Hammett Reaction Constant Research Articles

Changes in Mechanism and Transition State Structure for Solvolysis Reactions of Ring Substituted Benzyl Chlorides in Aqueous Solution.

Rate and product data are reported for the solvolysis reactions of twenty-seven mono, di (3,4) and tri (3,4,5) ring-substituted benzyl chlorides. The first order rate constant for solvolysis in 20% acetonitrile in water decrease from k solv = 2.2 s-1 for 4-methoxybenzyl chloride to 1.1 x 10-8 s-1 for 3,4-dinitrobenzyl chloride. The product rate constant ratios k MeOH/k TFE for solvolysis in 70/27/3 (v/v/v) HOH/TFE/MeOH range from a minimum of k MeOH/k TFE = 8 to a maximum of 110. The rate data were fit to a four-parameter Hammett equation that separates the resonance and polar effects of the aromatic ring substituents on the reaction rate. Increases in the values of the Hammett reaction constants and are observed as the substituent constants or are increased. A sharp decrease in the product selectivity k MeOH/k TFE = 26 for stepwise solvolysis of 4-methoxybenzyl chloride is observed as electron-withdrawing meta-substituents are added to 4-methoxybenzyl ring due to a Hammond-effect on the position of the transition state for solvent addition to the substituted 4-methoxybenzyl carbocation reaction intermediates. Sharp increases in the selectivity k MeOH/k TFE are observed with decreasing reactivity of other 3,4,5-subsituted benzyl chlorides due to anti-Hammond shifts on a two-dimensional More-O'Ferrall reaction coordinate diagram in the position of the transition state for a concerted solvolysis reaction.

Taming of the DIBAL Promoted Debenzylation of α-Cyclodextrin. Kinetics, Substituent Effects and Efficient Synthesis of Lings Tetrol.

The kinetics of the reaction of perbenzyl α‐cyclodextrin was studied varying the concentration of DIBAL and substrate, and the temperature. The initial debenzylation was found to be 1st order in substrate and follow the relationship 0.0675+0.179[DIBAL]2 with respect to the concentration of DIBAL. The second and the third debenzylation which led to the 3A,6A,6D‐triol (Lings triol) were both found to be 1st order in substrate concentration and zero order in DIBAL concentration. Longer reaction times with DIBAL in high concentration gave further debenzylation to comparatively complex mixtures containing the 2B,3A,6A,6D‐tetrol and the 3A,6A,6C,6D‐tetrol. In contrast reaction at 0.1 M DIBAL gave the symmetrical 3A,6A,3D,6D‐tetrol (Lings tetrol) in 60 % yield. The effect of chlorine or methyl substitution of the phenyl groups of perbenzyl α‐cyclodextrin was also investigated. Per 4‐chlorobenzyl slowed down the reaction with DIBAL, while 4‐methylbenzyl increased the reaction rate, but still gave the corresponding 6 A‐monool or 6A,6D‐diol products. A Hammett reaction constant of −4.9 was found for the first debenzylation showing a high degree of positive charge in the transition state. The per(2,4‐dichlorobenzyl)‐α‐cyclodextrin‐derivative was completely resistant to DIBAL, however upon addition of trimethyl aluminium this derivative also reacted to give the 6A,6D‐diol product.

Comparison of Substituent Effects in Benzenes (XC<SUB>5</SUB>H<SUB>5</SUB>C), Pyridines (XC<SUB>5</SUB>H<SUB>4</SUB>N) and Phosphorines (XC<SUB>5</SUB>H<SUB>4</SUB>P) and their Protonated Species

Collection of interesting and stimulative data led us to construct Hammett plots for different properties like proton affinities, gas phase basicities, solvation free energies of free and protonated benzenes (I), pyridines (II) and phosphorines (III), and for pKa values of protonated pyridines and phosphorines. Trends in Hammett reaction constants () for all these processes were discussed.

