13 7 aldehydes and ketones l.
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13.7 Aldehydes and Ketones. O. R-C-H. O. R-C-R'. 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. aldehyde. ketone. O. CH -CH -C-H. 3. 2. O.

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structures

O

R-C-H

O

R-C-R'

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.

aldehyde

ketone

aldehydes

O

CH -CH -C-H

3

2

O

CH -CH -CH -CH -C-H

3

2

2

2

Aldehydes

Now try these:-

  • Named using suffix -al.
  • E.g. HCHO methanal
  • CH3CHO ethanal

propanal

pentanal

ketones
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

Ketones
preparation of aldehydes and ketones
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.
redox reactions
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
reactions oxidation
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.
reactions reduction
Reactions - reduction
  • Powerful reducing agent needed.
  • NaBH4 - sodium tetrahydridoborate (III)
  • Aldehydes reduced to primary alcohols.
  • Ketones reduced to secondary alcohols.
reactions addition

O

OH

H

H

+

H-C-C-H

H -CN

H-C-C-H

H

CN

H

Reactions - addition
  • Carbonyl groups can undergo addition reactions.
  • HCN in presence of alkali.
  • Adds across C=O to form 2 hydroxynitriles (cyanohydrins)
  • Nucleophilic addition