The Tiffeneau–Demjanov chain extension reaction was modelled using the B3LYP/6-31G* basis set for both diazonium and water leaving groups. The chain extension of 12 with the diazonium leaving group occurs with an early transition state, a low activation barrier, and very little participation by the silicon substituent. Chain extension of 15, involving displacement of the less reactive leaving group water, occurs with a later transition state, a significantly higher activation barrier, and with greater participation by the silicon substituent.