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SNAr addition-elimination through the Meisenheimer complex, benzyne mechanism, and substituent effects
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Aromatic rings can undergo nucleophilic substitution when (a) a strong electron-withdrawing group (NO₂, CN, C=O) is positioned ortho or para to the leaving group, or (b) very harsh conditions form a benzyne intermediate.
Addition–Elimination (SNAr) mechanism
Reactivity order of leaving groups in SNAr: F > Cl > Br > I (opposite of SN2; rate-determining step is nucleophilic addition, not C–LG cleavage).
Benzyne (elimination-addition) mechanism — strong base (NaNH₂) on aryl halides without activating EWGs. Goes through a strained benzyne intermediate; nucleophile can add to either sp carbon, giving substitution at both positions.
(a) Rank these aryl chlorides toward NaOMe / MeOH: chlorobenzene, 4-chloronitrobenzene, 2,4-dinitrochlorobenzene, 2,4,6-trinitrochlorobenzene. (b) Explain why fluoride is a better leaving group than iodide in this reaction series.
(a) Reactivity order (fastest → slowest): 2,4,6-trinitrochlorobenzene > 2,4-dinitrochlorobenzene > 4-chloronitrobenzene ≫ chlorobenzene
Each ortho/para nitro group accepts the negative charge of the Meisenheimer intermediate by resonance, lowering the activation barrier for nucleophilic addition.
(b) F vs I leaving group — In SNAr the rate-determining step is nucleophilic addition, not C–LG cleavage. Fluorine is the most electronegative leaving group, so it activates the ipso carbon most strongly toward attack. Loss of fluoride happens only in the (fast) second step. Therefore F > Cl > Br > I in SNAr, the opposite of SN2.
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Treatment of 4-bromotoluene with NaNH₂ in liquid NH₃ at –33 °C gives a roughly 50:50 mixture of meta- and para-toluidine (3- and 4-aminotoluene). Explain mechanistically.
Benzyne (E2-type elimination–addition) mechanism.
Because the methyl group is mildly electron-donating, the two addition sites are nearly isoenergetic and the product ratio is close to 1:1.