Question

In an sn1 substitution of triphenylmethyl bromide where we had to synthesize triphenylmethyanol from triphenylmethyl bromide using acetone and water (water was the nucleophile) what would the following effect on the experiment be (increase, decrease or no effect on reaction) if we; a) increased water concentration b) halide is decreased in concentration c) the solvent is changed to diethyl ether and d) triphenylmethyl chloride is used? PLEASE type if possible!! THX!

Answer 1

Conditions 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 where intermediate is a stable carbocation which is formed during the course of 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.

Favoured by, polar protic solvents as they can stabilize the carbocation intermediates.

Now, triphenylmethyl bromide is a tertiary bromide and hence,

a) increased water (nucleophile) concentration will not affect the reaction rate.

b) halide is decreased in concentration then reaction rate will decrease.

c) the solvent is changed to diethyl ether (less polar, non-protic solvent) so reaction rate will decrease.

d) triphenylmethyl chloride is used then reaction rate will decrease as Cl- is not a good leaving group as Br-.