Question

In: Chemistry

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)

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.

(c) The conditions for both type of reactions are explained in above points.

queen_honey_blossom answered 1 year ago

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