From: Paul Handley (p.handley$##$chemistry.uq.edu.au)
Date: Wed Sep 06 2000 - 20:04:24 EDT
As far as I know, the double bonds in allenes are at right angles to each
other, and hence cannot overlap or conjugate in any way. My guess is that
the proton behaves as if it is attacking an isolated double bond, and gives
the normal markovnikov (spelling?) addition of water, and this product can
rearrange to give acetone. The stability of an allylic carbocation results
from conjugation of the positive charge with the p-orbitals of the double
bond, but the developing carbocation in allene protonation cannot sense the
other double bond because it is out of plane and no significant orbital
overlap can occur.
Comments please on this..
Paul Handley
Dept. of Chemistry
University of Queensland
Brisbane, Australia
>I have just read in a book that allene undergoes hydration to give
>acetone:
> H+
>CH2=C=CH2 + H2O -> CH3-CO-CH3
>(To see correctly use courier font)
>
>But this means that when protonated, allene forms the less stable
>cation:
>
> H+ +
>CH2=C=CH2 -> CH2=C-CH3 -> CH3-CO-CH3
>
>
>instead of the more stable allilic one:
>
> +
>CH2=C=CH2 -X-> CH2-CH=CH2
>
>Does anybody know why is this so? Or maybe
>there is an error in the book?
>
>With kindest regards,
>
>Witek Mozga
>
>mozgaw$##$rocketmail.com
>mozga$##$trimen.pl
>http://www.trimen.pl
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