Benzylic carbocation

This carbocation does not receive the extra increment of stabilization that its benzylic isomer does and so is formed more slowly The regioselectivity of addition is controlled by the rate of carbocation formation the more stable benzylic carbocation is formed faster and is the one that determines the reaction product... 

Section 11 10 Chemical reactions of arenes can take place on the ring itself or on a side chain Reactions that take place on the side chain are strongly influ enced by the stability of benzylic radicals and benzylic carbocations... 

Section 11 14 Benzylic carbocations are intermediates in SnI reactions of benzylic halides and are stabilized by electron delocalization... 

Electrophilic addition (Section 11 16) An aryl group stabilizes a benzylic carbocation and con trols the regioselectivity of addition to a double bond involving the benzylic carbon Markovni kov s rule is obeyed... 

Benzylic carbon (Section 11 10) A carbon directly attached to a benzene nng A hydrogen attached to a benzylic carbon is a benzylic hydrogen A carbocation in which the benzylic carbon is positively charged is a benzylic carbocation A free radical in which the benzylic carbon bears the unpaired electron is a benzylic radical Benzyne (Section 23 8) The compound... 

Scheme 10. Mechanislic possibililies for PF condensalion. Mechanism a involves an SN2-like attack of a phenolic ring on a methylol. This attack would be face-on. Such a mechanism is necessarily second-order. Mechanism b involves formation of a quinone methide intermediate and should be Hrst-order. The quinone methide should react with any nucleophile and should show ethers through both the phenolic and hydroxymethyl oxygens. Reaction c would not be likely in an alkaline solution and is probably illustrative of the mechanism for novolac condensation. The slow step should be formation of the benzyl carbocation. Therefore, this should be a first-order reaction also. Though carbocation formation responds to proton concentration, the effects of acidity will not usually be seen in the reaction kinetics in a given experiment because proton concentration will not vary.

Figure 11.12 Resonance forms of the allyl and benzyl carbocations. Electrostatic potential maps show that the positive charge (blue) is delocalized over the ir system in both. Electron-poor atoms are indicated by blue arrows.

Because of resonance stabilization, a primary allylic or benzylic carbocation is about as stable as a secondary alkyl carbocation and a secondary allylic or benzylic carbocation is about as stable as a tertiary alkyl carbocation. This stability order of carbocations is the same as the order of S l reactivity for alkyl halides and tosylates. 

Benzylic acid rearrangement, 836 Benzylic carbocation, electrostatic potential map of, 377 resonance in, 377 SN1 reaction and, 376-377... 

Electrostatic potential map—cont d ammonia, 145 aniline, 925 anilinium ion, 925 anisole, 111 18]annulene, 535 arene, 74 azulene, 541 benzaldehyde, 565, 704 benzene, 44, 521, 565 benzenediazonium ion, 945 benzoquinone, 631 benzyl carbocation, 377 benzyne, 576 borane, 223... 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

Chain transfer by the HS CHa CF CI CI Cl would be energetically unfavorable as it would involve Cle transfer from the initiator to the propagating cation, i.e., the creation of a relatively less stable initial benzyl carbocation by sacrificing a more stable propagating tertiary benzyl cation58. ... 

When the bromination of the unsubstituted P-methy 1-styrenes (ref. 19) is carried out in methylene chloride, the two diastereoisomeric dibromides are obtained in ratios of 72 threo/28 erythro and 20 threo/80 erythro for the cis and trans isomers, respectively. This result agrees fairly well with a partially bridged intermediate, since the corresponding benzylic carbocation leads to a 65 erythro/35 threo ratio (ref. 20). When the same reactions are carried out in methanol, the... 

It was previously observed that with a catalytic amount of FeCls, benzylic alcohols were rapidly converted to dimeric ethers by eliminating water (Scheme 14). In the presence of an alkyne this ether is polarized by FeCls and generates an incipient benzylic carbocation. The nucleophilic attack of the alkyne moiety onto the resulting benzyl carbocation generated a stable alkenyl cation, which suffer the nucleophilic attack of water (generated in the process and/or from the hydrated... 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

Entry 6 involves formation of a stabilized benzylic carbocation and results in a very efficient closure of a six-membered ring. Entry 7 involves an activated ring. The reaction was done using enantiomerically pure alcohol, but, as expected for a carbocation intermediate, the product was nearly racemic (6% e.e.). This cyclization was done enantiospecifically by first forming the Cr(CO)3 complex (see Section 8.5). 

Fig. 12.7a,e) it is important to realize that a protonated quinone methide QM1H + is actually a benzylic carbocation (Fig. 12.7a). Water will also add to the quinone methide under fairly neutral conditions.86-88 The isomer distribution of the resulting compounds, PI can be determined directly from or 13C (or 2D 13C/1H correlation)... 

The results indicate that in homogenous medium it is not possible to reach high guaiacol conversion. Under the conditions which are necessary to activate formaldehyde, the condensation reaction between vanillols or between p-vanillol and guaiacol is very quick. This is likely due to the high stability of the benzyl carbocation which forms by protonation of the hydroxymethyl group on p-vanillol [8],... 

For the alternant PAH that have been studied extensively, bay-region diol epoxides are important metabolically activated forms. Studies of the chemical and biological activity of a variety of diol epoxides have provided insight into the factors related to reactivity and biological activity. Chemical reactivity, as measured by spontaneous hydrolysis, correlated well with calculated quantum chemical parameters that estimate ir-electron stabilization upon conversion of the epoxide to a benzylic carbocation, provided... 

The determinations of absolute rate constants with values up to ks = 1010 s-1 for the reaction of carbocations with water and other nucleophilic solvents using either the direct method of laser flash photolysis1 or the indirect azide ion clock method.8 These values of ks (s ) have been combined with rate constants for carbocation formation in the microscopic reverse direction to give values of KR (m) for the equilibrium addition of water to a wide range of benzylic carbocations.9 13... 

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