Question

For a particular nucleophillic substitution reaction, if the rate of reactants were found in the order $RI>RBr>RCl>RF$, then the reaction could be:

• A. $S_{N}1$ only
• B. $S_{N}2$ only
• C. either $S_{N}1$ or $S_{N}2$
• D. neither $S_{N}1$ nor $S_{N}2$

Answer 1

The correct option is C either $S_{N}1$ or $S_{N}2$

$S_{N}1:$ The kinetics of tertiary alkyl halides $R_{3}C-X$ are found to be first order overall, consistent with a unimolecular slow step as in the $S_{N}1$ mechanism.

$Rate=k[R_{3}C-X]$, where $k$ is a rate constant for a particular alkyl halide and temperature (the square brackets are used to indicate molar concentration).

$S_{N}2:$ The kinetics of primary alkyl halides $(R-C{H}_2-X)$ are found to be second order overall, consistent with a bimolecular slow step as in the $S_{N}2$ mechanism.

$Rate=k[R-C{H}_2-X]\left[O{H}^{-}\right]$

This evidence indicates that different mechanisms are occurring depending on the nature of the alkyl halide. The main factors that govern the pathway for this reaction are the steric hinderance of the alkyl halide and the stability of the carbocation intermediate.

As iodine is a better leaving group due to large size, it is released at a faster rate in the presence of incoming nucleophile.

So more stable is the leaving group more fast is the nucleophilic attack.

Option C is correct.