A reader check you might actually find interesting


I’ve been arguing with my inorganic lab TA over a problem we’ve both been assigned in an organic chemistry class, and I’d like to see what the world out there thinks. The most obvious mechanism of the transformation above is the intramolecular one, in which the negative charge on carbon attacks the methoxy carbon and the C-O bond electrons go to oxygen. In reality, or so the problem states, it is an intermolecular mechanism that predominates. What is this intermolecular mechanism, and why is it favored over the intramolecular one?

Presumably, the intermolecular mechanism involves attack by the carbanion on the methyl group of another molecule, and simultaneous attack of that other molecule’s carbanion on the methoxy carbon of the first molecule:

According to me, this process is favored over the intramolecular mechanism because the tramsformation has to be essentially an SN2 substitution reaction, and the geometry of the molecule prevents the negative charge from lining up correctly to assume the typical SN2 transition state. Not to mention the lone pair is likely in resonance with the nearby pi system, which would put it nearly at right angles to (read: very far away from) the carbon it’s supposed to attack during the intramolecular mechanism. Intermolecular attack is thus much more likely in this case.

My TA claims that the question is somehow flawed, and that the intramolecular mechanism amounts to more or less a six-membered ring closing. I disagree with the idea that it’s a ring closing, because, well, no ring gets closed, but I’m not convinced my answer’s right. Any ideas readers (if you are out there…)?

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3 Comments

  1. I’m so unspatial it hurts sometimes, but I see what you’re saying and I think you’re right. The lone pair on the carbanion should be in a p orbital. Seems like if you take two of these, one on top of the other, you can get the intermoecular mechanism to look reasonable. As far as anything intramolecular, the geometry should be way wrong, and (IIRC) while moving electrons is pretty quick, changing those bond angles isn’t going to happen so fast. After all, with that carbon sp2-hybridized like it is, there should be a hydrogen atom kinda sticking in the direction of that methyl group…unless the

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  2. Basically, it is what you have said about poor orbital overlaps. The reaction is a 6 endo tet cyclization which is a disfavored process. See Baldwins’s Rules.

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