6.6 carbonyl compounds

Question 1

explain why alkenes undergo electrophilic substitution while carbonyl compounds undergo nucleophilic addition

Answer 1

the pi electrons in electron-rich C=C bond of alkenes serve as an electron-rich source to attract electrophiles
the C=O bond in carbonyl compounds is polar where the carbonyl carbon atom is electron deficient and bonded to a highly electronegative oxygen atom, hence bears a partial positive charge, attracting nucleophiles

Question 2

what are the methods of preparation of carbonyl compounds (2)?

Answer 2

1. oxidation of primary alcohols to aldehydes (K2Cr2O7 in dilute H2SO4, heat with immediate distillation), oxidation of secondary alcohols to ketones (KMnO4/K2Cr2O7 in dilute H2SO4, heat under refluex)
2. strong oxidation of alkenes (conc KMnO4 in H2SO4, heat under reflux)

Question 3

what are the factors affecting reactivity of compounds by nucleophilic addition?

Answer 3

1. electronic considerations:
   - electron-donating alkyl groups decrease the partial positive charge on carbonyl carbon atom, weaker electrophile
   - resonance stabilisation weakens electrophile
2. steric hindrance of bulky substituent groups

Question 4

what are the reactions of carbonyl compounds (3)?

Answer 4

1. nucleophilic addition (-CN)
2. reduction (LiAlH4 in dry ether, NaBH4 in methanol, H2 with Ni catalyst, heat under high pressure)
3. oxidation of aldehyde to carboxylic acid (KMnO4/K2Cr2O7 in dilute H2SO4, heat under reflux)

Question 5

describe the mechanism of nucleo addition of -CN

Answer 5

step 1:
HCN + NaOH -> H2O + CN-

step 2:
CN- attacks partial positive carbon to form tetrahedral carbocation intermediate

step 3:
attacks HCN to form CHOHCN + CN- regenerated

Question 6

what are the chemical tests for carbonyl compounds (4)?

Answer 6

1. 2,4-DNPH for ketones (orange ppt)
2. tollen’s reagent for aldehydes (silver mirror/grey ppt)
3. fehling’s reagent for aliphatic aldehydes (reddish-brown ppt)
4. iodoform test (brown decolourises, pale yellow ppt)