Reaction mechanisms Part (iii) Polar reactions Anna K. Croft* DOI: 10.1039/b927077a This review presents highlights from papers published in 2009 reporting the mechanistic studies of polar reactions, predominantly in solution. Articles discussed will be focused on those primarily describing experimental studies. Subheadings utilised in this review only indicate a rough guide to the specific topics covered—as is typical of modern mechanistic chemistry, many studies can fall under a number of possible categorisations. 1. Hydrogen transfer Many reactions require a hydrogen transfer step in their mechanisms, and this section maintains a broad definition, including reactions where either a proton transfer, or hydride transfer play a key role. Proton transfer across long distance can be facilitated by water, and this has been probed in the 7-hydroxyquinoline (7HQ) system, utilising both absorption and emission spectra. 1 In this system, solvent isotope effects were measured when excited 7HQ underwent keto-enol tautomerisation in solution (Scheme 1). The triple proton transfer was confirmed through a proton inventory experiment, and established asymmetric motion (or tunnelling) in the rate-limiting step. The polarity of the cosolvent ether (either diethyl ether or di-n-propyl ether) was found to impact on the lifetime of the transfer, affording different KIEs. Hydrogen atom transfer and solvation dynamics have also been examined for the spice curcumin. 2 Steady state absorption spectra and time-resolved fluorescence were used to obtain fluorescence upconversion decay parameters in a range of deuterated and non-deuterated solvents, namely methanol, ethylene glycol and chloroform. A prominent isotope effect was observed in deuterated solvents. Changes in solvent viscosity did not substantially alter the measured timescale for proton transfer, indicating that small amplitude molecular motions are involved. Scheme 1 Triple proton transfer (schematic) for excited 7HQ in mixed water-solvent system. School of Chemistry, University of Wales Bangor, Bangor, Gwynedd LL57 2UW, UK. E-mail: a.k.croft@bangor.ac.uk; Fax: +44 1248 370528; Tel: +44 1248 382375 304 | Annu. Rep. Prog. Chem., Sect. B, 2010, 106, 304–324 This journal is  c The Royal Society of Chemistry 2010 REVIEW www.rsc.org/annrepb | Annual Reports B Author copy Published on 08 November 2010 on http://pubs.rsc.org | doi:10.1039/B927077A View Online