Electrophilic Aromatic Substitution Question

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I was studying electrophlic aromatic substitution, and I had a small question about deactivating and activating groups. So is an alkyl group with electron-withdrawing substituents a meta or ortho-para director? An example would be CH2NO2. The carbon would be directly attached to the benzene, not the NO2 (which I know is a meta director by itself if it were directly attached to the benzene).

Also, do we typically look at the atom at the alpha position when determining if the group is going to be an ortho-para or a meta director? Thanks!

 

[Pre-Med Village]

Alkyl groups are only weakly activating. Adding an electron withdrawing substituent to the alkyl group will cause it to lose the tendency to donate electrons by induction. Hence, cyano group and trifluoromethyl group are both strongly deactivating meta directors. I've never seen methyl nitronate discussed, but I strongly suspect this is a meta director.

 

[UrshumMurshum]

Alkyl group is a stronger directing group than a NO2 group. So it would take precedence over the NO2 group. I had a chart in my organic chemistry textbook that went over the priority rules, I can't seem to find it right now.

 

[TheMightyBoosh]

Just so you know OP, EAS or any other benzene reactivity won't show up on the MCAT.

 

[Isoprop]

Alkyl group is a stronger directing group than a NO2 group. So it would take precedence over the NO2 group. I had a chart in my organic chemistry textbook that went over the priority rules, I can't seem to find it right now.

OP is talking about an electron withdrawing group attached to an alkyl group. In this case, I agree with JohnWetzel, it will probably be deactivating. A good example is a triflourinated methyl group (CF3-), which is strongly deactivating.

I would also suspect that the position of the EWG on the alkyl chain has some effect also. 2 F atoms on the first carbon will deactivate the ring. Not sure if those 2 F atoms were 3-4 atoms away. This is hard to predict.

In any case, all of this is information beyond the scope of the MCAT, but I want to remind people that it is still fair game to explain EAS in a passage and ask questions based on the passage.

 

[BerkReviewTeach]

...but I want to remind people that it is still fair game to explain EAS in a passage and ask questions based on the passage.

How is it fair game? The officials at AAMC removed benzene chemistry following the 2003 exam and made all sorts of announcements that it would no longer appear on the MCAT. Not once since that time has anyone in a thread here at SDN mentioned it showing up on their exam. You can bet that if it did show up, there would be a few threads complaining how unfair it was.

Just because it appears in a prep book doesn't mean it's fair game; it more likely means that the prep book hasn't been updated in a while.

 

[Isoprop]

I am surprised at your response BRT. There was a thread a while back ago about alkenes where I have said the same thing and we were in agreement.

The MCAT writers like to write passages based on material that students haven't seen before and ask questions based on the basic science behind the passage. With organic chemistry on the MCAT, there have been passages of organic chemistry or biochemistry reactions that most students have never seen or heard. But a lot of the concepts are explained, and the questions will test things like stereochemistry, acid-base properties, SN1 vs SN2, etc.

For instance, there may be a passage on EAS synthesis techniques, but the questions will ask things like "rank the compounds in order of increasing acidity" or "which resonance form explains the reactivity of xyz" or "is the product in reaction 1 enantiomers, diastereomers..." or whatever.

Is a passage like this likely? Probably not. But is it fair? I think so.

I really want to emphasize that prior knowledge of EAS will not be tested. In other words, this is not information that MCAT students need to bring with them into the exam. But I want students who are taking the MCAT to be aware that reactions you are not supposed to know may appear in a passage and you may have to interpret that passage with knowledge you DO know.

If what I said earlier made people think that they should study and memorize EAS reactions, I am sorry because that is wrong. But I just want people to not freak out if they see EAS or another reaction they are not supposed to know in a passage. Not only is that fair but the MCAT writers LIKE doing that.

 

[Erythropoietin]

It would also be wise for test companies to take out stuff that is not included on the MCAT.