Fragmentations

Bond polarization - Carbonyl sink

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NOTE: Important charges and non-bonding electrons are shown throughout the animation except during the transition phase

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The bicyclic methoxy ketone compound shown above fragments to a seven-membered ring in acid. Numbering the atoms shows which bond fragments (2-3 bond). We need to identify the source and the sink for the electrons to polarize the bond. The protonated carbonyl group acts as the sink, and the lone pair of electrons on the other oxygen acts as the source.

You can think of the electrons in the C-C bond being 'pushed' by the oxygen's lone pair and 'pulled' by the protonated carbonyl group until the bond breaks. The C-C bond is also weaker than normal C-C bonds due to ring strain.

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