Which among the following halide/s will not show SN1 reaction:

Step 1: Understand $S_N1$ mechanism requirements

$S_N1$ (unimolecular nucleophilic substitution) requires:

1. Formation of a stable carbocation intermediate
2. Good leaving group
3. Polar protic solvent

Carbocation stability order:

• Tertiary (3°) > Secondary (2°) > Primary (1°) > Methyl
• Allylic and benzylic carbocations are stabilized by resonance
• Vinyl and aryl carbocations are extremely unstable

Step 2: Analyze each option

(a) $\ce{H3C-CH(CH2Cl)-CH2Cl}$ (1,2-dichloropropane or similar):

• Primary or secondary halide
• Can undergo $S_N1$ if secondary position

(b) $\ce{CH3-CH(CH3)-CHCl-CH3}$ (secondary halide):

• Secondary carbocation forms
• Can undergo $S_N1$ ✓

(c) Benzyl chloride ($\ce{C6H5-CH2Cl}$):

• Forms benzylic carbocation (resonance stabilized with benzene ring)
• 4 resonance structures
• Excellent $S_N1$ substrate ✓

(d) Vinyl chloride ($\ce{CH2=CHCl}$) or aryl chloride:

• Would form vinyl carbocation (sp² hybridized)
• Extremely unstable (no resonance, high s-character)
• Cannot undergo $S_N1$ ✗
• Also, C-Cl bond has partial double bond character (resonance with π system)

Step 3: Identify the answer

Vinyl and aryl halides do not undergo $S_N1$ because:

1. Vinyl/aryl carbocations are extremely unstable
2. C-X bond is stronger (sp² carbon)
3. Resonance between halogen lone pair and π system

Answer: (d)

Key Points:

• Vinyl halides ($\ce{C=C-X}$): No $S_N1$ or $S_N2$
• Aryl halides ($\ce{C6H5-X}$): No $S_N1$ or $S_N2$ (without activation)
• Benzylic halides ($\ce{C6H5-CH2-X}$): Excellent $S_N1$ substrates
• Allylic halides ($\ce{C=C-C-X}$): Good $S_N1$ substrates