Experimental and theoretical approaches to the study of the reactivity and mechanism of the substitution of phenyl 1‐(2,4‐dinitronaphthyl) ether with anilines derivatives

AbstractReactions of aniline derivatives in dimethyl sulfoxide with phenyl 1‐(2,4‐dinitronaphthyl) ether yield aryl 1‐(2,4‐dinitronaphthyl) amine, which results in substitution of the phenoxy groups at the naphthyl ipso carbon atom. Rate constants were measured spectrophotometrically, and reaction proton transfer was rate limiting. The values of the rate coefficients indicate a rate‐limiting proton transfer mechanism with significant substituent effects. The calculated activation parameters were of regular variation with substituents in 4‐ and 3‐position in the aniline nucleophile, and the reaction proceeded through a common mechanism. Hammett's reaction constant showed that the reaction rate constants depend on the electron density of the nitrogen atom of aniline derivative, whereas the coefficient value obtained from the Brönsted relation indicated that the reaction was significantly associative and quite zwitterion like. Computational studies of the substitution were carried out based on density functional theory, and theoretical to the experimental agreement was achieved.

Can Non-bonded Pair of Electrons of Sp3 Nitrogen with Two Single σ-Bonds on Either Side Still Transmit Substituent Electronic Effects to the Reaction Site? Reversal of Attenuation Effect by Sp3 Nitrogen–A Chemical Education Perspective

Dependence of reactivity of organic molecules on substituents was a well-established phenomenon in terms of Hammett and Taft equations in physical-organic chemistry. The well-known Hammett and Taft linear free energy relationships were extensively used in elucidating the organic reaction mechanisms. The exponential depletion of Hammett reaction constant (ρ), as a function of distance in terms of increasing the number of sp3 carbon atoms (-CH2-) between the reaction center and the substituent, is understood from our laboratory experiments. But, introduction of sp2 carbon atoms (-CH=CH-) between the reaction center and the substituent enhances the Hammett reaction constant (ρ). In the present work, we have tried and observed the same and even little more effective transmission of substituent effect through sp3 nitrogen (-NH-). However, the presence of a sp3 carbon by the side of sp3 nitrogen (-NH-CH2-) depletes the substituent effect in the usual manner in N-phenyl glycines. Probable explanations were presented in support of our observation. In the present work, the observations were manifested in terms of Taft ρ* values instead of Hammett ρ value as the pKa values of only 4-nitopheylcarbamic acid and carbamic acids and the Taft σ* values of 4-nitrophenyl and H are available from literature.

Synthesis, kinetics, and mechanism of bromophenols by N‐bromophthalimide in aqueous acetic acid

AbstractThe kinetics and mechanism of bromination of phenol and its substituents, viz. 4‐chlorophenol, 4‐bromophenol, 4‐methylphenol, and 4‐methoxyphenol by N‐bromophthalimide (NBP) in the presence of mercuric acetate in the temperature range of 303–318 K in aqueous acetic acid medium have been investigated. The reaction follows first‐order dependence on [NBP] and fractional order dependence of rate on [Phenol]. The activation parameters have been evaluated, and based on the observed kinetic results the probable mechanism has been proposed. Observed kinetic features and Hammett's reaction constant (ρ) suggests that bromination occurs through electrophilic substitution of bromonium ion (Br+) into the aromatic ring in the transition state. Large negative entropy of activation values probably suggests the rigid nature of transition state.

Ionic Liquids as "Masking" Solvents of the Relative Strength of Bases in Proton Transfer Reactions.

Equilibrium constants for the proton transfer reaction between pyridines and trifluoroacetic acid were measured in room-temperature ionic liquids (ILs) of different cation-anion compositions. The experimental equilibrium constants for ion-pair formation were corrected according to the Fuoss equation. The calculated equilibrium constants for the formation of free ions were taken as a quantitative measure of the base strength in IL solutions and compared with the relative constants in water. The effect of IL composition is discussed for a series of fixed IL anions and fixed IL cations. Finally, the sensitivity of the proton transfer reaction to the electronic effects of the substituent groups on the pyridine ring was quantified by applying the Hammett equation. A more marked levelling effect on the base strength was observed in ILs than in water. The Hammett reaction constants ρ were then correlated with solvent parameters according to a multi-parametric analysis, which showed that both specific hydrogen-bond donor/acceptor and non-specific interactions play an important role, with α and permittivity being the main parameters affecting the ability of the IL to differentiate the strength of the base.

Regioselective Carbyne Transfer to Ring-Opening Alkyne Metathesis Initiators Gives Access to Telechelic Polymers.

