The electrophile, $E^{\oplus}$ attacks the benzene ring to generate the intermediate $\sigma$-complex. Of the following, which $\sigma$-complex is of lowest energy?

JEE Main · 2008 · 27 Apr 2008 · Offline · Hydrocarbons

The electrophile,

E^{\oplus}

attacks the benzene ring to generate the intermediate

σ

-complex. Of the following, which

σ

-complex is of lowest energy?

(A)(see structure)
(B)(see structure)
✓ Correct
(C)(see structure)
(D)(see structure)

Answer: (B)

Solution

The arenium-ion intermediate is stabilised when the ring bears electron-releasing groups and destabilised by electron-withdrawing ones, because the positive charge of the

σ

-complex needs to be spread out. Among the substituted cases, the one whose substituent withdraws electron density least strongly lets the positive charge sit most comfortably and so gives the intermediate of lowest energy, so that least-destabilised arenium ion is the most stable of the set.

JEE Main · 2008 · 27 Apr 2008 · Offline · Single correct option

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