Why ketones react with PCl5

I'm doing my chemistry coursework: identifying an organic unknown. Both of my unknowns which are ketones reacted with PCl5 and produced HCl gas, I was wondering if anyone can tell me why this is the case.

My teacher hinted that its something to do with bonds not being static and something about resonance, and HCl gas has lots of energy and drives the reaction...entrphy change (if this is any help)

thanks!! :p:

Reply 1

charco

Study Forum Helper

your teacher seems to be hinting that some ketones exist in resonance with enols (tautomerism) and that the -ol group will produce a positive test for alcohols i.e. fumes of HCl with PCl5

Tautomerism:

CH3CH2COCH3 <==> CH3CH=C(OH)CH3

IMHO its far more likely that the ketone has some water impurity that makes it SEEM as if there is a contribution from the enol

Reply 2

jocelyn

thanks for your help
I've done some research on tautomerism, and one of the websites that i've looked at says that it is catalysed by an acid or base. But if the acid is PCl5 which only acts as an acid in water, is it still possible that when pcl5 reacts with a liquid ketone (which is normally a liquid at room temp) that pcl5 reacts with the ketone to become an acidic solution. Then is this acid that catalyses the tautomeris

ie pcl5 + 4H2O = H3PO4 +5HCl

Reply 3

charco

Study Forum Helper

Yes, but you still have a seemingly positive test for an alcohol if water is present because white fumes are formed as the HCl is liberated. How do you know that the PCl5 is reacting with alcohol (from tautomerism) or water?
What I am saying is that the test is pretty crap as small quantities of water impurity interfere with it.
Any acidity could stimulate the formation of the enol and then further addition of PCl5 would give a positive test.
Many of the 'simple' tests for organic groups are vague and it's no wonder that spectroscopy has virtually replaced wet chemistry in this respect.
IR stretching frequencies soon spots the C=O group and MS and NMR are pretty good at identifying carbon chains and other structural units.

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