Organic: carbonyl chemistry

Question 1

The carbonyl bond structure

Answer 1

• both C and O are sp2 hybridised
• the bond has a sigma and pi bond
• the O has two lone pairs of electrons which are perpendicular to the bonding orbitals

Question 2

Explain why nucleophiles attack the carbon of the carbonyl group

Answer 2

• O is more electronegative than C
• electron density lies closer to O
• the larger filled pi bond lies on the oxygen and the antibonding pi orbital lies on C

therefore the nucleophile attacks C

Question 3

The nucleophile angle of attack

Answer 3

• for best orbital overlap, the interaction is head on
• but this direction of overlap causes interference with the orbitals involved in the pi bond
• the nucleophile attacks at 107º

Question 3

The orbital consideration of nucleophilic attack

Answer 3

• the HOMO of the nucleophile interacts with the LUMO of the C=O, this is the antibonding pi orbital which resides close to the C atom
• Filling the antibonding orbital of the carbonyl group causes the pi bond to break and a new C-Nu bond to form
• the electrons from the pi bond move to the electronegative O to give an oxyanion

Question 3

Why is 107º ideal nucleophilic attack?

Answer 3

it is a compromise between maximising orbital of the HOMO and the antibonding pi, and minimising repulsion between the HOMO and e- density in the C=O in the pi bond

Question 4

What factors affect the degree of hydration of a carbonyl in water?

Answer 4

1. sterics
2. electron withdrawing substituents
3. ring strain

Question 5

The influence of sterics on the degree of hydration

Answer 5

• the more bulky the group, the lower the tendency to be hydrated
• in carbonyl form, the compound is sp2 hybridised so the substituents are 120º from one another
• the addition of the OH group changes the hybridisation from sp2 to sp3, where bond angle is reduced to 107º
• this angle compression causes steric repulsion to increase thus decreasing how favourable hydration is

Question 6

The influence of electron-withdrawing substituents on the degree of hydration

Answer 6

• when a more electronegative atom is bonded to a carbon adjacent to the carbonyl, the C becomes more positive
• the electronegative atoms act as an inductively electron withdrawing group, increasing polarisation of C
• C is more electrophilic and more prone to attack
• the more electron withdrawing the group, the greater the degree of hydration

Question 7

Degree of hydration: which has the more profound effect, sterics or EWG?

Answer 7

Electronic effects (EWG)

Question 8

The influence of ring strain on the degree of hydration of a carbonyl

Answer 8

• 3 and 4 membered rings have high degree of strain
• for a 3 membered ring, the sp2 is forced to have a bond angle of 60º which is half the bond angle the compound would prefer to have
• by hydrating such a compound, the bond angle becomes more ideal so drives the equilibrium to the hydrated product as it relieves some ring strain