PROBLEM 7.59 - Organic Chemistry

Question

Question: Pick the reactant or solvent in each part that gives the faster S_N1 reaction.

1. Reaction of  H₂O with (CH₃)CCl  or (CH₃)₃Cl
2. Reaction of  CH₃OH with (CH₃)CBr or  (CH₃)₂CHCH₂Br
3. Reaction of   CH₃CH₂CH(I)CH₃ with CH₃CH₂OH  in water or DMSO

Step-by-Step Solution

Answer

Answer 

1. The reaction of  H₂O   with (CH₃)₃Cl  will give the faster  S_N1   reaction.
2. The reaction of   CH₃OH  with (CH₃)CBr  will be faster because it is a tertiary alkyl halide. 
3. The reaction of   CH₃CH₂CH(I)CH₃  with CH₃CH₂OH  in water is a faster reaction.

Factors affecting the S N 1 reactions

The S_N1   reaction mechanism is a two-step mechanism in which the carbocations are formed as intermediates.

 The   reaction mechanism depends upon the following factors:

• The presence of a neutral or weak nucleophile is favorable for the   reactions.
• The polar aprotic solvents also increase the rate of the   reactions.

Reagent or solvent

a.The reaction of water with (CH₃)₃Cl  is faster as compared to (CH₃)CCl. This is because the I^- group is a better leaving group than the Cl^- .

 

b.The reaction of ethanol with  (CH₃)CBr  will be faster. This is because the (CH₃)CBr is a tertiary alkyl halide which is very reactive toward the S_N1  reaction.

Tertiary alkyl halide

The (CH₃)₂CHCH₂Br  is a primary alkyl halide that is least reactive toward the S_N1 reactions.

Primary alkyl halide

c. The reaction of CH₃CH₂CH(I)CH₃  with CH₃CH₂OH  in water is faster. Water is a protic solvent, and the S_N1  reactions are faster in the presence of a polar protic solvent.

Secondary alkyl halide

On the other hand, the DMSO (dimethyl sulfoxide) is a polar aprotic solvent. The S_N1  reactions are slower in aprotic solvents.