Question

Reactivity of alkyl halides towards ${\mathrm{SN}}_1$ nucleophilic substitution reaction.

Answer 1

${\mathrm{SN}}_1$ reaction:

1. A nucleophile replaces a leaving group in ${\mathrm{SN}}_1$ reactions.
2. This reaction occurs in two steps i.e. First the carbon and halogen bond is slowly cleaved to form a carbocation in the first phase of the reaction. The halide ion's solvation energy with the proton of the protic solvent causes the carbon and halogen bond to break.
3. The first step is the most difficult and reversible. The rate of reaction is determined by the concentration of alkyl halide rather than the concentration of hydroxide ion in this stage, which is also known as the rate-determining step.
4. In the second step, a nucleophile attacks the carbocation that has been produced.
5. A carbocation formation exists. The ease of forming a bond with the nucleophile is determined by the carbocation's stability.
6. As a result of the strong stability of $3^{°}$carbocation, an $3^{°}$alkyl halide will undergo the ${\mathrm{SN}}_1$ action relatively quickly. followed by the $2^{°}$ alkyl halide and $1°$alkyl halide i.e. $3°$alkyl halide$>$$2^{\circ }$alkyl halide$>$$1^{\circ }$alkyl halide.
7. Example:${\left({\mathrm{CH}}_3\right)}_3{\mathrm{C}}^{+}>{\left({\mathrm{CH}}_3\right)}_2{\mathrm{C}}^{+}>{\mathrm{CHCH}}_2^{+}>{{\mathrm{CH}}_3}^{+}$ methyl is the least stable.
8. A strong leaving group accelerates an SN1 reaction as well. This is so because the rate-determining step includes the group that is leaving.
9. A good leaving group seeks to leave so that the bond between carbon and the leaving group can be broken more quickly. As soon as the bond is broken, the carbocation is created; the faster the carbocation is created, the faster the nucleophile can enter, and the faster the reaction will be finished.
10. Halides are a good example of a leaving group, and as you move down the column, their leaving-group ability increases. The order of sequence of halides are ${\mathrm{I}}^{-}>{\mathrm{Br}}^{-}>{\mathrm{Cl}}^{-}>{\mathrm{F}}^{-}$

Therefore, the reactivity of alkyl halides towards ${\mathrm{SN}}_1$ nucleophilic substitution reaction is $3^{\circ }$alkyl halide$>2^{\circ }$alkyl halide$>1^{\circ }$alkyl halide.