Beckmann fragmentation

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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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In the reaction above the starting material is bicyclic and the product monocyclic, so a C-C bond must have been broken. This is known as a fragmentation reaction. C-C bonds are very strong and therefore difficult to break, but the reason this reaction works is because of polarization. It follows that to break a strong C-C bond, it helps if it is polarized.

For polarization to occur there needs to be a 'source' of electrons at one end, and an electron 'sink' (into which they can flow) at the other.

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