Question

1. Which alkyl halide in each of the following reactions will react faster in an S_N2 reaction with hydroxide (^-OH)? Explain. Hint, some of these compounds are unreactive in S_N2 reactions- be sure to think about what type of alkyl halide undergoes S_N2 reactions!
   1. Bromobenzene or benzyl bromide
   2. CH₃Cl or (CH₃)₃Cl
   3. CH₃CH=CHBr or H₂C=CHCH₂Br

Answer 1

The SN2 reaction is nucleophilic substitution biomolecular reaction, in which simultaneous bond making and breaking takes place. SN2 reaction of alkyl halides with hydroxide ion involves simultaneous backside attack of hydroxide ion and loss of halide ion from other end. In these reactions intermediate formation does not take place.

a.

Bromobenzene has a bromine atom directly attached to benzene ring . In this case backside attack of hydroxide ion is not possible. Thus, bromobenzene does not undergo SN2 reaction with hydroxide ion.

Benzyl bromide has bromine atom attached to benzylic carbon atom, which is SP3 hybridized. Backside attack of hydroxide ion on benzylic carbon atom is possible giving benzyl alcohol as the product. Thus, Benzyl bromide undergo SN2 reaction with hydroxide ion.

b.

Methyl chloride(CH3Cl) has a chlorine atom attached to methyl group. Methyl carbon is SP3 hybridized and backside attack by hydroxide ion is possible resulting in formation of methyl alcohol as product. Thus, CH3Cl undergoes SN2 reaction with hydroxide ion.

Tertiary butyl chloride((CH3)3Cl) has a chlorine atom attached to tertiary butyl group. Although carbon attached to chlorine atom is SP3 hybridized, it is very bulky due to presence of three methyl groups. Thus, backside attack of hydroxide ion is not favored. Therefore, (CH3)3Cl does not undergo SN2 reaction with hydroxide ion.

c.

CH3CH=CHBr has a bromine atom attached to SP2 hybridized vinylic carbon atom. Backside attack of hydroxide ion on vinylic carbon is not feasible. Thus, CH3CH=CHBr does not undergo SN2 reaction with hydroxide ion.

H2C=CH2CH2Br has a bromine atom attached to SP3 hybridized carbon of allylic CH2 group. Backside attack of hydroxide ion on this allylic carbon is possible. Thus, H2C=CH2CH2Br undergos SN2 reaction with hydroxide ion.