Regioselective carbyne-transfer reagents derived from (3,3,3-trifluoroprop-1-yn-1-yl)benzene give access to functionalized ring-opening alkyne metathesis polymerization (ROAMP) initiators [R-C6H4C≡Mo(OC(CH3)(CF3)2)3] featuring electron-donating or -withdrawing substituents on the benzylidyne. Kinetic studies and linear free-energy relationships reveal that the initiation step of the ring-opening alkyne metathesis polymerization of 5,6,11,12-tetradehydrobenzo[a,e][8]annulene exhibits a moderate positive Hammett reaction constant (ρ = +0.36). ROAMP catalysts featuring electron-withdrawing benzylidynes not only selectively increase the rate of initiation (ki) over the rate of propagation (kp) but also prevent undesired intra- and intermolecular chain-transfer processes, giving access to linear poly-(o-phenylene ethynylene) with narrow molecular weight distribution. The regioselective carbyne transfer methodology and the detailed mechanistic insight enabled the design of a bifunctional ROAMP-reversible addition-fragmentation chain-transfer (RAFT) initiator complex. ROAMP followed by RAFT polymerization yields hybrid poly-(o-phenylene ethynylene)-block-poly-(methyl acrylate) block copolymers.

Prussian Blue/NaNO2as an Efficient Reagent for the Nitration of Phenols in Aqueous Bisulfate and Acetonitrile Medium: Synthetic and Kinetic Study

The reaction kinetics of Prussian blue (PB)/NaNO2 initiated for the nitration of phenols by in aqueous bisulfate and acetonitrile medium indicated first-order dependence on [phenol], [NaNO2], and [PB]. An increase in [KHSO4] accelerated the rate of nitration under otherwise similar conditions. The rate of nitration was faster in the solvent of higher dielectric constant (D). Observed results were in accordance with Amis and Kirkwood plots [log k′ vs. (1/D) and [(D − 1)/(2D + 1)]. These findings together with the linearity of plots, log k′ versus (vol% of acetonitrile (ACN)) and mole fraction of (nx) ACN, probably indicate the importance of both eloctrostatic and nonelctrostatic forces, solvent–solute interactions during nitration of phenols. Reaction rates accelerated with the introduction of electron-donating groups and retarded with electron-withdrawing groups, which are interpreted by Hammett's theory of linear free energy relationship. Hammett's reaction constant (ρ) is a fairly large negative (ρ < 0) value, indicating attack of an electrophile on the aromatic ring. Furthermore, an increase in temperature decreased the reaction constant (ρ) values. This trend was useful in obtaining isokinetic temperature (β) from Exner's plot of ρ versus 1/T. Observed β value (337.8 K) is above the experimental temperature range (303–323 K), indicating that the enthalpy factors are probably more important in controlling the reaction.

크롬(VI)-피라진 착물을 이용한 알코올류의 산화반응과 메카니즘

Cr(VI)-pyrazine complex (PZCC) was synthesized by the reaction of pyrazine with chromium (VI) trioxide in 6 M HCl. The structure was characterized using IR spectroscopy and inductively coupled plasma (ICP). The oxidation of benzyl alcohol using PZCC in various solvents showed that the reactivity increased with the increase of the dielectric constant, in the order: N,N’-dimethylform- amide > acetone > chloroform > cyclohexene. In the presence of N,N’-dimethylformamide solvent with an acidic catalyst such as sulfuric acid (H2SO4 solution), PZCC oxidized benzyl alcohol (H) and its derivatives (p-OCH3, m-CH3, m-OCH3, m-Cl, m-NO2). Electron-donating substituents accelerated the reaction rate, whereas electron acceptor groups retarded the reaction rate. Hammett reaction constant (ρ) was -0.70 (308 K). The observed experimental data were used to rationalize the hydride ion transfer in the rate-determining step.

English

The kinetics of oxidation of some substituted N,a-diphenylnitrones by N-chlorosuccinimide (NCS) to yield benzaldehyde and nitrosobenzene was studied. The reaction follows first order kinetics with respect to (NCS) and fractional order with respect to (nitrone). The order with respect to alkali was inverse fractional order. The reaction exhibited both negative and positive Hammett reaction constants depending upon the nature of substituents. A plot of rate constant (log k) and s gave a concave downward curve. The electron releasing substituents fall on one side of curve and electron withdrawing substituents fall on another side. The activation parameters were calculated. A probable mechanism consistent with experimental observations has been proposed.

