Keywords: Nucleophilic substitutions / Benzyl bromides / Substituent effect / Solvent effect / Density functional calculations / Activation parameters DFT computations on the mechanisms of nucleophilic substitutions on benzyl bromides were performed and the calculated activation parameters were compared with experimentally acquired data. In vacuo, the presence of electron-withdrawing (e-w) groups on the benzyl bromides accelerated the reactions and the calculated ∆G ‡ /∆H ‡ /∆S ‡ vs. σ plots were linear, while in solvents there were breaks in the calculated and measured ∆G ‡ /∆H ‡ vs. σ + plots of the reactions between the benzyl bromides and Br -, both with electron-donating (e-d) and with e-w substituents accelerating the reactions. The calculated ∆S ‡ values appeared to be independent of the substituents. In solvents, the calculated ∆G ‡ /∆H ‡ /∆S ‡ vs. σ + plots for the reactions between benzyl bromides and pyridine were linear, whereas breaks were observed in the plots of the measured data. These reactions were promoted by e-d substituents, but the measured reactivities of sub-
A series of model compounds capable of intramolecular sulfur(II)-axygen interaction of the 1,5-type was investigated by the ab initio SCF-MO method. Compounds of the type X-S-A=B-Y(Z) =O with cis configuration about A=B, containing X = F, OH, NH2, CH,, and SH; A = CH; B = CH and N; and Z = H, 0, as well as lone pair, were studied. This resulted in four families containing a total of twenty compounds. The geometries of two basic planar conformations of these compounds, s-cis/s-trans (CT) and s-trans/s-trans (TT), obtained by internal rotations about the B-Y and S-C bonds, respectively, were optimized by using three different basis sets. These energy minimum conformations represent optimum geometries with and without S.-0 interactions, respectively. Four quantities were used to measure the extent of S.-0 interactions. These included the lengthening of the Y=O bond on going from TT to CT [AR(Y=O)], the shortening of the S.-O distance [AR(S-.O)], the energy change between TT and CT [ A E ( S -O ) ] , and the Mayer type bond order in the CT conformations [B(S-O)]. The linear interdependence of these four quantities indicated that each of these four parameters was about equally good measure of the extent of S-0 interaction. With the aid of these numerical measures earlier qualitative principles may now be evaluated quantitatively. Depending on the constitution of the individual compounds studied, the covalent character of the S.-O "bond" was found to be IO-30% of a usual single bond when sulfur d orbitals were included in the split-valence basis set. In compounds with X = OH, the O-S-.O moiety can be related to the nonequivalent 0-S-0 hypervalent bond of unsymmetrical sulfuranes.Intramolecular interactions between a pair of nonbonded fragments of a molecule have an important role in governing properties like conformation, spectroscopic behavior, or chemical reactivity. Moreover, after the pioneering work of Biirgi and Dunitz,' it has also been pointed out2 that the relative positions of reactive moieties in the crystalline state are in fact along the direction that one expects from reactants in the early stage of a given reaction. This of course offers an invaluable tool for the study of the initial part of a chemical reaction and yields important pieces of information about the reaction coordinate itself.An extremely interesting case, which is not yet fully understood, is the interaction of nonbonded S and 0 atoms, since these atoms do not escape from a very close contact. In fact a large number of organosulfur compounds are known, where the actual conformation is controlled by intramolecular sulfur-oxygen (S-0) interaction^.^*^ In these molecules the S-0 nonbonded distances are significantly shorter (2.03-3.00 A) than the sum of the corresponding van der Waals radii (3.25 A). The interest in these kinds of compounds is at least twofold. First, the stability of S-0 close contact conformations in c r y~t a l l i n e~~~ and gas4 phase may explain the unusual spectroscopic and physicochemical properties 'Permanent a...
The kinetics of the reaction of R1R2S (R1 = Ra = alkyl or aryl) and ArS0,NCI-Naf were investigated in buffered water-ethanol solution. Analytical methods have been elaborated to follow the reaction which affords R W -SNS0,Ar and R1R2S0. From the reaction scheme proposed. kinetic equations are deduced and supported experimentally. In fast and slow (kd)) equilibrium reactions, respectively, ArSO,NCI-Na+ in water is transformed into ArS0,NHCI and ArSO,NCI, which are reactive electrophiles and which, in their turn, react with R1R2S, the first in a slow (kl') and the second in a relatively fast (k,) step to form reactive intermediates. The rate constants kd', kl ' , and k2 have been determined for different reactants and the observed substituent effects are correlated with the formation of an (RIRBSCI+ArSO,NQ-) ion-pair intermediate (Q = H or CI). Me, S is converted byPh2S by TsNCI, (kl' < k i ) , and MeSPh by both chlorinating agents (kl'k i ) . For the reactions of MeSPh with TsNCI, and TsNHCl k,/kl' is 3.5 x lo6. MeSPh is chlorinated by TsNCI, 1500 times faster than Ph&. For XC6H4SMe, electron-donating X groups increase the rate of reaction with both TsNHCl and TsNCI, (p -4.25 and -3.56, respectively). For YC6H4S02NHCI, electron-withdrawing Y groups produce an increase in reactivity toward MeSPh (p +I -76). From the ion-pair intermediate R1R2SNS0,Ar and R1R2S0 are formed, probably via a sulphurane intermediate, in fast, competitive SN reactions under different steric control. The formation of sulphoxides is more hindered by bulky R groups than that of sulphimides. Other productcontrolling factors are also discussed. MANN AND POPE found that sulphides react with salts alkaline medium, HOCl, ArS0,NCl-as reactants may be of N-chlorosulphonamides (e.g. with chloramine T) to neglected. Several authors have considered ArS0,NHCl yield sulphimides [equation (l)]. In media containing as the effective electr~ph:ile.~J~ Nevertheless, this water, sulphoxides are also produced [equation (2)]. species cannot be the only electrophile reacting with
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