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Could find the mechanism in brief online. Still have few queries. What makes the D2O not attack the the central carbon (to which OH is attached) and choose other the carbon instead (both have a double bond). Again, in the 'repeat' part of mechanism, there's only one carbon with double-bond (carbonyl group). Why does D2O attack the second carbon again and not the carbonyl group one? I'm really confused.

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  • $\begingroup$ While I would encourage the questioner to offer additional thoughts about the mechanism of this process, I don't think this question should be closed. There is a good question contained here that shows some understanding of the process (i.e., why doesn't water attack the carbonyl directly). $\endgroup$ – jerepierre Jul 31 '18 at 23:07
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The oxygen of the water will attack the carbon of the C=O forming the hydrate, but this is in equilibrium and completely reversible and a dead end so of no consequence when considering the enol reaction.

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Short answer - Electrphile seeks an electron rich site to attack

Explanation - D2O contains partial charges on the atoms because of the difference in the electronegativity of D and O. Oxygen being more electronegative than Deuterium has partial negative charge and D has partial positive charge.

In the enol form the carbon atom directly attached to Oxygen also has partial positive charge (difference in electronegativity O being more electronegative than C). So, the already partially positive charged Deuterium can't attack a site which also has electron deficiency. So, it attacks the carbon next to it which is slightly electron rich when Oxygen lone pairs attack the adjacent carbon.

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