Question

4. An energy diagram for electrophilic aromatic substitution of benzene with Br2 with FeBr3 as catalyst below. The rate-limiting step is formation of the sigma complex (Step 1). Generally, any factor that stabilizes the sigma complex also stabilizes the transition-state leading to that complex. This effect leads to a lower transition-state energy and a faster reaction. rate-limiting transition state reactants + Br + FeBrz Η Br FeBr + O energy intermediate products Br + HBr + FeBr 3 -45 kJ/mol reaction coordinate – a. (3) Draw all possible intermediate sigma complexes formed by attack of l*, the electrophile in this reaction on vanillin at C-2, C-5, C-6. Then for each sigma complex, draw all resonance forms. H _CHz attacks C-2 OH

Answer 1

И Н и C:2 attack НО ор Resonance structures о но и и 7 , ochh БСС oh Tot Loch M3 C-5 attack ОСН ОН Он Resonance structures т оси тос 1 бон, нсон 1 ОН и - и ОН I+ C-6 attack - ОС но оси ОН Resonance structures ОН и на он 1 0 - осон ОСН, 1 애 OH Ber och ОН

Evaluating the relative stabilities of these 3 possibilities

A. # of resonance structures.

More the # of resonance structures, more the stability. The C5 attack gives the intermediate with 4 possible resonance structures. Based on this, C5 attack will be most favoured

B. Presence of positive charge next to the electron withdrawing group

Aldehyde group is electron withdrawing group. In the case of C2 and C6 attacks, there is a resonance structure that has a positive charge on the carbon atom next to the aldehyde group. This will be high energy intermediate and will be less stable. No such resonance structure is possible for C5 attack where positive charge is on the carbon next to the aldehyde group. Therefore, C5 attack will be most favoured.

Both these effects point towards the higher stability of the intermediate formed from the C5 attack. Therefore the sigma complex formed from C5 will be more stable and also will form faster.