Q.Benzophenone can be obtained by ___________. (Two or more than two options may be correct.)

The key idea is that benzophenone ($\text{Ph}_2\text{C=O}$) can be prepared by Friedel–Crafts acylation, organocadmium reactions, and the Gattermann–Koch reaction, but not by Grignard addition (which gives a tertiary alcohol).

Step 1: Option (i) is a standard Friedel–Crafts acylation: benzoyl chloride reacts with benzene in the presence of $AlCl_3$ to give benzophenone directly.

Step 2: Option (ii) uses diphenylcadmium, which reacts with benzoyl chloride to give the ketone without over-addition — organocadmium reagents are less reactive than Grignards. …

(i) Benzoyl chloride + benzene + $\mathrm{AlCl_3}$ — Friedel–Crafts acylation. $\mathrm{AlCl_3}$ generates the acylium ion $\mathrm{C_6H_5CO^+}$, which acylates benzene to give benzophenone. Correct.

(ii) Benzoyl chloride + diphenylcadmium — organocadmium reagents convert acyl chlorides to ketones and are unreactive enough to stop at the ketone. $\mathrm{C_6H_5COCl + (C_6H_5)_2Cd \rightarrow (C_6H_5)_2CO}$. Correct. …

Method: Retrosynthetic Analysis & Named Reaction Mapping

This question tests your ability to identify which reactions produce benzophenone — a ketone with the structure $C_6H_5COC_6H_5$.

Step 1: Recall the structure of benzophenone

Benzophenone = diphenyl ketone:

It has two phenyl rings attached to a carbonyl carbon.

Step 2: Map each option to a known reaction

Option (A): Benzoyl chloride + benzene + $AlCl_3$

→ This is Friedel-Crafts acylation.

Benzoyl chloride ($C_6H_5COCl$) reacts with benzene in presence of $AlCl_3$ to give benzophenone.

✓ Correct

Option (B): Benzoyl chloride + diphenylcadmium

→ This is a cadmium reagent reaction (related to organometallic synthesis).

Diphenylcadmium $(C_6H_5)_2Cd$ reacts with benzoyl chloride to give benzophenone.

✓ Correct

Option (C): Benzoyl chloride + phenylmagnesium chloride

→ This is a Grignard reaction.

Phenylmagnesium chloride ($C_6H_5MgCl$) reacts with benzoyl chloride.

But Grignard reagents react twice with acid chlorides — the first addition gives a ketone, but the second addition gives a tertiary alcohol (triphenylmethanol), not benzophenone.

✗ Incorrect

Option (D): Benzene + carbon monoxide + $ZnCl_2$

→ This is the Gattermann-Koch reaction (formylation). …

Let’s break this down step-by-step — first the concept, then the common mistakes, and finally how to avoid them.

🧪 Concept Recap: Making Benzophenone

Benzophenone is a diaryl ketone ($\text{Ph}_2\text{C=O}$).

It can be made by several methods — but not all reactions that look right actually work.

✓ Correct Options

(A) Benzoyl chloride + benzene + $\text{AlCl}_3$

→ Friedel–Crafts acylation — works perfectly.

Benzoyl chloride acts as acylating agent, benzene as substrate, $\text{AlCl}_3$ as catalyst.

(B) Benzoyl chloride + diphenylcadmium

→ Cadmium reagent method — works.

Diphenylcadmium ($\text{Ph}_2\text{Cd}$) reacts with acid chlorides to give ketones.

(C) Benzoyl chloride + phenylmagnesium chloride

→ ✗ Does NOT give benzophenone — gives a tertiary alcohol (triphenylmethanol) because Grignard reagent adds twice.

(D) Benzene + carbon monoxide + $\text{ZnCl}_2$

→ ✗ Does NOT work — this is not a standard method.

(Compare: Gattermann–Koch uses $\text{CO} + \text{HCl} + \text{AlCl}_3$ to give benzaldehyde, not benzophenone.)

Correct answers: (A) and (B)

✗ Common Mistakes & How to Avoid Them

1. Thinking Grignard + acid chloride always gives ketone

• Mistake: Students assume one equivalent of Grignard stops at ketone.
• Why it’s wrong: Grignard reagents are very reactive — they attack the ketone product immediately to form a tertiary alcohol.
• Avoid: Remember:

  Grignard + acid chloride → tertiary alcohol (unless you use a special reagent like cadmium or copper).

2. Confusing Gattermann–Koch with benzophenone synthesis

• Mistake: Option (D) looks like Gattermann–Koch, so students think it gives benzophenone.
• Why it’s wrong: Gattermann–Koch uses $\text{CO} + \text{HCl} + \text{AlCl}_3$ and gives benzaldehyde, not benzophenone. Also, $\text{ZnCl}_2$ is not the correct catalyst here.
• Avoid: Memorise the exact reagents for each named reaction — don’t swap catalysts or skip HCl.

3. Assuming any organometallic with acid chloride gives ketone

• Mistake: Students think “organometallic + acid chloride = ketone” is universal.
• Why it’s wrong: Only less reactive organometallics (like $\text{R}_2\text{Cd}$, $\text{R}_2\text{CuLi}$) stop at ketone. Grignard and organolithium go further.
• Avoid: Classify organometallics by reactivity:
  • Low reactivity (Cd, Cu) → ketone
  • High reactivity (Mg, Li) → tertiary alcohol …