Ring activator vs deactivator (MCAT question of day)

[TommyTman]

Hey guys,

I'm not sure if anyone does Kaplan's MCAT question of the day? Today's question was:

Which of the following compounds would react the slowest in electrophilic nitration?

I thought the answer was phenyl acetate because esters strongly deactivate the ring, but the answer was chlorobenzene. I'm wondering why as halogens only weakly deactivate rings.

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[DrknoSDN]

Activating groups are usually electron donors. Halogens have lone pairs of electrons that they could donate to the ring to form resonance with a bonding substituent but the electron withdrawing induction is more powerful than the lone pair donating potential. (overall weak deactivating, ortho-para directing)

[Edit]Phenyl acetate reacts faster due to the donating electron inductive effects of the ester oxygen. (Credit to stigus)

 

[deleted380953]

Just to add, you're right, in that ester groups can deactivate the ring, but only when the ring is bonded to the carbonyl carbon. In the case of Phenyl Acetate, the benzene group is bonded to a an oxygen that has a a partial negative charge, rather than the carbonyl carbon. This gives the oxygen ability to donate its charge to the ring. Note the difference of a substituent that's --CO2R (withdrawing group) and --OCOR (donating group)


http://www.mhhe.com/physsci/chemist...ics/carey04oc/ref/ch12substituenteffects.html

If you look at the list from the link, compare the 7th blue one, with the 4th green one

 

[TommyTman]

Thanks for the help guys. The link was especially helpful. I was really just looking at the functional group as a whole rather than the bonding to the ring which was a mistake. Thanks again.

 

[Blueprint MCAT Tutor]

I'm like 95% sure you won't get this question on the MCAT anymore. The official list of orgo topics doesn't include the whole ring activator / deactivator thing anymore. It's one of those topics that the big MCAT prep companies have just left in their books from when it was on the list of official topics like a decade ago.