond
2) Electrons in C=O bond move to O
3) Lone pair on O moves to H+ from dilute hydrochloric acid
Forming hydroxynitrile

Question 10

Why does nucleophilic addition lead to formation of a racemic mixture?

Answer 10

The cyanide ion can attack either side of the planar C=O bond
So theoretically forms a 50/50 mixture of each enantiomer

Question 11

How can aldehydes be oxidised to carboxylic acids?

Answer 11

Using acidified potassium dichromate (K2Cr2O7/H+)

Question 12

Why can’t ketones be easily oxidised?

Answer 12

Because the C-C bond would have to be broken which would require more energy
So only very strong oxidising agents can break the C-C bond and would form carbon dioxide and water

Question 13

What are the two tests you would use to distinguish between aldehydes and ketones?

Answer 13

Tollen’s
Fehling’s

Question 14

Explain how the Tollen’s test works.

Answer 14

Tollen’s reagent contains the complex ion [Ag(NH3)2]+, which is formed when aqueous silver nitrate is reacted with aqueous ammonia
When an aldehyde is warmed with Tollen’s reagent, Ag+ is reduced to metallic Ag(s) forming a silver mirror
Ketones give no reaction as they don’t have a hydrogen atom attached to the carbonyl carbon

Question 15

Explain how the Fehling’s test works.

Answer 15

Fehling’s reagent contains a solution containing Cu2+ ions (blue)
When an aldehyde is warmed, it is oxidised by Fehling’s reagent, reducing the Cu2+ ions to Cu+ forming an insoluble red precipitate of copper(I) oxide
Ketones give no reaction as they don’t have a hydrogen atom attached to the carbonyl carbon

Question 16

What reducing agent is used to reduce aldehydes and ketones?

Answer 16

Aqueous NaBH4: sodium tetrahydridoborate (III)
Generates the nucleophile :H- (hydride ion)
It is attracted to the delta positive carbon on C=O bonds

Question 17

What are aldehydes reduced to?

Answer 17

Primary alcohols

Question 18

Give a general equation for the reduction of aldehydes using NaBH4.

Answer 18

RHCO + 2[H] –> RCH2OH
Nucleophilic addition reaction

Question 19

What are ketones reduced to?

Answer 19

Secondary alcohols

Question 20

Give a general equation for the reduction of ketones using NaBH4.

Answer 20

RR’CO + 2[H] –> RR’CHOH
Nucleophilic addition reaction