Cr(VI)-6-Methylquinoline을 이용한 치환 벤질 알코올류의 산화반응과 속도론에 관한 연구

MQCC (Cr(VI)-6-methylquinoline) complex was synthesized by the reaction of 6-methylquinoline with chromium(VI) trioxide in 6 M HCl. The structure was characterized using IR (Infrared Spectroscopy) and ICP (Inductively Coupled Plasma) analysis. The oxidation of benzyl alcohol using 6-MQCC in various solvents showed that the reactivity increased with the increase of the dielectric constant, in descending order of DMF > acetone > chloroform > cyclohexene. In the presence of DMF solvent with acidic catalyst such as sulfuric acid (H2SO4 solution), 6-MQCC oxidized benzyl alcohol (H) and its derivatives (p-OCH3, m-CH3, m-OCH3, m-Cl, m-NO2) were effectively oxidized. Electron-donating substituents accelerated the reaction rate, whereas electron acceptor groups retarded the reaction rate. The Hammett reaction constant (ρ) was -0.69 (308 K). The observed ex- perimental data was used to rationalize the fact that the hydride ion transfer occurred at the rate-determining step.

크롬(VI)-헤테로고리 착물(2,4'-비피리디늄 클로로크로메이트)에 의한 치환 벤질 알코올류의 산화반응에서 속도론과 메카니즘

Cr(VI)-heterocyclic complex (2,4’-bipyridinium chlorochromate) was synthesized by the reaction between heterocyclic compound(2,4’-bipyridine) and chromium trioxide, and characterized by IR and ICP analysis. The oxidation of benzyl alcohol using 2,4’-bipyridinium chlorochromate in various solvents showed that the reactivity increased with the increase of the dielectric constant (e), in the order : N,N-dimet-hylformamide (DMF) > acetone > chloroform > cyclohexene. In the presence of DMF solvent with acidic catalyst such as hydrochloric acid (HCl solution), 2,4’-bipyridinium chlorochromate oxidized benzyl alcohol (H) and its derivatives (p-CH3, m-Br, m-NO2). Electron-donating substituents accelerated the reaction rate, whereas electron acceptor groups retarded the reaction rate. The Hammett reaction constant (ρ) was -0.67 (303 K). The observed experimental data have been rationalize the proton transfer occurred followed the formation of a chromate ester in the rate-determining step.

Cr(VI)-헤테로고리 착물(Cr(VI)-Isoquinoline)를 이용한 치환 벤질 알코올류의 산화반응과 속도론에 관한 연구

Cr(VI)-heterocyclic complex[Cr(VI)-isoquinoline] was synthesized by the reaction between of heterocyclic compound(isoquinoline) and chromium trioxide, and characterized by IR and ICP analysis. The oxidation of benzyl alcohol using Cr(VI)-isoquinoline in various solvents showed that the reactivity increased with the increase of the dielectric constant(), in the order : cyclohexene, m-Br, m-). Electron- donating substituents accelerated the reaction, whereas electron acceptor groups retarded the reaction. The Hammett reaction constant() was -0.69(308K). The observed experimental data have been ratiolized. The hydride ion transfer causes the prior formation of a chromate ester in the rate-determining step.

