Fragmentation – Bond polarization (Hydroxy Sink)

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

^{Click the structures and reaction arrows in sequence to view the 3D models and animations respectively}

Both diastereoisomers of the cyclic diol above fragment in acid to give an aldehyde and water. Numbering the atoms shows which bond fragments (1-4 bond). We need to identify the source and the sink for the electrons to polarize the bond. The protonated hydroxy 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 departing water until the bond breaks. The C-C bond is also weaker than normal C-C bonds due to ring strain.

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M. A. Drahl, M. Manpadi and L. J. Williams, Angew. Chemie – Int. Ed., 2013, 52, 11222–11251.