Question

Write a short note on Nucleophilic substitution reactions in haloalkanes.

Hint:

Remember: Primary haloalkanes → SN2, Tertiary haloalkanes → SN1, Secondary can follow either depending on conditions.

Answer 1

Step 1: Introduction.
Haloalkanes (alkyl halides) undergo nucleophilic substitution reactions in which the halogen atom (a good leaving group) is replaced by a nucleophile (electron-rich species such as OH$^-$, CN$^-$, NH$_3$, etc.). These reactions are important for the preparation of a wide variety of organic compounds. Step 2: Types of nucleophilic substitution.
\begin{enumerate} \item
SN1 mechanism (Unimolecular): - Occurs in two steps. - First, the C–X bond breaks heterolytically to form a carbocation. - Then, the nucleophile attacks the carbocation. - Favoured in tertiary haloalkanes due to carbocation stability. Example: \[ (CH_3)_3C{-}Br \;\xrightarrow{H_2O}\; (CH_3)_3C{-}OH + HBr \] \item
SN2 mechanism (Bimolecular): - Occurs in one step (simultaneous bond breaking and bond forming). - Nucleophile attacks from the backside, leading to inversion of configuration (Walden inversion). - Favoured in primary haloalkanes. Example: \[ CH_3{-}CH_2{-}Br + OH^- \;\longrightarrow\; CH_3{-}CH_2{-}OH + Br^- \] \end{enumerate} Step 3: Importance.
Nucleophilic substitution in haloalkanes is a key synthetic method for preparing alcohols, amines, nitriles, ethers, etc. Conclusion:
Haloalkanes undergo nucleophilic substitution reactions mainly by SN1 or SN2 pathways depending on the structure of the alkyl group. These reactions form the basis for a wide range of organic syntheses.