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

Abstract

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.

Hydrolytic Stabilization of Protected p-Hydroxybenzyl Halides Designed as Latent Quinone Methide Precursors

“…Analysis of Hammett σ values reasonably predicts the effect of functional groups on hydrolytic stability in compounds containing benzylic leaving groups . As the benzyl ring becomes more electron withdrawing, the rate of hydrolysis decreases.…”

Positron annihilation studies in solid substituted aromatic compounds

“…The authors defined new substituent constants (r þ p ) based on the specific rates of solvolysis of cumyl chloride and its ring substituted homologues (k 0 and k, respectively). The so-called Brown-Okamoto constants (r þ p ), in fact, show that these constants take into account only resonance effects and it is possible to confirm which effect (resonance or inductive) it is the most predominant one in the studied system, as in the study of hydrolysis of 2-aryl-2-chloro-4,4-dimethylpentanes [17]. The most remarkable and important contributions are in Medicinal Chemistry [18].…”

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

“…The reaction constant ρ + gives a measure of the sensitivity of the reaction to perturbation by factors such as those pertaining to the substituent and the solvent; k H is the rate constant of the parent (unsubstituted) cumyl chloride ( 3a ). The Hammett–Brown equation proved to be useful in correlating substituents' effects in various substituted benzenes 3–5 irrespective of the substituent position in the ring (meta or para), the type of carbocation formed, or the solvent used 6. In systems where strong deactivating groups are attached to the carbocation intermediate, such as the CF 3 group, strong departures from the Hammett–Brown linear plots take place.…”

“…The Hammett–Brown equation hence takes the following extended form: where Σσ + is the sum of the contributions of all substituents present. Apart from their classical area of application in the structure–reactivity relationship 4, the Hammett–Brown parameters or the Hammett equation on which they are based have recently found applications in various fields such as environmental toxicology, drug metabolism 15, chromatography 16, oxidation studies 17, mechanistic studies 18, mass spectrometry 19 to name but a few. Some earlier studies based on the original Hammett equation found disubstitution effects of various substituted benzoic acids to be additive 20.…”

“…This work therefore undertook to investigate the behavior of the electronegative atom‐bearing halogeno and methoxy substituents in disubstituted cumyl chlorides under the same conditions used by Brown for deriving ρ + σ + values. A fewer studies to the author's knowledge dealt with these effects of disubstitution 4,25 in such systems studied by Brown. The compounds chosen to be investigated were, namely the halogeno 3‐flouro‐4‐methyl‐ ( 3b ), 3‐bromo‐4‐methyl‐ ( 3c ), and 3‐iodo‐4‐methyl‐ ( 3d ) derivatives of cumyl chloride (2‐chloro‐2‐phenylpropane) together with the 3‐methoxy‐4‐methyl‐ ( 3e ) and 3,4‐dimethyl‐ ( 3f ) derivatives.…”