Question

Which of the following aryl halides will react with sodamide/ liquid ammonia leading to substitution via benzyne mechanism

Answer 1

Answer:

Previously [see: Nucleophilic Aromatic Substitution] we saw that electron-poor aromatic rings containing a leaving group can undergo substitution with electron-rich nucleophiles.  We saw that the mechanism proceeds through addition of a nucleophile to the aromatic ring (via an electron-rich intermediate) followed by loss of a leaving group, in a process sometimes called, “addition-elimination”.

nucleophilic aromatic substitution mechanism summary addition elimination

Importantly, the only substitution product is the one where the nucleophile ends up attached to the same carbon as that bearing the leaving group.  (This differentiates it from electrophilic aromatic substitution, where a mixture of ortho-, para–  and meta- products can be obtained.)

2. A “Nucleophilic Aromatic Substitution” In Name, But By A Different Mechanism

Although the “addition-elimination” mechanism for nucleophilic aromatic substitution has been known since at least 1902  (when Meisenheimer isolated a key intermediate) , it became increasingly clear in the first half of the twentieth century that certain reactions classified as “nucleophilic aromatic substitution” appeared to proceed through a different mechanism altogether.

For example, it was found that treating chlorobenzene with sodium amide (NaNH2) in liquid ammonia (boiling point = –33°C) resulted in the rapid formation of aminobenzene (“aniline”):

nucleophilic aromatic substitution of chlorobenzene at an unusually low temperature knh2 nh3

An addition-elimination mechanism here doesn’t seem right, considering that nucleophilic aromatic substitution reactions with far stronger electron withdrawing groups (e.g. NO2, rather than Cl) require higher temperatures and longer reaction times.

Another observation was that no reaction occurred under these conditions when the ortho- positions were attached to alkyl groups. A hydrogen is necessary at one of these positions for the reaction to proceed.

no nucleophilic substitution of bromobenzene happens without ortho hydrogens

(note – NaNH2 and KNH2 can be considered to be essentially the same for our purposes)

A second observation was that in the case below only the ortho- and meta- products formed, and never the para– .

in nucleophilic aromatic substitution of orthochlorotoluene no para product is observed

3. The Benzyne Intermediate

Various intermediates were proposed to explain these results, but then in 1953 John D. Roberts (then at MIT) nailed it by  publishing one of the most elegant chemical experiments of all time.

Explanation:

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Answer 2

answer: written below-

explanation:

Previously [see: Nucleophilic Aromatic Substitution] we saw that electron-poor aromatic rings containing a leaving group can undergo substitution with electron-rich nucleophiles. We saw that the mechanism proceeds through addition of a nucleophile to the aromatic ring (via an electron-rich intermediate) followed by loss of a leaving group, in a process sometimes called, “addition-elimination”.

nucleophilic aromatic substitution mechanism summary addition elimination

Importantly, the only substitution product is the one where the nucleophile ends up attached to the same carbon as that bearing the leaving group. (This differentiates it from electrophilic aromatic substitution, where a mixture of ortho-, para– and meta- products can be obtained.)

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