Question

For alkyl halides used in SN1 and SN2 mechanisms, rank the leaving groups in order of r

For alkyl halides used in SN1 and SN2 mechanisms, rank the leaving groups in order of reaction rate. Iodide Chloride Bromide Fluoride

eaction rate.

Answer 1

Concepts and reason

S_N1 and S_N2 reactions are nucleophilic substitution reactions. Here, the numbers 1 and 2 indicates unimolecular and bimolecular respectively. The rate of reaction is influenced by the nature of leaving group. The rate of both S_N1 and S_N2 mechanism is high, if the substrate has good leaving group.

Fundamentals

In alkyl halides, the halide ion is substituted by the incoming nucleophile in both S_N1 and S_N2 mechanisms. Hence, here the leaving group is halide ion.

S_N1 reaction is a two-step reaction. The rate of the reaction depends only on the concentration of substrate. It does not depend on the concentration of incoming nucleophile.

S_N2 reaction is a single step reaction. InS_N2 mechanism, the rate of the reaction depends only on the concentration of substrate.

The order of basicity of the halide ions is as follows:

$\begin{array}{l}\\\,\,\,\,\,\,\,\,\,{{\rm{F}}^ - }\,\,\,{\rm{ > }}\,\,\,\,\,{\rm{C}}{{\rm{l}}^ - }\,\,\,{\rm{ > }}\,\,\,{\rm{B}}{{\rm{r}}^ - }\,\,\,{\rm{ > }}\,\,\,{{\rm{I}}^ - }\\\\{\rm{strong}}\,{\rm{base}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{weak}}\,{\rm{base}}\\\end{array}$

The order of leaving groups with respect to the reaction rate in S_N1 and S_N2 is as follows:

${{\rm{I}}^ - }\,{\rm{ > }}\,{\rm{B}}{{\rm{r}}^ - }\,{\rm{ > }}\,\,{\rm{C}}{{\rm{l}}^ - }\,{\rm{ > }}\,{{\rm{F}}^ - }$

Ans:

Rank the leaving groups in order of reaction rate as follows: