Ochem questions

[Persistence101]

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Can anyone please tell me what these reactions are:

1. pinacol rearrangement of polyhydroxalcohols

2. haloform reactions

Thanks

 

[usmaple]

Haloform:
It forms a carboxylic acid from a methyl ketone. Add a base (OH-) with Br2 or Cl2. The base will pull off all of the protons on the methyl group of the ketone (they're alpha protons, so they're mildly acidic). After each one is pulled off, it's replaced with a halogen via the X2. In the last step, an OH- attacks the carbonyl carbon, and CX3- is kicked out (it's a stable anion since the halogens inductively draw away electron densities).

So, main reaction:
Methyl Ketone + Excess OH- + X2 -> Carboxylic Acid + etc.


Pinacol:
Forms a ketone (and possibly aldehydes?) from a vicinal diol. I don't remember the exact mechanism, and there are rules for favored rearrangements. But, I think the chances of this being on is pretty slim (I know it's in the AAMC guide, but still....), and if it is on I imagine it would either be in the context of a passage or you would have to know the very broad basics. Same probably goes for the haloform.

Good luck.

 

[Mudd]

The Pinacol rearrangement involves protonating one of the two hydroxyl groups on a vicinal diol. The water that is formed is a good leaving group, which generates a carbocation after it leaves. An alkyl group from the neighboring carbon (the other carbon that has a hydroxyl group) migrates to the carbocation, leaving a carbocation on the carbon with the hydroxyl. The oxygen can donate an electron pair, which results in a protonated carbonyl. Upon deprotonation, the carbonyl is the final product (usually a ketone). There was a passage where the migrating alkyl group resulted in a ring expansion from five to six carbons. I assume that is what they mean by "synthetic use" in the AAMC MCAT guideline.