Carbonyl Compounds

Question 1

Why aldehyde and ketone have higher boiling point than alkanes but lower than alcohols

Answer 1

• pd-pd attraction between polar aldehyde and ketone molecules are stronger than the id-id attraction between non-polar alkane molecules
• but weaker than hydrogen bonds between polar alcohol molecules

Question 2

Preparation of aldehydes and ketones

Answer 2

• oxidation of primary alcohol to form aldehyde
• oxidation of secondary alcohol to form ketone

Question 3

Why do aldehydes and ketones undergo nucleophilic addition reaction

Answer 3

• C=O is polar with carbonyl carbon having partial positive charge
• carbonyl carbon is electron deficient and thus is susceptible to attack by a nucleophile
• the attack on the electrophilic carbon breaks the carbonyl pi bond and the nucleophile is added to the carbonyl C

Question 4

Why do aldehydes and ketones not undergo nucleophilic substitution reaction

Answer 4

• to undergo nucleophilic substitution, then either C-C bond or C-H bond has to be broken
• breaking of these two types of bonds is energetically demanding and the carbocation formed is a poor leaving group

Question 5

Stereochemistry of nucleophilic addition reaction

Answer 5

• carbonyl molecule is trigonal planar with respect to carbonyl atom
• the nucleophile is equally likely to attack from either side of the plane
• thus an optically inactive racemic mixture, containing equimolar amounts of the two enantiomers will be formed

Question 6

Factors affecting rate of nucleophilic addition reaction

Answer 6

• catalyst
• pH of reaction medium
  — slow in acidified solution, slow in very alkaline solution, rapid in slightly alkaline solution

Question 7

Why are ketones less reactive than aldehydes in nucleophilic addition reaction

Answer 7

• additional alkyl group has electron-donating effect which reduces the partial positive charge on carbonyl carbon, making it less susceptible to attack by nucleophiles
• additional alkyl group is bulky and results in greater steric hindrance which hinders the approach of nucleophiles to the carbonyl carbon

Question 8

Reactions of carbonyl compounds

Answer 8

• nucleophilic addition
• reduction of aldehyde to primary alcohol
• reduction of ketone to secondary alcohol
• oxidation of aldehydes by strong oxidation agents
• oxidation of ketones by mild oxidising agents , Tollens’ reagent and Fehlings’s solution
• oxidation with I₂ in NaOH(iodoform reaction)
• condensation with 2,4-dinitrophenylhydrazine(2,4-DNPH)