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a. Why are overline(e) donating groups, ...

a. Why are `overline(e)` donating groups, such as `(-NH_(2))` (`-`OR), `(-R) o-` and `p-` directing towards `SE` reaction? Explain by an example.
b. Why are `overline(e)` withdrawing groups, [such as `(-NO_(2), (-SO_(3)H),(-CN),(-COOH)`, etc].`m-` direection towards `SE` reaction? explain by example.
c. Although halogens are `overline(e)` withdrawing yet they are `o-` and `p-` directing towards `SE` reaction. Why?

Text Solution

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a. Due to `+M` effect (or `oveline(e)` donating effect) of `(-NH_(2))` group, aniline is resonance stablised as shown:

Due to this, the ring is activated at `o-` and `p-` positions. Thus, `overline(e)` density is increased at these positions. Hence, the incoming electrophile attacks at these positions giving `o-` and `p-` products. b. Due to `overline(e-)` withdrawing effect of the `(-COOH)` group, benozoic acid is resonance stablised as shown:

due to this, the benzene ring is deactived at `o-` and `p-` positons, thus `overline(e)` density is decreased at these positon. So, comparatively `overline(e)` density at `m-` position is increased, and hence the incoming electrophile attacks at `m`-postion giving `m-` products,
C. Halogens are said to be `pi` donating but `sigma` with-drawing, i.e., they withdraw `overline(e)s` thorugh resonance. The intermediate carbocation formed during `SE` reaction is destablised by the `-1` effect of the halogens.

Resonane stablises the intermediate carbocation and this effect is more prominent at `o-` and `p-` positons, `-I` effect is stronger (due to high `EN` of halogens ) than resonance and causes net `overline(e)` withdrawal, and thus causing net deactivation. The resonance effect for the attack at `o-` and `p-` positons and hence makes the deactivation lesss for `o-` and `p-` subsitution.
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