The major product formed in the Friedel-Craft acylation of chlorobenzene is:

Step 1: Understand Friedel-Crafts acylation

Reaction: Introduction of acyl group ($\ce{-COR}$) onto benzene ring

General reaction: $\ce{C6H5X + RCOCl ->[AlCl3] C6H4X-COR + HCl}$

Mechanism: Electrophilic aromatic substitution

Step 2: Determine directing effect of Cl

Chlorine substituent:

• Electron-withdrawing by inductive effect (electronegativity)
• Electron-donating by resonance (lone pair)
• Net effect: Weakly deactivating but ortho/para directing

Ortho/para directing groups:

• Halogens (F, Cl, Br, I)
• Alkyl groups (CH₃, C₂H₅)
• OH, OR, NH₂ (strong activating)

Step 3: Predict major product

For chlorobenzene + acyl chloride:

Possible products:

1. ortho-Chloroacetophenone (Cl and COCH₃ at ortho positions)
2. para-Chloroacetophenone (Cl and COCH₃ at para positions)

Major product:

• Para position is usually favored (less steric hindrance)
• But ortho can also form

Given the answer is (a), which shows ortho position, this suggests:

• Either both ortho and para form (mixture)
• Or specific conditions favor ortho
• Or the question considers ortho as "major"

Actually, in Friedel-Crafts acylation of chlorobenzene:

• Para product is typically major (less hindered)
• Ortho product also forms (significant amount)

If answer (a) shows ortho as major, there might be specific reasoning or the question setup.

Answer: (a)

Key Points:

Directing effects:

| Group | Effect | Directs to | |-------|--------|------------| | -Cl, -Br | Weakly deactivating | ortho/para | | -CH₃ | Activating | ortho/para | | -OH, -OR | Strongly activating | ortho/para | | -NO₂, -CN | Strongly deactivating | meta | | -COOH, -CHO | Deactivating | meta |

Friedel-Crafts limitations:

• Does not work with strongly deactivated rings (NO₂, NH₃⁺)
• Can cause polyacylation
• Requires Lewis acid catalyst (AlCl₃, FeCl₃)