Chemistry - 3.3.8: Aldehydes and Ketones

What is the carbonyl group?

C=O

What is the ketone suffix?

-one

What is the aldehyde suffix?

-al

Where can the aldehyde group only occur at?

The end of the chain

What intermolecular forces are present in aldehydes and ketones and why?

Van der Waal forces and permanent dipole-dipole forces - the C=O group is polar

2 physical properties of aldehydes and ketones

1. Higher boiling point than alkanes due to permanent dipole-dipole forces
2. Short chain aldehydes and ketones mix completely with water as hydrogen bonds form between the oxygen of C=O and water - become less soluble as carbon chain length increases

What states of matter are aldehydes and ketones

Methanal is a gas, other short chain ones are liquids

Are carbonyl molecules reactive and why?

Yes as the C=O bond is strongly polar (there's a large difference in electronegativity between C and O), so nucleophilic reagents can attack Cδ+ + double bond means addition reactions are possible

What does the nucleophilic addition of KCN to aldehydes/ketones produce?

Hydroxynitriles

3 steps of nucleophilic addition of CN-

1. C=O bond breaks and the electrons go to Oδ- (arrow from C=O bond to Oδ-) Lone pair of electrons on CN- attracted to Cδ+ (arrow from lone pair on CN- to Cδ)
2. This results in the O having a negative charge, so the lone pair of electrons on O- are attracted to a H+ ion (arrow from lone pair on O- to H+)
3. This produces a hydroxynitrile (with -CN and -OH groups)

What is needed for the nucleophilic addition of aldehydes and ketones with CN-?

KCN / NaCN and dilute HCl

What is special about the products of the nucleophilic addition of CN- ions to aldehydes and asymetrical ketones?

The products are a racemic mixture of two enatiomers / racemate

What are aldehydes oxidised to?

Carboxylic acids

What are ketones oxidised to?

Nothing

What can oxidise aldehydes but not ketones?

Weak oxidising agents

2 tests to distinguish between aldehydes and ketones?

Fehling's solution and Tollens' reagent

What reducing agent can be used to reduce aldehydes and ketones?

Sodium borohydride (NaBH4)

What type of reaction is the reduction of aldehydes and ketones?

Nucleophilic addition

2 steps of reduction of aldehydes and ketones

1. C=O bond breaks and the electrons go to Oδ- (arrow from C=O bond to Oδ-) Lone pair of electrons on H- attracted to Cδ+ (arrow from lone pair on H- to Cδ)
2. This results in the O having a negative charge, so the lone pair of electrons on O- are attracted to a H+ ion (arrow from lone pair on O- to H+)
3. This produces an alcohol

In the nucleophilic addition of CN-, where does the H+ come from?

A dilute acid (e.g. dilute HCl)

In the reduction of aldehydes and ketones, where does the H+ come from?

The solvent

What symbol is used to represent reduction in equations?

[H]

What are aldehydes reduced to?

Primary alcohols

What are ketones reduced to?

Secondary alcohols

What is the hazard associated with the use of KCN?

It's toxic

Why is KCN used insstead of HCN?

HCN is weak (KCN dissociates to provide CN-)

Why can aldehydes/ketones be reduced by NaBH4 while alkenes cannot?

1. H- ion is attracted to Cδ+ of C=O
2. In contrast, H- is repelled by C=C as it is electron rich / an area of high electron density

Why does the nucleophilic addition of CN- to aldehydes/ketones result in a racemate?

There is a planar carbonyl group, so there is an equal probability of attack from either side, above or below, by CN-.

How can you separate two stereoisomers of hydroxynitriles?

Use plane polarised light - enantiomers would rotate light in opposite directions

Why might the nucleophilic addition of CN- not produce a compound that shows stereoisomerism?

No chiral centre / the product is symmetrical

Write the equation for the reduction of butanone to butan-2-ol.

CH3CH2COCH3 + 2[H] -> CH3CH2CH(OH)CH3

Write an equation for the reduction of propanal to propan-1-ol.

CH3CH2CHO + 2[H] -> CH3CH2CH2OH