Rates of Solvolysis of the Nitro- and the Methoxyphenyldimethylcarbinyl Chlorides<sup>1-3</sup>

Okamoto, Y.; Brown, Herbert C.

doi:10.1021/ja01565a038

Cited by 26 publications

(6 citation statements)

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“…40%. h Solvolysis of o -MeO and o -Me cumyl chlorides, ref ; steric effects are expected to be greater here than for α-phenylethyl chlorides; the brackets indicate that the value is subject to uncertainty.…”

Section: Resultsmentioning

confidence: 99%

Unexpectedly Small Ortho-Oxygen Substituent Effects on Stabilities of Benzylic Carbocations

et al. 2004

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Equilibrium constants are reported for the ionization of benzylic alcohols to carbocations stabilized by cyclic or acyclic o-alkyl or o-oxygen substituents. The measurements were stimulated by the observation of small or inverse effects of replacement of an o-CH2 group by O or S in the cyclopentyl ring of indanol (kO/kCH2 = 1.2) or in the cyclohexyl ring of tetralol (kO/kCH2 = 0.6, kS/kCH2 = 0.3) on rates of carbocation formation. Values of pKR (KR = [ROH][H+]/[R+]) have been obtained by combining rate constants, kH, for the acid-catalyzed ionization of the alcohols with kH2O for attack of water on the carbocation measured by the azide clock method. For carbocations derived from the following alcohols, values of pKR are as indicated: 1-indanol, -11.7; 2,3-dihydro-3-hydroxybenzofuran (benzofuran hydrate), -9.3; 1-tetralol, -12.2; 4-chromanol, -12.0; 4-thiochromanol, -12.3; o-methyl-1-phenylethanol, -13.8; o-methoxy-1-phenylethanol, -11.7. The measurements show that, in contrast to its small kinetic effect, the equilibrium effect of replacing the o-CH2 group by O in the cyclopentyl ring of indanol is 250-fold, whereas the effect of the same replacement in the cyclohexyl ring of tetralol is only 1.6. It is concluded (a) that the efficiency of conjugation of annular o-oxygen substituents to a benzylic carbocation center is sensitive to conformational restrictions arising from ring strain and (b) that, in the case of indanol, the kinetic effect of the same oxygen atom is subject to an imbalance of favorable resonance and unfavorable inductive effects at the transition state.

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Section: Resultsmentioning

confidence: 99%

Unexpectedly Small Ortho-Oxygen Substituent Effects on Stabilities of Benzylic Carbocations

et al. 2004

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“…Built‐in dipole interactions could also, perhaps, contribute to this enhanced reactivity. However, none of the results obtained by direct measurement of the reactivity of the position meta to the methoxy group indicated larger activation than expected 37–39. As it seems, the additivity principle perhaps fails to accommodate adequately the variation in π‐complex character of the transition state resulting from substituent–substituent interactions, leading to larger rate values than predicted.…”

Section: Resultsmentioning

confidence: 84%

Solvolysis of 2‐chloro‐2(3,4‐disubstituted) phenylpropanes: Validity of Hammett–Brown σ⁺ constants in assessing additive effects of substituents

Taha

2012

Int J of Chemical Kinetics

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The objective of this study is to test the suitability of the extended Hammett-Brown equation, log (k XX /k HH ) = ρ + σ + , in depicting satisfactorily additive effects of electronegative atom-bearing substituents, which are known to possess diverse and multicomponent influences on the side chain reactions of polysubstituted benzenes. The equation has been used to correlate, for the first time, the additive effect of substituents in the specific rates of solvolysis of 2-chloro-2-phenylpropanes (3b-3f) having 3-F,4-Me, 3-Br,4-Me, 3-I,4-Me, 3-Me,4-Me, or 3-MeO,4-Me substituents. The rates were determined titrimetrically at 288, 298, and 308 K using 90% aqueous acetone as solvent. Measured additive effects of these substituents on the solvolysis rate and activation parameters of the parent cumyl chloride (2-chloro-2phenylpropane) are found to be well correlated using the equation given above. Plots of log (k XX /k HH ) of 3b-3f together with mainly di-, but also tri-and mono-substituted cumyl chlorides from previous studies against σ + give a linear correlation coefficient of 0.990 as a measure of the validity of the equation to depict such systems. The halogen substituents' extent of conformity with additivity reflected in their relative (k obsd /k calcd ) rate ratios is found to correlate with the steric size of substituents. Plots of rate ratios against Taft's steric factor of each halogen give a linear correlation coefficient of 0.994 for the 3-halo substituents. The 3,4-dimethyl substituents' relative rate ratio of 1.03 shows excellent additivity, whereas the

