The Dissociation Constants of p-Alkylmercaptobenzoic Acids

Baliah, V.; Shanmuganathan, Sp.; Varadachari, R.

doi:10.1021/j150553a044

Cited by 13 publications

(8 citation statements)

References 10 publications

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“…A zero value of β has been found for the addition of thiol anions to methylcobaloxime where the “d” orbital participation was suggested 69. The reversal order of dissociation constants for the mercaptobenzoic acids in the series 4‐CH 3 SC 6 H 4 COOH (p K a = 5.80), 4‐C 2 H 5 SC 6 H 4 COOH (p K a = 5.71), and 4‐ i C 3 H 7 SC 6 H 4 COOH (p K a = 5.66) has been attributed to the hyperconjugation effect involving the “d” orbitals of sulfur 70. Similar effect can be expected to operate in the present reactions wherein the sulfur atom accommodates the electron density from the attacking nucleophile in its vacant “d” orbitals, which subsequently lowers the basicity of PhS − compared to PhO − making the former a better nucleofuge from T ± than the latter.…”

Section: Resultsmentioning

confidence: 99%

Enhanced reactivity in the ammonolysis of phenyl thiolacetates in aqueous medium

Rajarathnam

Babu

Nadar

2001

Int J of Chemical Kinetics

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The ammonolysis of several substituted phenyl thiolacetates is kinetically studied in aqueous medium, 18 • C, ionic strength 0.1 M (KCl). By following the leaving groups spectrophotometrically (λ max = 260-410 nm), under excess free ammonia, pseudo-first-order rate coefficients (k obs ) are obtained. The plots of (k obs − k H ) against free ammonia concentration are linear at constant pH. The macroscopic nucleophilic substitution rate coefficients (k N ) are obtained as the slopes of these plots and found to be pH-independent for all the thiolesters. The Brönsted-type plot (log k N against pK a of leaving groups) and the Hammett plot (log k N against σ values of substituents) obtained for the title reactions of thiolesters are linear with slope values of β lg = −0.34 and ρ = 0.74 respectively. From the magnitude of these values, experimental data, the kinetic law, and the analysis of products, it is deduced that the ammonolysis of thiolesters proceeds through a simple bimolecular nucleophilic substitution pathway with a zwitterionic tetrahedral addition intermediate (T ± ), whereby its formation is rate-determining (k 1 step). Comparison of this reaction of thiolesters with a similar reaction of analogue oxyesters shows a mechanistic difference. Further, for thiolesters there is a rate enhancement with larger k N values. The change in mechanism and enhanced reactivity observed by substitution of the oxygen atom by sulphur atom on the phenyl moiety are discussed in detail.

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Section: Resultsmentioning

confidence: 99%

Enhanced reactivity in the ammonolysis of phenyl thiolacetates in aqueous medium

Rajarathnam

Babu

Nadar

2001

Int J of Chemical Kinetics

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“…Infra-red 5 9 23 and ultra-violet 24 spectra of some saturated and Unsaturated sulphones indicate that the 51-0 bond in sulphoxides and sulphones is a semi-polar bond. Szmant and his co-workersY2~ from the cryoscopic behaviour of a number of sulphones in sulphuric acid, proposed that the electronic structures of the SO2 group range from those in which both sulphur-oxygen bonds are doubly covaleat to those in which both bonds are semipolar (structures I and 111).…”

Section: Discussionmentioning

confidence: 98%

Kinetics of bromine addition to unsaturated sulphones. Part 2.—Effect of conjugation

Shanmuganathan¹,

Subramanian²

1967

Trans. Faraday Soc.

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The rates of bromine addition to several gypand P,y-unsaturated sulphones in glacial acetic acid at 30" follow second-order kinetics. P,y-unsaturated sulphones add bromine at a slightly faster rate than the corresponding a$-unsaturated sulphones, indicating that the electrophilic mechanism of addition is operating. The absence of significant decrease in the rate of bromine addition to an gyp-, as compared to the corresponding fl,y-unsaturated sulphone, indicates the absence of significant conjugation of the sulphonyl group with the ethylenic link in the ground state of or,&unsaturated sulphones.

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“…The driving force would be the stabilization of the carbanion XX by sulfur: c6h6schch2r -> c8h5s=chch2r XX The fact that metalation of methyl phenyl sulfide occurs in the methyl group (87) and not in the ring, as in the case of anisóle, has been explained (15) by postulating contribution of hyperconjugated structures such as XXIa, which lead to activation^o f the hydrogen atoms.…”

Section: VImentioning

confidence: 99%

“…(X = 0, S) ¿OOH Indeed, if the inductive effect were the dominant factor, the reverse order should have been observed. However, structures such as H+ appear to be unimportant because no condensation takes place when phenylthioglycolic acid is replaced by alkylthioglycolic acids (16). Further, analogous structures have been invoked again ( 14) to account for the fact that in alkylmercaptobenzoic acids the pKa decreases when R changes from RS^^>COOH methyl to ethyl to isopropyl, that is, with the decreasing number of hyperconjugated structures.…”

Section: S=¿-hmentioning

confidence: 99%