Question

The mechanism of S_N¹ reaction is given as:

$\mathrm{R}-\mathrm{X}\underset{\text{lon pair}}{\to {\mathrm{R}}^{\oplus }{\mathrm{X}}^{\Theta }}\to {\mathrm{R}}^{\oplus }$$\Vert X^{\Theta }\stackrel{Y^{\Theta }}{⟶}R-Y+X^{\Theta }$

$SolventSeparatedionpair$

A student writes general characteristics based on the given mechanism as:

(a) The reaction is favored by weak nucleophiles.

(b) R would be easily formed if the substituents are bulky.

(c) The reaction is accompanied by racemization.

(d) The reaction is favored by non-polar solvents. Which observations are correct?

• A. (a) and (b)
• B. (a), (b) and (c)
• C. (a) and (c)
• D. (b) and (d)

Answer 1

The correct option is B

(a), (b) and (c)

Explanation for correct option:

B. (a), (b) and (c)

(a) The reaction is favored by weak nucleophiles.

(b) R would be easily formed if the substituents are bulky.

(c) The reaction is accompanied by racemization.

${\text{SN}}_1$(Nucleiophillic Substitution Reaction 1)

Mechanism:

$\mathrm{R}-\mathrm{X}\underset{\text{lon pair}}{\to {\mathrm{R}}^{\oplus }{\mathrm{X}}^{\Theta }}\to {\mathrm{R}}^{\oplus }$$\Vert X^{\Theta }\stackrel{Y^{\Theta }}{⟶}R-Y+X^{\Theta }$

$SolventSeparatedionpair$

Separated ion pair:

1. A cation and an anion that are present in a solution of an ionizable substance combine to produce a distinct chemical species known as an ion pair.
2. Depending on how much the two ions have been solvated, there are three different forms of ion pairs.

Characteristics of ${\text{SN}}_1$:

1. ${\text{SN}}_1$ reactions almost usually include weak nucleophiles, as strong nucleophiles are too reactive to allow a carbocation to develop.
2. It results in a racemic mixture of enantiomers in the product because the nucleophile can attack the carbocation from either side (front or back).
3. As, they considerably aid in the creation of carbocations, which are much more easily attacked by nucleophiles, polar protic solvents will favor an $S{N}_1$ reaction even at the condition when the nucleophile is constrained.
4. The $S{N}_1$mechanism starts with the production of a carbocation, and as the stability of the carbocation rises, the reaction favors the $S{N}_1$mechanism. A carbocation is stabilized by polar solvents, favoring reaction.
5. Due to the following process, racemic products are formed in this reaction.
6. Because of the following characteristics, polar liquids favor the $S{N}_1$ mechanism.

Hence option B is correct.