Rearrangements – Aziridinium opening – water nucleophile

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

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Water opens the aziridinium intermediate at the more hindered end, so the amine migrates from the secondary to the primary position. Water is a weak nucleophile and therefore cannot undergo S_N2 reactions. Secondary centres can undergo either S_N1 or S_N2 reactions. Water waits until the leaving group has left of its own accord, to give a cation, which rapidly grabs any nucleophile. This can only happen at the secondary centre as the primary cation is too unstable and cannot form.

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F. Durrat, M. V. Sanchez, F. Couty, G. Evano and J. Marrot, European J. Org. Chem., 2008, 2008, 3286–3297.