Question

1a. Draw the two chair conformations for 1b. Draw the dominant frontier orbital interaction for the bromide in the reaction below. Which the SN2 reaction of NaCN with CH,Br. (Hint: the one do you think is more reactive? Why? HOMO of cyanide interacting with the LUMO of CH,Br). Clearly show the optimum alignment and NaCN + NaBr in-phase overlap of these orbitals. DMF 1e. Draw the enol intermediate in the 1d. Draw a specific example (reactants and products) hydration reaction below and its of a stereospecific substitution reaction. corresponding tautomer. cat. HgSO4 cat. H20 H2O enol tautomer 2. Rank the following as indicated by placing the appropriate letter in the respective box a. Increasing order of reactivity in an SN2 reaction with CH,CHI most reactive least reactive b. Increasing reactivity in a Sn2 reaction with KCN least reactive most reactive

Answer 1

1a)

The two conformations are diaxial (Structure I) and diequatorial (Structure II).S_N²reactions occur by the rear attack of the nucleophile on the alkyl halide. In the case of structure I, there is significant steric hinderance due to 3,5-diaxial interactions between the two methyl groups. When a nucleophile attacks structure I from rear (which is from axial side on C-1), the steric interactions increases further. As a result, this conformer is expected to undergo S_N² reactions slowly. On the other hand, Structure II does not have such unfavourable interactions. Also the attack of the nucleophile in Structure II is expected from equatorial side on C-1. Since this does not increase steric interactions, this conformer is expected to undergo S_N² reactions faster.

CH3 di eqretano

1b)

The HOMO of the CN^- interacting with the LUMO of CH₃Br and forming the bond in S_N² reaction between CH₃Br and CN^-

JEN H

1c)

Ти од На о ело

1d) SN2 reactions are stereospecific substitution reactions. The substitution occurs by inversion of configuration

Naon Br chz H CH3 2-octano) 2- Bromo octare

2a) Order of decreasing nucleophilicity is

OH^- > H₂O > NH₃ > SH^-

S_N² reactions are faster with better nucleophiles

2. Rank the following as indicated by placing the appropriate letter in the respective box a. Increasing order of reactivity in an SN2 reaction with CH,CHI SH least reactive most roactive

2b) Order of reactivity towards SN2:

primary bromide > Secondary bromide > allyl bromide > 1-Bromoethyne

Primary bromide has lesser steric hinderance compared to secondary bromide, therefore the former undergo S_N² reactions faster than the latter. Allyl bromide prefers S_N¹ reaction since the corresponding carbo cation is stable. acetylide formed from 1-bromoethyne itself is a nucleophile and does not under nucleophilic reactions.

b. Increasing reactivity in a SN2 reaction with KCN terasa maray a siarancia van Ke - Br least reactive most reactive