EAS Reaction

[NavyDDS1990]

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How would you tell if a group is withdrawing or donating group?

is it something you would just have to memorize or still able to distinguish by referring to the periodic table?

I know it has to do something with electronegativity, but what I dont understand is that If there is OCH3 on a benzene ring, isn't OCH3 more electronegative, therefore pulls the electrons towards itself?

I am totally confused

 

[StumpMT]

I think it would be easiest and most beneficial to just memorize.

In order of strongest to weakest the donating/activating/ortho-para directors are NR2 > OR2 > alkyl/aryl.

In order of strongest to weakest the withdrawing/deactivating/meta directors are NO2/SO3H/CN/CF3/NR3+ > carbonyls > halides.

Donating groups are ortho/para directors and withdrawing groups are meta directors with one exception: halogens are withdrawing but are weak ortho/para directors.

 

[emitpeels]

Majority of the time, if the group has more than one electronegative element in it, its going to be withdrawing. Just be sure to remember the Nitrile will be withdrawing and halogens are o/p. Hope this helps!

 

[customx]

Just memorize them in order of strongest to weakest. The strength of a group, or comparing strengths is a type of question you're bound to see on the DAT.

 

[cbeck4]

Yea, just memorize them especially if your time is limited. To answer your question, however, an OCH3 group IS more electronegative, thus pulling electron density towards itself, however it's resonance ability outweighs the electronegativity factor way more. Hope that helps.

 

[gobinoob]

I was taught that it has much to do with two factors, which tend to outweigh inductive effects:

1. Does the substituent have an electron pair to donate?
2. Is the substituent, itself, electron deficient (electron starved)?

The strongest donating groups, OH, OCH3, NH2, have lone pairs to donate. Some of the weaker donating groups, such as CH3 (and other R groups), have no lone pairs but instead feature a "delta negative", which indicates an excess of electrons and a willingness to donate. In CH3, for example, note that since C is more electronegative than H, it will pull electrons toward itself (we indicate this with a delta negative). This excess of electrons can then be shared with the aromatic ring to increase either nucleophilicity or arenium ion stability.

In contrast, a strong electron withdrawing group is one with a formal + charge and inductive groups attached to it. An important example is NO2; if we google NO2 to see its structure, we realize it has a + formal charge (it is electron deficient), and is surrounded with O's, which, being more electronegative, further withdraw electrons from our N, who in turn withdraws from the aromatic ring. This same principle applies for other strong deactivators, such as NR3+, NH3+, SO3H, and nitriles. Weaker electron withdrawing groups, such as carbonyls, which all feature the same C=O bond, follow the same concept but have a delta + due to the more electronegative O with respect to C, instead of a formal + charge.

Also, just memorize the "halogen exception" as it is important. I hope this helps give some insight into the reasons behind activating and deactivating groups!