1. 13.7 Aldehydes and Ketones

2. Structures.
Two series of organic compounds containing C=O carbonyl group.
Aldehydes have a hydrogen attached to the carbonyl group.
Two groups react differently and can be distinguished.
R-C-H O
aldehyde
R-C-R' O
ketone

3. Aldehydes 2 3 2 3 Named using suffix -al. O E.g. HCHO methanal
Now try these:-
Named using suffix -al.
E.g. HCHO methanal
CH3CHO ethanal
CH -CH -C-H O 2 3
propanal 2
CH -CH -CH -CH -C-H O 3
pentanal

4. Ketones Now try these: Name use suffix -one. E.g. CH3COCH3 propanone
Ketones with 5 or more carbons have structural isomers eg.
CH3COCH2CH2CH2CH3
& CH3CH2COCH2CH2CH3
Draw the structures and name these isomers.
Now try these:
CH3CH2CH2CH2COCH2CH2CH3
CH3CH2CH2COCH2CH2CH3
CH3CH2COCH2CH2CH2CH2CH3
CH3COCH2CH3
CH3CH2COCH2CH2CH2CH3

5. Preparation of aldehydes and ketones
Oxidation of alcohols
Acidified potassium dichromate - heat under reflux.
Orange dichromate Cr2O72- is reduced to green Cr3+ in solution.
Primary alcohol  aldehyde  carboxylic acid
Secondary alcohol  ketone.

6. Redox reactions Oxidation of alcohols are redox reactions.
Oxidation of ethanol;
CH3-CH2-OH CH3-CHO + 2H+ + 2e-
CH3-CHO + H2O CH3COOH + 2H+ + 2e-
reduction;
Cr2O72-(aq) + 14H+(aq) + 6e- 2Cr3+ (aq) +7H2O
orange  green

7. Reactions - oxidation
Aldehydes have a hydrogen atom next to the carbonyl group - easily oxidised.
Fehlings solution - Cu2+(aq) ions (in alkali) reduced to Cu+ - blue to brick red.
Aldehyde oxidised to carboxylic acid.
Ketones not oxidised easily by either Fehlings solution or acidified dichromate.

8. Reactions - reduction Powerful reducing agent needed.
NaBH4 - sodium tetrahydridoborate (III)
Aldehydes reduced to primary alcohols.
Ketones reduced to secondary alcohols.

9. Reactions - addition Carbonyl groups can undergo addition reactions.
HCN in presence of alkali.
Adds across C=O to form 2 hydroxynitriles (cyanohydrins)
Nucleophilic addition
H-C-C-H O H +
H -CN  OH CN