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“…Solvolysis solvents were prepared as previously described . 2-Aryl-2-chloropropanes, 1h ,13a 1j , and 1l , were prepared following the reported procedures. The 13 C NMR and boiling point data for 2-aryl-4,4-dimethyl-2-pentanols ( 3-OH ) are summarized in Supporting Information.…”

Section: Methodsmentioning

confidence: 99%

Solvolysis of 2-Aryl-2-chloro-4,4-dimethylpentanes. Confirmation of Validity of Brown−Okamoto σ⁺Constants

et al. 1999

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Specific rate constants of solvolysis in 90% aqueous acetone have been determined at 25 °C for 10 2-aryl-2-chloro-4,4-dimethylpentanes (3) having a substituent p-cycPr, p-Me, m-Me, p-F, (H), p-Cl, m-Cl, m-CF3, p-CF3, or p-NO2. The Hammett−Brown relation is satisfactorily linear (r = 0.999) with ρ+ of −4.51, showing that Brown−Okamoto σ+ values based on the solvolysis of the cumyl chloride system 1 are widely usable for the substrates solvolyzing without appreciable nucleophilic solvent participation. Solvolysis of 3 having 3,5-Cl2 or 3,5-(CF3)2 substituents suggests the use of σ+ values of +0.740 and +0.982 instead of respective 2σm + values of +0.798 and +1.040. The large negative ρ+ values in TFE of −5.91 and −5.63 for 1 and 3, respectively, reflect decreased nucleophilic (or Brønsted base type) solvation. Comparison of solvolysis rates of 1 with 3, both having m-Cl, p-CF3, or 3,5-(CF3)2 substituents, in aqueous ethanol, aqueous acetone, TFE, and TFE−EtOH suggests that nucleophilic intervention by solvent in the cumyl system increases as the intermediate carbocation becomes more electron-demanding.

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Rates of Solvolysis of the Nitro- and the Methoxyphenyldimethylcarbinyl Chlorides^1-3

Cited by 26 publications

References 0 publications

Unexpectedly Small Ortho-Oxygen Substituent Effects on Stabilities of Benzylic Carbocations

Unexpectedly Small Ortho-Oxygen Substituent Effects on Stabilities of Benzylic Carbocations

Solvolysis of 2‐chloro‐2(3,4‐disubstituted) phenylpropanes: Validity of Hammett–Brown σ⁺ constants in assessing additive effects of substituents

Solvolysis of 2-Aryl-2-chloro-4,4-dimethylpentanes. Confirmation of Validity of Brown−Okamoto σ⁺Constants

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Rates of Solvolysis of the Nitro- and the Methoxyphenyldimethylcarbinyl Chlorides1-3

Cited by 26 publications

References 0 publications

Unexpectedly Small Ortho-Oxygen Substituent Effects on Stabilities of Benzylic Carbocations

Unexpectedly Small Ortho-Oxygen Substituent Effects on Stabilities of Benzylic Carbocations

Solvolysis of 2‐chloro‐2(3,4‐disubstituted) phenylpropanes: Validity of Hammett–Brown σ+ constants in assessing additive effects of substituents

Solvolysis of 2-Aryl-2-chloro-4,4-dimethylpentanes. Confirmation of Validity of Brown−Okamoto σ+Constants

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Rates of Solvolysis of the Nitro- and the Methoxyphenyldimethylcarbinyl Chlorides^1-3

Solvolysis of 2‐chloro‐2(3,4‐disubstituted) phenylpropanes: Validity of Hammett–Brown σ⁺ constants in assessing additive effects of substituents

Solvolysis of 2-Aryl-2-chloro-4,4-dimethylpentanes. Confirmation of Validity of Brown−Okamoto σ⁺Constants