A student ran two separate substitution reactions, Reaction A and Reaction B, in lab that utilized...

A student ran two separate substitution reactions, Reaction A and Reaction B, in lab that utilized an alkyl halide and a nucleophile. However, he did not know if the reactions proceeded through an S_N1 or S_N2 mechanism. To determine the mechanism for both reactions, the student increased the concentration of the nucleophile in Reactions A and B. The rate of reaction for Reaction A increased. The rate of reaction for Reaction B remained the same.

(a) Which reaction is the S_N1 reaction? (1 pt)

(b) Which reaction is the S_N2 reaction? (1 pt)

(c) Explain both of your choices. (4 pts)

Solutions

Expert Solution

Condition for SN1 reaction:

SN1 reaction is an unimolecular and hence, a first order reaction. So, only substrate such as electrophile affects the reaction rate.

For reaction: RX + Y ---> RY + X

The reaction rate is expressed as rate = k [RX]

This reaction has two steps.

Intermediate is a stable carbocation which is formed during the course of reaction.

This generally has two transition states as it is a two step reaction.

The stability of carbocation is the key factor of the reaction. Hence, reactivity of alkyl halides are as: 3^o > 2^o > 1^o

For this reaction, weak or neutral nucleophiles are sufficient.

Conditions for SN2 reaction:

It is a bimolecular or a second order reaction. Hence, both substrate such as electrophile and nucleophile affect the rate of reaction

For reaction: RX + Y ---> RY + X

The reaction rate is expressed as rate = k[RX][Y]

This reaction has only one step.

This reaction goes through a transition state, there is no intermediate cation formation.

Presence of steric hindrance affect the reaction as transition state becomes more crowded. Hence reactivity of alkyl halides are as: 1^o > 2^o > 3^o

In this pathway strong nucleophiles are desired.

Based on these conditions:

(a) Reaction B is SN1 reaction as there is no effect of nucleophile concentration.

(b) Reaction A is SN2 reaction as there is an increase in rate of reaction as increasing nucleophile increases rate of reaction.

queen_honey_blossom answered 1 year ago

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