Indian Journal of Chemistry Vol. 45A, May 2006, pp. I 099 - 1112 Is electrophilicity a kinetic or a thermodynamic concept? P K Chattaraj"" *, U Sarkar", DR Ro y", M Elango", R Parthasarathi b & V Subramanian"·* " Chemi stry Department , Indian Institute of T ec hnology. Khara gpur 721 302. India bC he mical Laboratory , Central Leather Resea rch In stitute, Adyar. Chennai 600 020 , India Email: pkc @chem.iitkgp.e rn et.in, s ubuch em@hotmail.com Received 21 November 2005; accepted 20 March 2006 An under standing of the precise na ture of a chemical reac ti vity descriptor is of utmost interest to quantum chemists. An attempt ha s been made here to analyze whether the electrophilicity index is a reliable descriptor of the kinetic behaviour or not. Relative experimental rates of Friedel-Crafts benz ylation, acetylation and benzoylation reactions correlate well with th e correspondin g calculated electrophilicit y values. Chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene have been studied as representative exa mpl es of electrophilic addition and s ubstitution reactions, respectively. The correlation is not ve ry good although it improves dra stically by remov ing a few data point s to show that the electrophilicity is a kinetic quantity with inherent thermody namic information. The correlation between the experime ntal and the calculated activation energies is studied for so me Markovnikov and anti-Marko vnikov addition reac ti ons and it turns out to be rea so nably well. Reaction electrophilicity, l oc al electrophilicity and activation hardne ss are used together to provide a trans parent picture of reac ti on rates as well as the orientation of aromatic electrophilic substitution reac ti ons. Ambiguity in the definition of the electrophilicity is hi ghlighted. Many of the organic reactions can be described in terms of the electro (nucleo) philic addition and substitution. These reactions have got large synthetic potentials and are most widely studied 1 - 4 . Traditionally, the electrophilicity is treated J.4 as a kinetic quantity, which explains the rate of a reaction through its correlation with activation energy or free energy of activation occasionally s upplemented by thermodynamic stabilities of various species involved. On the other hand, the nucleophilicity and the ba sicity are often analyzed at par 1 .4, since both involve the amount of electron density present in it and its potential to donate that, although the former correlates with I:!.G+ (a kinetic quantity) and the latter with I:!.G , (a n equilibrium or thermodynamic property ). Although, it has been known for a long time that the electrophilicity is a cardinal index of reactivity and selectivity, an acceptable definition of it was lacking. Based on the work of Maynard er a/. 5 , a theoretical definition of electrophilicity has been introduced recently by Parr et a/ 6 . It may be noted that Ma ynard et al. 5 and Parr et a/ . 6 have prescribed the sa me definition of electrophilici ty through essentially kinetic (via correlation with reaction rates) and thermodynamic (in terms of the energetically favourable charge transfer processes) routes, respectively and hence it is expected that it will contain both kinetic and thermodynamic information. This electrophilicity, however , does not correlate we ll with the electron affinity<'. Density Functional Theory (DFT) 7 · 8 has bee n quite successful in providing theoretical background of popular qualitative chemical concepts . In this context, several reactivity descriptors have been proposed and used to analyze chemical reactivity and s ite selectivity. Hardness, global softness. electronegativity and polarizability are the global reactivity descriptors widely used to understand the global nature of molecules in terms of their stability and it is possible to gain knowledge about the react1v1ty of molecules. Atomic charge s, Fukui functions ( FF) and local softnesses are the l oca l reactivity descriptors, which provide information about the site selectivity. In addition to these reactivity descriptors, Hard and Soft Acids and Bases ( HSAB) principle has been employed in number of cases in analyzing both nucleophilicity and ba sic it y. which encapsulates both thermodynamic and kinetic properties of numerous molecules <J.JO _ A . hard (soft) nucleophile prefers to react with a hard (soft) electrophile for both kinetic and thermodYnamic considerations and for two s pecies of comp arable electronegativity values (s ame strengths of the acids/base s) 9 " 10 .