New ab initio calculations are reported for two typical organic reactions of polar species, the Wittig reaction of the stabilized ylide Ph₃PCHCO₂CH₃ with benzaldehyde, and the Morita–Baylis–Hillman reaction of the Michael acceptor methyl acrylate with benzaldehyde, catalyzed by quinuclidine. The vacuum ab initio data is combined with continuum solvent models in an attempt to predict solution rate constants. The results show (a) that calculations can now be carried out using accurate local correlation methods with large basis sets for systems with over fifty atoms; (b) that many ‘standard’ computational methods, while qualitatively useful, and known to give accurate results for small molecular systems, can be very inaccurate for large reactive partners; and (c) that predicting rate constants quantitatively is very difficult.