Halogens: Deactivating Question

[emminent]

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Hey guys, so I know that halogens direct o/p but are deactivating. Does that still mean they are also withdrawing and withdraw electrons like NO2/COOH/etc.? or are the halogens electron donating?

 

[discensdentibus]

Both 🙂 draw some resonance structures. Its not favorable to put a (+) charge on a halogen, but it still happens. Every substituent is withdrawing except H and alkyl substituents. Knowing that its still O/P directing means that a halogens ability to donate some charge into the ring trumps its withdrawing capabilities. Also, Och3>akyl>H>F>Cl

It may be important to note that a fluorobenzene is more reactive to EAS than a Chlorobenzene

 

[emminent]

So let's see if I got this right, it's more donating (hence o/p) than withdrawing but still deactivates the ring b/c it likes to attract electrons?

 

[discensdentibus]

Its kinda getting a bit picky logic wise, but here is the reasoning straight from Bruice text "The halogens are weakly deactivating substituents. Like all strongly and moderately activating substituents, the halogens donate electrons to the ring by resonance and withdraw electrons from the ring inductively. Because halogen substituents have been found experimentally to make benzene less reactive, they must withdraw electrons more strongly than they donate electrons by resonance"

"Since fluorine is the weakest deactivator of the halogens, electron donation by resonance must be the more important factor in determining the relative reactivities of halo-substituted benzenes"

(fluorine is better at donating because its a 2p orbital and they got better overlap)

 

[emminent]

Much thanks man; just wondering, you done w/ your DAT?

 

[discensdentibus]

July 10 🙂

 

[emminent]

Good luck man; post your breakdown after lol

 

[syeh3]

I would just memorize halogen are the weakly deactivating ones and NO2 is stronger deactivating