Cr(VI)-헤테로고리 착물[Cr(VI)-2-methylpyrazine]를 이용한 치환 벤질 알코올류의 산화반응과 메카니즘에 관한 연구

Cr(VI)-헤테로고리 착물[Cr(VI)-2-methylpyrazine]를 합성하여, 적외선 분광광도법(IR), 유도결합 플라즈마(ICP)등으로 구조를 확인하였고, 여러 가지 용매 하에서 Cr(VI)-2-methylpyrazine를 이용하여 벤질 알코올의 산화반응을 측정한 결과, 유전상수(<TEX>${\varepsilon}$</TEX>) 값이 큰 용매 순서인 시클로헥센 < 클로로포름 < 아세톤 < N,N-디메틸포름아미드 용매 하에서 높은 산화반응성을 보였다. 산 촉매(HCl)를 이용한 DMF 용매 하에서 Cr(VI)-2-methylpyrazine은 벤질 알코올(H)과 그의 유도체들(p-<TEX>$CH_3$</TEX>, m-Br, m-<TEX>$NO_2$</TEX>)을 효과적으로 산화시켰다. 전자받개 그룹들은 반응 속도가 감소한 반면에 전자주개 치환체들은 반응속도를 증가시켰고, Hammett 반응상수(<TEX>${\rho}$</TEX>) 값은 Cr(VI)-2-methylpyrazine= -0.65(308K) 이였다. 본 실험에서 알코올의 산화반응 과정은 먼저 크로메이트 에스테르 형성과정을 거친 후, 속도결정단계에서 수소화 전이가 일어나는 메카니즘임을 알 수 있었다. Cr(VI)-heterocyclic complex[Cr(VI)-2-methylpyrazine] was synthesized by the reaction between of heterocyclic compound(2-methylpyrazine) and chromium trioxide, and characterized by IR and ICP analysis. The oxidation of benzyl alcohol using Cr(VI)-2-methylpyrazine in various solvents showed that the reactivity increased with the increase of the dielectric constant(<TEX>${\varepsilon}$</TEX>), in the order : cyclohexene<chloroform<acetone <N,N-dimethylformamide(DMF). In the presence of DMF solvent with acidic catalyst such as hydrochloric acid(HCl solution), Cr(VI)-2-methylpyrazine oxidized benzyl alcohol(H) and its derivatives(p- CH3, m-Br, m-NO2). Electron-donating substituents accelerated the reaction, whereas electron acceptor groups retarded the reaction. The Hammett reaction constant(<TEX>${\rho</TEX><TEX>}$</TEX>) was Cr(VI)-2-methylpyrazine= -0.65(308K). The observed experimental data have been ratiolized. The hydride ion transfer causes the prior formation of a chromate ester in the rate-determining step.

크롬(VI)-4-(Dimethylamino)pyridine에 의한 알코올류의 산화반응과 반응속도에 관한 연구

6M-HCl 용매 하에서 헤테로고리 화합물인 4-(dimethylamino)pyridine과 chromium(VI) trioxide의 반응을 통하여 4-(dimethylamino)pyridinium chlorochromate[<TEX>$C_7H_{10}N_2HCrO_3Cl$</TEX>] 착물을 합성하여, 적외선분광광도법(IR), 유도결합 플라즈마(ICP) 등으로 구조를 확인하였다. 여러 가지 용매 하에서 4-(dimethylamino)pyridinium chlorochromate를 이용하여 벤질알코올의 산화반응을 측정한 결과 유전상수(<TEX>${\varepsilon}$</TEX>) 값이 큰 용매 즉, 시클로헥센<클로로포름<아세톤<N,N'-디메틸포름아미드순으로 높은 산화반응성을 보였다. 산 촉매(<TEX>$H_2SO_4$</TEX>)를 이용한 N,N'-디메틸포름아미드 용매 하에서 4-(dimethylamino)pyridinium chlorochromate는 벤질 알코올과 그의 유도체들(p-<TEX>$OCH_3$</TEX>, m-<TEX>$CH_3$</TEX>, H, m-<TEX>$OCH_3$</TEX>, m-Cl, m-<TEX>$NO_2$</TEX>)을 효과적으로 산화시켰다. 그리고 전자받개 그룹들은 반응속도가 감소한 반면에 전자주개 치환체들은 반응속도를 증가시켰다. 또한 Hammett 반응상수(<TEX>${\rho}$</TEX>) 값은 -0.68(303K) 이었다. 그러므로 본 실험에서 알코올의 산화반응 과정은 속도결정단계에서 수소화 전이가 일어나는 메카니즘임을 알 수 있었다. Cr(VI)-4-(dimethylamino)pyridine[4-(dimethylamino)pyridinium chlorochromate] was synthesized by the reaction of 4-(dimethylamino)pyridine with chromium trioxide in 6M-HCl, and characterized by IR, ICP. The oxidation of benzyl alcohol using 4-(dimethylamino)pyridinium chlorochromate in various solvents showed that the reactivity increased with the increase of the dielectric constant(<TEX>${\varepsilon}$</TEX>), in the order: cyclohexene<chloroform<acetone<N,N-dimethylformamide. In the presence of hydrochloric acid (<TEX>$H_2SO_4$</TEX> solution), 4-(dimethylamino)pyridinium chlorochromate oxidized benzyl alcohol and its derivatives(p-<TEX>$OCH_3$</TEX>, m-<TEX>$CH_3$</TEX>, H, m-<TEX>$OCH_3$</TEX>, m-Cl, m-<TEX>$NO_2$</TEX>) smoothly in DMF. Electron-donating substituents accelerated the reaction, whereas electron acceptor groups retarded the reaction. The Hammett reaction constant(<TEX>${\rho}$</TEX>) was -0.68(303K). The observed experimental data was used to rationalize the hydride ion transfer in the rate-determining step.

