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Effect of Hetero Atom on the Hammett's Reaction Constant () from the Physical Basis of Dissociation Equilibriums of (Dithio) Benzoic Acids and (Thio) Phenols and Its Application To

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Effect of Hetero Atom on the Hammett's Reaction Constant () from the Physical Basis of Dissociation Equilibriums of (Dithio) Benzoic Acids and (Thio) Phenols and Its Application To Hindawi Publishing Corporation Advances in Physical Chemistry Volume 2012, Article ID 598243, 4 pages doi:10.1155/2012/598243 Research Article Effect of Hetero Atom on the Hammett’s Reaction Constant (ρ) from the Physical Basis of Dissociation Equilibriums of (Dithio) Benzoic Acids and (Thio) Phenols and Its Application to Solvolysis Reactions and Some Free Radical Reactions Jagannadham Vandanapu1 and Sanjeev Rachuru2 1 Department of Chemistry, Osmania University, Hyderabad 500 007, India 2 Department of Chemistry, Mizan-Tepi University, Tepi Campus, Tepi, Ethiopia Correspondence should be addressed to Jagannadham Vandanapu, [email protected] Received 3 April 2012; Accepted 11 May 2012 Academic Editor: Leonardo Palmisano Copyright © 2012 J. Vandanapu and S. Rachuru. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The emergence of putative Hammett equation in mid 1930s was a boon to physical-organic chemists to elucidate the reaction mechanisms of several organic reactions. Based on the concept of this equation several hundreds of papers have emerged in chemical literature in the last century on the effect of structure, on reactivity, and very few on thermodynamic stability and kinetic reactivity of intermediates. In this article an attempt is made to explain the effect of hetero atom on Hammett’s reaction constant (ρ) taking the dissociation equilibriums of benzoic acids, dithiobenzoic acids, phenols, and thiophenols. 1. Introduction which we tried to explain in the present work taking the title equilibriums as staple examples. Ever since the Hammett equation was developed [1, 2], there were several hundreds of redox, condensation, dispropor- tionation, nucleophilic and electrophilic substitution, and 2. Results and Discussion addition reactions with meta- and para-substituted benzene derivatives in the literature, for which the Hammett reaction The effect of substituent either in meta- or para-position in (ρ) constants were reported. Inclusion of those references the benzene ring on the rate or equilibrium constant is given here is beyond the scope this article as they run into by Hammett [2] in the form of a formula: several pages. However the readers can find many articles and reviews in several standard physical-organic chemistry A B text books. In addition to these numerous reactions, a ◦ 1 −RT ln K + RT ln K = ΔF = + B2 , (1) few reactions were reported by one of the authors (V. d2 D Jaganndham) from elsewhere [3] and from our laboratory [4, 5] on the solvolysis and reactions of intermediate K carbocations with nucleophilic solvent water. An effect of where is the equilibrium or rate constant of the substituted K ◦ ΔF α-hetero atom substitution on kinetic and thermodynamic reactant, is that of unsubstituted reactant, “ ” is the free d stability of intermediate carbocations were also reported energy change for equilibrium process or rate process, “ ”is from elsewhere [6, 7] and from our laboratory [8]. But in the distance between the substituent and the reaction center, D A B these reactions [3–5] no attempt is made to explain the effect “ ” is the dielectric constant of the medium, and , 1,and B A of α-hetero atom on the Hammett reaction constant (ρ), 2 are the constants. Here depends on the substituent and B1 and B2 depend on the nature of the reaction. Later, based 2 Advances in Physical Chemistry −0.5 − O − ρ = OH O O O O Slope = hammett 1.5 C C C r = 0.9964 4-CN −1 Ka + H+ X − X X 1.5 4-Br 3-Cl 4-Cl Hammett ρ = 1 3-MeO a K p −2 3-Me Scheme 1 H 4-Me S − −2.5 SH S S− S S 4-MeO C C C 4-amino Ka −3 + H+ −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 X X X Hammett σ values Hammett ρ = 1.5 Figure 1: Plot of pKa values versus Hammett sigma values for Scheme 2 dithiobenzoic acid dissociation equilibriums. on some experimental observations Hammett rearranged (1) to the form: XH X− log K = log K ◦ + ρσ, (2) Ka σ =−A/ . R ρ = /d2T B /D B where 2 303 and (1 )( 1 + 2), (2)is + H+ now known as famous Hammett equation. The magnitude of σ depends on the substituent and ρ depends on the nature Y Y of the reaction, medium, and temperature. Now the question is the evaluation of Hammett substituent constant (σ). For X = O and S K K ◦ this the values of , , and Hammett reaction constant Y = H and 4-NO2 (ρ) are needed. K and K ◦ are experimentally determinable = quantities. Therefore the choice of a value of unity for Hammett reaction constant 2.5 ρ when X = S the reaction constant ( ) in the ionization equilibriums of and it is 2.1 when X = O substituted benzoic acids in water solution at 25◦Cwas determined by the large amount of accurate data available Scheme 3 from the work of Dippy and his coworkers [9–11]. With the core of σ values thus obtained, the Hammett reaction constants (ρ) were obtained for several other reactions. Thus in turn using the Hammett reaction constant (ρ), the H O H unknown or accurately not known Hammett substituent XX X + H C constants (ρ) were determined for other substituents. In conclusion it is understood from the Hammett’s work on ksolv k effect of substituents on reaction equilibriums and rates, that s ρ − the reaction constant ( )isone for the dissociation of benzoic k − + H2O, HX acids in aqueous solution at 25◦C(Scheme 1). X [X ] Y Y When the hetero atom is changed from oxygen to Y sulfur that is for the dissociation of dithiobenzoic acids When X = Cl the Hammett reaction constant (ρ) was found to be 1.5 Hammett’s ρ for solvolysis step is −2.05 (Scheme 2). for water reaction it is 1.17 AplotofpKa [12] versus Hammett substituent constant When X = Br (σ) was excellently linear with a slope of 1.5 and correlation Hammett’s ρ for solvolysis step is −5.49 coefficient of 0.9964 (Figure 1). for water reaction it is 3.74 The reasons for high Hammett reaction constant (ρ)are: The Hammett reaction constant (ρ) for dithiobenzoic Scheme 4 acids dissociation equilibriums is one and half times greater Advances in Physical Chemistry 3 O H H C •− NO2 R NO2 • O O k N k • r S + + RCHOH ++RCHO H Reaction Heterolysis X X RA•− X Nitroxide radical Radical anion 7 −1 −1 2 −1 When X = C and R = H: kr < 10 M s and ks < 10 s 8 −1 −1 3 −1 When X = C and R = CH3: kr = 3.3 × 10 M s and ks = 1.2 × 10 s 8 −1 −1 3 −1 When X = N and R = H: kr = 1 × 10 M s and ks = 3.2 × 10 s 9 −1 −1 5 −1 When X = N and R = CH3: kr > 2.4 × 10 M s and ks = 2.6 × 10 s Scheme 5 than that of benzoic acids dissociation equilibriums. The gem-adducts and on the reaction of the cations (ks)with magnitude of ρ depends on several factors like stability of nucleophilic water. The Hammett reaction constant (ρsolv) transition state. Since the ρ value of dithiobenzoic acid is for ksolv step in the solvolysis of gem-dichlorides is −2.05 greater than that of benzoic acid dissociation equilibrium [4]. The same (ρs) for addition of water to the cation is 1.17 series, an implicit conclusion is that the transition state is [4], while in gem-dibromide reactions ρsolv is −5.49 and ρs far more stable than the transition state of benzoic acid dis- is 3.74 [5]. Therefore it is very clear that there was a three- sociation equilibrium series. This is tacitly comprehensible time increase in Hammett’s ρ value in the formation of the from the ease with which sulfur can involve its lone pair of cations when we move the hetero atom from chlorine to electrons in resonance than the ease with which oxygen can bromine and for the reaction of the cation with water the involve its lone pair of electrons (sulfur 3s23p4 and oxygen increase is about three and half times in the Hammett ρ value 2s22p4). Here we are referring to resonance in S=C–S− ↔ S−– (Scheme 4). C=S that is far more pronounced than O=C–O− ↔ −O–C=O These Hammett reaction constants depend largely on the because of the relative ease with which sulfur can donate stabilities of the intermediate α-chloro and α-bromobenzyl its lone pair of electrons than oxygen. In general, sulfur is carbocations, that is, their formation from neutral halide very nucleophilic because of its large size, which makes it ion adducts and their reaction with nucleophile (water). readily polarizable, and its lone pairs of electrons are readily The intrinsic barrier for capture of resonance stabilized accessible. The same observations were made in the study carbocations by nucleophiles results largely from loss of of kinetic and thermodynamic stability of α-oxygen- and α- resonance interactions in the transition state by bond sulfur-stabilized carbocations in solution [6, 7]. formation to the nucleophile. The lower intrinsic barrier Similarly the Hammett reaction constant (ρ) for thiophe- for formation and larger intrinsic barrier for capture of nol dissociation equilibriums was computed from the pKa α-bromobenzyl carbocations by solvent water (than of α- [12] values of thiophenol and 4-nitro thiophenol dissocia- chloro stabilized benzyl carbocations) was consistent with tion equilibriums and it came out to be 2.5 (Scheme 3).
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