EWG and EDG

[MedPR]

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Is there a list somewhere? I'm having some trouble remembering which are which, or being able to tell if a less commonly seen substituent is an EDG or EWG.

For example, NO2 is an electron withdrawing group right? I know it can stabilize electrons by resonance, but doesn't NO2 have a 2- charge? So wouldn't it want to donate the extra electrons?

 

[chiddler]

Is there a list somewhere? I'm having some trouble remembering which are which, or being able to tell if a less commonly seen substituent is an EDG or EWG.

For example, NO2 is an electron withdrawing group right? I know it can stabilize electrons by resonance, but doesn't NO2 have a 2- charge? So wouldn't it want to donate the extra electrons?

http://en.wikipedia.org/wiki/Polar_effect

a mini list. some of the more common groups.

The N in NO2 is saturated with electrons. It's a cation so it can't really give or accept electrons very much. The only moment it can is when both oxygens hold a negative charge, allowing the N to have its lone pair...not very favorable.

NO2 can withdraw due to inductive effect of 2 oxygen atoms, and the positively charged nitrogen also pulls in electrons.

 

[milski]

I had to memorize them a few days ago for a final. The general trend seemed to be:

N/O with C substituents on their other side. They can be EDG since the lone pair can resonate and give an electron. Any possible resonance in the other direction (away from the group that you care about) decreases their EDG qualities.

Then you move through alkyls groups and halides. Then anything with carboxyl group attached to C atom and NO2 is at the end as the most EWG.

Not sure how helpful that is but it sort of worked for me.

 

[gettheleadout]

Just snapped this out of my organic book, it lays them out nicely:

 

[MedPR]

Cool thanks for that image! EDG = activating and EWG = deactivating, right?

 

[gettheleadout]

Cool thanks for that image! EDG = activating and EWG = deactivating, right?

Yep! Because the aromatic ring is the nucleophile in EAS.

 

[chiddler]

Cool thanks for that image! EDG = activating and EWG = deactivating, right?

but we don't have to know benzene chemistry, according to BRteach

 

[SaintJude]

Berk Review Teacher offered a great trend:

Consider the first atom attached to the pi-system. Does it have a lone pair of electrons? If the answer is yes, then it can be an EDG by resonance. Does it have an atom in a pi-bond that has no electron pair (such as the C of a carbonyl)? if yes, then it can be an EWG by resonance. If it has no lone pair and is not a part of a pi-bond, then you need to consider the inductive effect.

This worked really well. Try to get a hand on the groups from a test prep like Kaplan or EK (PM me if you need to) b/c you don't want to memorize groups that haven't been traditionally on the MCAT. But yeah, If the atom had a lone pair next to the ring, then it can donate the electrons. Otherwise, if the atom has no electron pair, then it can be EWG.





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[gettheleadout]

but we don't have to know benzene chemistry, according to BRteach

BS Topics list says "effect of substituents on basicity of aromatic amines" so maybe that's where it's relevant? No idea

 

[chiddler]

BS Topics list says "effect of substituents on basicity of aromatic amines" so maybe that's where it's relevant? No idea

yeah but activating or deactivating is specifically benzene reactivity. these terms describe if the ring becomes more or less reactive.

on the other hand, the original question i think is perfectly valid, namely, is X group EWD or EDG? these affect basicity and acidity.

 

[gettheleadout]

yeah but activating or deactivating is specifically benzene reactivity. these terms describe if the ring becomes more or less reactive.

on the other hand, the original question i think is perfectly valid, namely, is X group EWD or EDG? these affect basicity and acidity.

Any aromatic ring can be activated or deactivated by an EDG or EWD respectively, just the ortho/para and meta directing nature of the substituents is confined to benzene.

Put a trifluoromethyl substituent on a non-benzene aromatic system and the nucleophilicity of the ring will still go down as it loses electron density. It's not like EAS can only occur on benzene.

 

[MedPR]

yeah but activating or deactivating is specifically benzene reactivity. these terms describe if the ring becomes more or less reactive.

on the other hand, the original question i think is perfectly valid, namely, is X group EWD or EDG? these affect basicity and acidity.

Yea I agree that background knowledge of benzenes won't be tested, but AAMC might still put them on there in a passage or something.

The passage might say "Electron donating groups attached to benzene are ortho-para directors and electron withdrawing groups are meta-directors."

Then the question might be

"If bromine is an ortho-para directing substituent, which of the following is true?

A. Bromine is an electron withdrawing group
B. Bromine is an electron donating group
C. Bromine blah blah
D. Bromine blah blah blah"

So the passage told you (correctly) that EDGs are ortho-para and EWGs are meta directing. If you don't know in the back of your mind that halides are EWG, the passage basically leads you to believe that halides (bromide, specifically) are EDG, so you will pick B and be wrong.

If you knew the benzene stuff you would know halides are an exception. If not, you at least have to know that halides are EWG.

 

[chiddler]

Any aromatic ring can be activated or deactivated by an EDG or EWD respectively, just the ortho/para and meta directing nature of the substituents is confined to benzene.

Put a trifluoromethyl substituent on a non-benzene aromatic system and the nucleophilicity of the ring will still go down as it loses electron density. It's not like EAS can only occur on benzene.

well come on. i'm sure that "benzene reactivity" doesn't mean ONLY C6H6 and all other aromatics are fair game.

 

[chiddler]

Yea I agree that background knowledge of benzenes won't be tested, but AAMC might still put them on there in a passage or something.

The passage might say "Electron donating groups attached to benzene are ortho-para directors and electron withdrawing groups are meta-directors."

Then the question might be

"If bromine is an ortho-para directing substituent, which of the following is true?

A. Bromine is an electron withdrawing group
B. Bromine is an electron donating group
C. Bromine blah blah
D. Bromine blah blah blah"

So the passage told you (correctly) that EDGs are ortho-para and EWGs are meta directing. If you don't know in the back of your mind that halides are EWG, the passage basically leads you to believe that halides (bromide, specifically) are EDG, so you will pick B and be wrong.

If you knew the benzene stuff you would know halides are an exception. If not, you at least have to know that halides are EWG.

how disappointing. you're right i suppose it can be in a passage.

 

[gettheleadout]

well come on. i'm sure that "benzene reactivity" doesn't mean ONLY C6H6 and all other aromatics are fair game.

Well I haven't seen the quote where BerkReviewTeach said that so I don't know how he meant it...