Step 1: Understanding the Concept:
Identify the fundamental reaction mechanism (substitution vs. addition, and electrophilic vs. nucleophilic vs. free radical) for each classic named organic reaction listed. Step 2: Key Formula or Approach:
Review the reagent behaviors: Alkoxide ions are nucleophiles. Lewis acids generate electrophiles. Light (hv) initiates free radical cascades. Step 3: Detailed Explanation:
Let's analyze each reaction:
A. Williamson Synthesis:
Reaction: $R-X + R'-O^-Na^+ \to R-O-R' + NaX$.
Mechanism: The alkoxide ion acts as a nucleophile attacking the alkyl halide in an $S_N2$ mechanism. This is Nucleophilic substitution. (A $\to$ III)
B. Friedel Craft Reaction:
Reaction: Benzene $+ R-Cl \xrightarrow{AlCl_3} \text{Alkylbenzene} + HCl$.
Mechanism: The Lewis acid $AlCl_3$ generates an alkyl carbocation (an electrophile), which attacks the electron-rich aromatic ring. This is Electrophilic substitution. (B $\to$ IV)
C. Bromination of vinyl benzene:
Reaction: Addition of $Br_2$ across the $C=C$ double bond of the vinyl group.
Mechanism: The pi electrons of the alkene attack the polarizable $Br_2$ to form a bromonium ion, followed by bromide attack. Alkene additions are classically Electrophilic addition. (C $\to$ I)
D. Chlorination of toluene in light:
Reaction: Toluene $+ Cl_2 \xrightarrow{h\nu} \text{Benzyl chloride}$.
Mechanism: The presence of UV light ($h\nu$) causes homolytic cleavage of $Cl_2$, producing chlorine free radicals that substitute the benzylic hydrogens. This is Free radical substitution. (D $\to$ II)
Matching sequence: A-III, B-IV, C-I, D-II. Step 4: Final Answer:
Option (C) is the correct match.
