Question

Discuss the mechanism of unimolecular and bimolecular nucleophilic substitution reactions in haloalkanes.

Hint:

Remember: SN1 = two steps with carbocation intermediate, rate depends only on substrate. SN2 = one step, backside attack, rate depends on both substrate and nucleophile.

Answer 1

Haloalkanes undergo nucleophilic substitution reactions in which a nucleophile replaces the halogen atom. These reactions proceed by two different mechanisms:
SN1 (unimolecular) and
SN2 (bimolecular). 1. Unimolecular Nucleophilic Substitution (SN1 Mechanism): \begin{enumerate} \item The reaction takes place in two steps. \item
Step 1 (slow step): The C–X bond breaks heterolytically, forming a carbocation. \[ R–X \;\longrightarrow\; R^+ + X^- \] \item
Step 2 (fast step): The nucleophile attacks the carbocation to form the product. \[ R^+ + Nu^- \;\longrightarrow\; R–Nu \] \item Since the rate-determining step involves only the haloalkane, the reaction rate is: \[ \text{Rate} = k [R–X] \] \item Follows first-order kinetics and usually occurs in
tertiary haloalkanes due to stable carbocation formation. \end{enumerate} 2. Bimolecular Nucleophilic Substitution (SN2 Mechanism): \begin{enumerate} \item The reaction takes place in a single step. \item The nucleophile directly attacks the carbon atom bonded to the halogen, from the backside, while the leaving group departs simultaneously. \[ R–X + Nu^- \;\longrightarrow\; [\text{Transition state}] \;\longrightarrow\; R–Nu + X^- \] \item The transition state has a partial bond between carbon, halogen, and nucleophile. \item Since both haloalkane and nucleophile participate in the rate-determining step: \[ \text{Rate} = k [R–X][Nu^-] \] \item Follows second-order kinetics and usually occurs in
primary haloalkanes. \end{enumerate} Conclusion: \begin{itemize} \item SN1 mechanism involves carbocation intermediate and is favored in polar protic solvents with tertiary haloalkanes. \item SN2 mechanism is a concerted reaction involving backside attack, favored in polar aprotic solvents with primary haloalkanes. \end{itemize}