A Model Reaction Assesses Contribution of H-Tunneling and Coupled Motions to Enzyme Catalysis

To assess the contribution of physical features to enzyme catalysis, the enzymatic reaction has to be compared to a relevant uncatalyzed reaction. While such comparisons have been conducted for some hydrolytic and radical reactions, it is most challenging for biological hydride transfer and redox reactions in general. Here, the same experimental tools used to study the H-tunneling and coupled motions for enzymatic hydride transfer between two carbons were used in the study of an uncatalyzed model reaction. The enzymatic oxidations of benzyl alcohol and its substituted analogues mediated by alcohol dehydrogenases were compared to the oxidations by 9-phenylxanthylium cation (PhXn(+)). The PhXn(+) serves as an NAD(+) model, while the solvent, acetonitrile, models the protein environment. Experimental comparisons included linear free energy relations with Hammett reaction constant (ρ) of zero versus -2.7; temperature-independent versus temperature-dependent primary KIEs; deflated secondary KIEs with deuteride transfer (i.e., primary-secondary coupled motion) versus no coupling between secondary KIEs and H- or D-transfer; and large versus small secondary KIEs for the enzymatic versus uncatalyzed alcohol oxidation. Some of the differences may come from differences in the order of microscopic steps between the catalyzed versus uncatalyzed reactions. However, several of these comparative experiments indicate that in contrast to the uncatalyzed reaction the transition state of the enzymatic reaction is better reorganized for H-tunneling and its H-donor is better rehybridized prior to the C-H→C transfer. These findings suggest an important role for these physical features in enzyme catalysis.

Kinetics and mechanism of oxidation of alcohols

This paper describes the kinetics and mechanism of oxidation of several monohydric alcohols to the corresponding aldehydes and ketones by bis(quinuclidine)bromine(I) bromide in the presence of pyridinium trifluoroacetate. All evidence from the thermodynamic parameters, deuterium kinetic isotope effect, and Hammett reaction constants supports a two-step mechanism and suggests that the transfer of hydride ion from the substrate to the oxidant is the rate-determining step.

2,2'-Bipyridinium Chlorochromate에 의한 치환 벤질 알코올류의 산화반응과 메카니즘에 관한 연구

2,2`-Bipyridinium chlorochromate[] was synthesized by the reaction of 2,2`- bipyridine with chromium(VI) trioxide in 6M HCl. The structure was characterized by IR and ICP analysis. The oxidation of benzyl alcohol using 2,2`-bipyridinium chlorochromate in various solvents showed that the reactivity increased with the increase in the order of the dielectric constant(), in the order: cyclohexene, H, m-Br, ) in N,N`-dimethylformamide. Electron-donating substituents accelerated the reaction, whereas electron acceptor groups retarded the reaction. The Hammett reaction constant() was -0.64(303K). The oxidation reactivity of alcohols can be a useful factor to study about physical properties such as thermal stability, when the polysilsesquioxane solution is ready for an applying coating agent. The observed experimental data was used to rationalized the hydride ion transfer in the rate-determining step.

크롬(VI)-4,4'-Bipyridine 착물에 의한 치환 벤질 알코올류의 산화반응 속도론과 메카니즘

Cr(VI)-4,4'-bipyridine complex(4,4'-bipyridinium dichromate) was synthesized by the reaction of 4,4'-bipyridine with chromium trioxide in H2O, and characterized by IR, ICP. The oxidation of benzyl alcohol using 4,4'-bipyridinium dichromate in various solvents showed that the reactivity increased with the increase of the dielectric constant(e), in the order: cyclohexene<chloroform<acetone<N,N'-dimethylformamide. In the presence of hydrochloric acid(HCl solution), 4,4'-bipyridinium dichromate oxidized substituted benzyl alcohols (p-CH3, H, m-Br, m-NO2) smoothly in N,N'-dimethylformamide. Electron-donating substituents accelerated the reaction, whereas electron acceptor groups retarded the reaction. The Hammett reaction constant(ρ) was -0.63(303K). The observed experimental data have been rationalized as follows; the proton transfer occurs after the prior formation of a chromate ester in the rate determining step.