Question

1. When crotyl chloride is treated with sodium iodide in acetone, it reacts much faster than n-butyl chloride, even though both are primary alkyl halides. Explain the rate difference.

2.The most significant difference between the S_N1 and S_N2 mechanism is the _______________ intermediate that is formed in an ______ mechanism.

3. List butyl chloride, sec-butyl chloride, and tert-butyl chloride in order of increasing relative reactivity with ethanolic AgNO­₃. Explain the reason one might expect these results.

4. Why should n-butyl bromide react faster than n-butyl chloride ?

Answer 1

1) First formation of carbo cation takes place in the reaction of crotyl chloride or n-butyl chloride with sodium iodide and this is the rate determminig step being slow one.

The rate depends on the stability of carbo cation formed. More stable the carbocation inrermediate, faster will be the rate.

Among the above two cases, the carbo cation formed from crotyl chloride is an allylic cation which is stabilised by the resonance with the double bond. But no such resonance is there for n-butyl chloride and hence the carbo cation formed from it is lesser stable.

Therefore crotyl chloride reacts faster than n-butyl chloride.

2) The most significant difference between the S_N1 and S_N2 mechanism is the ___carbo cation_ intermediate that is formed in an --SN₁__ mechanism.

3) butyl chloride < sec-butyl chloride < and tert-butyl chloride

One of the reasons for this order is stability of the carbo cation formed from alkyl halide. More is the stability of carbo cation, faster will be the rate of it's formation.

4) The reactivity reflects the strengths of the carbon-halogen bonds. The carbon-bromine bond is the weaker and the carbon-chlorine is stronger. In order for a halide ion to be produced, the carbon-halogen bond has to be broken. The weaker the bond, faster will be the rate of it's cleavage.