PROBLEM 7.62 - Organic Chemistry

Question

Question: (a) Rank A, B, and C in order of increasing  S_N2 reactivity. (b) Rank A, B, and C in order of increasing S_N1 reactivity.

Step-by-Step Solution

Answer

Answer 

1. The order of the increasing S_N2   reactivity is as follows:

A < C < B 

 

     b.The order of the increasing  S_N1  reactivity is as follows:

A < B < C

The vinyl halides are the least reactive toward both S_N1 and S_N2   substitution reactions.

Reactivity order of alkyl halides toward S N 1 mechanism

The tertiary alkyl halide is more reactive toward the S_N1  reactions, followed by the secondary alkyl halides. 

The primary alkyl halides are least reactive toward the S_N1 substitution. 

This is due to the formation of a carbocation intermediate in S_N1  reactions. The tertiary carbocations are more stable than the secondary carbocations. The primary carbocations are the least stable.

Reactivity order of alkyl halides toward S N 2 mechanism

The S_N2  reaction mechanism is a one-step mechanism in which the bond formation and the bond breakage take place simultaneously. The reaction proceeds by the formation of a transition state. 

The primary alkyl halides are most reactive toward the S_N2  substitution, followed by the secondary alkyl halides. 

The tertiary alkyl halides are least reactive toward the S_N2  substitution due to stearic hindrance.

The order of reactivity of the given compounds

The given compounds consist of one primary alkyl halide (B), one secondary alkyl halide (C), and one vinyl alkyl halide (A).

a. The order of the increasing S_N2 reactivity is as follows:

A < C < B 

Compound B is a primary alkyl halide. It will be the most reactive toward S_N2   substitution. The compound C will be less reactive than B.

Compound A is a vinyl alkyl halide. It will be the least reactive toward S_N2 substitution. This is because the carbon-halogen bond in vinyl halides is stronger due to the high s-character of the sp₂ hybrid orbitals of the carbon atom.  

Due to high s-character, greater energy is required to break the bond, and, therefore, the vinyl halides do not undergo the S_N2  substitution.

 

b. The order of the increasing S_N1  reactivity is as follows:

A < B < C

The secondary alkyl halides are very reactive compared to the primary alkyl halides due to the stability of the carbocation intermediate. 

The vinyl halides are the least reactive toward the S_N1  reactions. This is because the loss of the halide ion results in the formation of a highly unstable vinyl carbocation. 

The sp-hybridized positively charged carbon atom has only two groups attached to it, which makes it even less stable than the primary carbocations.