Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Lo.Lee.Ta. on March 31, 2015, 05:39:12 PM
-
How do I know if the nucleophile would attack the carbonyl carbon or deprotonate the alpha carbon?
Someone else has tried to ask this question on a different website, but I don't think the answer was satisfactory... :/
I would like to intuitively understand why something else is not the product instead.
It doesn't much sense to say that forming a hydrate, for instance, is not synthetically useful- and that's why it doesn't happen.
This is where the question was originally asked:
http://www.coursesaver.com/showthread.php?3890-How-do-I-know-if-the-nucleophile-would-attack-the-carbonyl-carbon-or-the-alpha-hydrogen
If I have a ketone + an OH-, it seems like either a hydrate or an enolate could result.
If I have a ketone + -OCH3, it seems like either an acetal or an enolate could result.
How am I supposed to know what happens in these situations...?
Thanks SO much! :)
-
It doesn't much sense to say that forming a hydrate, for instance, is not synthetically useful- and that's why it doesn't happen.
It's not that it doesn't happen, it does, the thing to understand is that it is reversible. If hydration is not a step in a pathway that leads to a stable product, then it is unproductive.
If you were to treat a ketone with Na18OD in D218O - i.e. aqueous sodium hydroxide with isotopically labelled O and H, you would see incorporation of 18O in the carbonyl (via hydration-dehydration) and D incorporation at the α-position (via deprotonation/deuteration).
If I have a ketone + an OH-, it seems like either a hydrate or an enolate could result.
Both occur, and both are reversible. The result of each will usually only be apparent if they are steps in productive pathways.
If I have a ketone + -OCH3, it seems like either an acetal or an enolate could result.
A hemiacetal, not an acetal. Again reversible. Non-cyclic hemiacetals are usually not stable enough to be isolated.
-
How do I know if the nucleophile would attack the carbonyl carbon or deprotonate the alpha carbon?
Also, somewhat linked, is there not a confusion here?
Deprotonation is related to acid-base behaviour, rather than nucleophilic behaviour. In many cases nucleophilicity is linked to basic behaviour but not always.