T-Cumyl chlorides

An example of a reaction series in which large deviations are shown by — R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides, ArCMe2Cl54. This reaction follows an SN1 mechanism, with intermediate formation of the cation ArCMe2 +. A —R para-substituent such as OMe may stabilize the activated complex, which resembles the carbocation-chloride ion pair, through delocalization involving structure 21. Such delocalization will clearly be more pronounced than in the species involved in the ionization of p-methoxybenzoic acid, which has a reaction center of feeble + R type (22). The effective a value for p-OMe in the solvolysis of t-cumyl chloride is thus — 0.78, compared with the value of — 0.27 based on the ionization of benzoic acids. 

The pn- and p -values for electrophilic bromine additions to arylolefins are in the same range as those for other reactions via analogous benzylic carbocations. However, generally the comparisons are only qualitative because of significant differences in the experimental conditions and in the mechanisms. For example, as has already been mentioned, the reaction constant of t-cumyl chloride methanolysis is —4.82 (Okamoto et al., 1958), i.e. slightly higher than that for a-methylstyrene bromination in methanol, where the intermediate resembles that in the solvolysis of cumyl derivatives (Scheme 13). 

An example of a reaction series in which large deviations are shown by —R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides. ArCMe2Cl82. This reaction follows an S l mechanism, with intermediate formation of the cation ArCMe2+. A —R para-substituent such as OMe may stabilize the... 

Reactions that occur with the development of an electron deficiency, such as aromatic electrophilic substitutions, are best correlated by substituent constants based on a more appropriate defining reaction than the ionization of benzoic acids. Brown and Okamoto adopted the rates of solvolysis of substituted phenyldimeth-ylcarbinyl chlorides (t-cumyl chlorides) in 90% aqueous acetone at 25°C to define electrophilic substituent constants symbolized. Their procedure was to establish a conventional Hammett plot of log (Ar/A ) against for 16 /wcfa-substituted t-cumyl chlorides, because meta substituents cannot undergo significant direct resonance interaction with the reaction site. The resulting p value of —4.54 was then used in a modified Hammett equation. 

Standard values were estimated by Brown-Okamoto [Brown and Okamoto, 1958a] from the rate constants k of the solvolysis of substituted t-cumyl chlorides (XC6H4C(CH3)2C1 ) in 90 % acetone-water at 25°C, as ... 

Meanwhile, other solvolytic data from the work of Brown and Peters has been interpreted as evidence for the lack of ability of norbornyl groups to stabilize electron deficiency. The rates of solvolysis of a series of p-alkyl-t-cumyl chlorides in 90 % aqueous acetone at 25 °C have been measured, and rate constants and Hammett-Taft a values compared. These results show a small differential electronic effect for the norbornyl substituents which is larger from the exo-direction than from the endo-direction, but also that these groups exhibit only a moderate ability to stabilize positive charge. Studies have been extended to the solvolysis of l-(p-cyclopropyi-phenyl)- and l-(p-isopropylphenyl)-l-arylethyl chlorides, and previous conclusions concerning the lack of a-participation in 2-norbornyl systems have been restated. ... 

For the reactivity parameters Y, n, a+ (but not a) andN+ the lack of curvature is not unexpected. This is because these parameters are defined with respect to the rate of some standard reaction (solvolysis of t-butyl chloride, substitution of methyl iodide, solvolysis of cumyl chlorides, combination reaction of nucleophiles with a standard electrophile). Therefore the resultant plot is of the type log k vs. log k, while the curvature shown in a typical Br nsted plot (Figure 5) results from a plot of log k vs. log K. This curvature is due to a gradual change from a reactant-like transition state, which is insensitive to a perturbation in the reactivity parameter, to a product-like transition state in which equilibrium perturbations are largely reflected in the transition state (and hence the rate). A log k — log k plot is not expected to show this effect and hence is not expected to show curvature. 

The use of the Brown equation as a probe of reaction mechanism is essentially based on the alternative use of substituent parameters tr and a- the better correlation with one of the reference scales, i.e. in this analysis, indicates closer similarity in the mechanism or in the structure of the transition state to that of the reference reaction, solvolysis of a-cumyl chlorides [2]. While the broad applicability of the Brown treatment is widely appreciated, this (T treatment has the inevitable limitations of a single reference parameter relationship. 

Any Sn1-Sn2 mechanistic complication should be absent in the solvolysis of a-t-butylbenzyl tosylates [15], which have a neopentyl-type structure (Tsuji et al., 1990). Indeed, the substituent effect in the solvolysis is accurately described by (2) with an r value of 1.09 which differs from the value r = 1.0 for the a-cumyl chlorides solvolysis. Based on the linearity of the correlation of the substituent effects on the solvolyses of [14] and [15] in 80% aqueous acetone, an Sn1-Sn2 mechanistic duality is also unlikely to be the cause of the exalted r value observed in the solvolysis of [14]. 

Problem 8.20 Give plausible explanation for the following facts. Primary and secondary alkyl halides are generally ineffective as initiators of cationic polymerization of monomers such as isobutene and styrene, but t-butyl and cumyl chlorides are effective. On the other hand, triphenylmethyl chloride and cyclo-heptatrienyl (tropylium) chloride are not very efBcient in polymerizing isobutylene and styrene but produces rapid polymerization of p-methoxystyrene, vinyl ethers and N-vinylcarbazole. 

Azobis (2-methyl-N-phenylpropionamidine) dihydrochloride 2,2 -Azobis (2-methylpropane) 2,2 -Azobis (2-methylpropionamide) dihydrate 2,2 -Azobis [N-(2-propenyl)-2-methylpropionamide] 2,2 -Azobis [2-(3,4,5,6-tetrahydropyrimidin-2-yI) propane] dihydrochloride 2,2 -Azobis (2,4,4-trimethylpentane) n-Butyl-4,4-bis (t-butylperoxy) valerate t-Butyl hydroperoxide t-Butyl peroxycrotonate t-Butyl peroxyneoheptanoate Cerium Cumene hydroperoxide Cumyl peroxyneodecanoate o-Cumylperoxyneoheptanoate 1 -[(1 -Cyano-1 -methylethyl) azo] formamide Decanoyl chloride Decanoyl peroxide Di-t-amyl peroxide 2,2-Di (t-butylperoxy) butane Dicetyl peroxydicarbonate Dicyclohexyl peroxydicarbonate Dimethyl 2,2 -azobis (2-methylpropionate) 2,5-Dimethyl-2,5-di (benzoylperoxy) hexane 2,5-Dimethylhexane-2,5-dihydro peroxide Dimyristyl peroxydicarbonate Di-n-propyl peroxydicarbonate Ethyidibutylperoxybutyrate 3,3,6,6,9,9-Hexamethyl 1,2,4,5-tetraoxa cyclononane Lauroyl chloride Pelargonyl peroxide, 2-Phenylazo-4-methoxy-2,4-dimethylvaleronitrile Phosphine Potassium persulfate Sodium persulfate Succinic acid peroxide... 

Hexamethylenelmlne Methoxy PEG-10 intermediate. Insect repellents Hexahydrophthallc anhydride intermediate. Insecticides Allyl chloride Arsenic trichloride Benzophenone n-Butylamine t-Butylamine Cashew nut shell oil m-Chloroaniline 4-Chloro-3-nitrobenzotrifluoride 4-Chlorophenol Cumyl phenol Cyclopentanone 2,3-Dibromo-1 -propanol Dibutylamine Dicyclopentadiene Dinonyl phenol... 

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