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13.7 Aldehydes and Ketones

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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  1. 13.7Aldehydes and Ketones

  2. 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

  3. 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

  4. 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

  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. 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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