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During aldol formation, I've learned that there is a second addition of water in many cases to cause a dehydration to occur that results in a double bond at the alpha and beta position to the carbonyl. Why does this specific reaction automatically dehydrate? Is it due to the benzene ring?

  • $\begingroup$ That certainly helps because you now have an extended $\pi$ system. $\endgroup$ – Zhe Apr 23 '17 at 20:43

@Biomed Boy

What do you mean by addition of 2nd molecule of water? Actually, that was not a clear statement. If you could show it in some of the mechanisms, it coul have been better to understand your doubt/question completely.
There is dehydration usually always associated with Aldol, if substrate has 2 or more (alpha) H atoms, even in absence of a Ph ring: in acid catalyzed as well as base catalyzed Aldols, though their mechanisms differ.

The reason is always a conjugated (with carbonyl) bond, which may be facilitated by a Ph ring, as in the given case.

The dehydration steps do involve:
I. In acid catalyzed Aldol, it is usual E2
Acid catalysed aldol
II. In base catalyzed Aldol, it is E1cB Base catalysed Aldol

  • $\begingroup$ Why do they vote down, for no apparent reason? $\endgroup$ – Che Mistry Apr 23 '17 at 23:07
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    $\begingroup$ While we appreciate your contribution to the site, it would be preferable to not answer questions, that do not comply with our homework policy or which need further clarification, like you imply with "Actually, that was not a clear statement." $\endgroup$ – Martin - マーチン Apr 24 '17 at 8:51

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