13 7 aldehydes and ketones
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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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13.7 Aldehydes and Ketones

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13 7 aldehydes and ketones

13.7Aldehydes and Ketones


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


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