Methylbenzyl cations

Write the principal resonance structures of o methylbenzyl cation and rn methylbenzyl cation Which one has a tertiary carbocation as a contnbuting resonance form ... 

The a-methylbenzyl cation (1) can be approached from the alcohol dehydration direction or the alkene protonation direction, as shown, and both of these processes have been the subject of ab initio molecular orbital calculations. It was found that the alcohol dehydration has a transition state about half way between the two stmctures shown, with the transition state and the carbocation having about the same amount of 7T-orbital overlap. However, the alkene protonation has an earlier transition state with less effective 7r-orbital overlap than that in the cation. This is held to explain the different Yukawa-Tsuno r+ values found for the two processes, 0.7-1.1 for alkene... 

The cumyl cation (4) has been the subject of an X-ray crystallographic study, as its hexafluoroantimonate salt at —124 °C.31 It is nearly planar (8 ° twist), with a short bond between the C+ and the ring (1.41 A), consistent with benzylic delocalization. The Me—C+ bonds are also shortened, indicative of hyperconjugative interaction.31 However, calculations are taken to show that hyperconjugation is not important in isolated benzyl cations e.g. structures such as (6) are not important contributors to the overall structure of (5).32 The stabilization provided by alkyl groups would thus be because of their polarizability, and the Baker-Nathan effect would be due to steric hindrance to solvation.32 The heats of formation of some a-methylbenzyl cations indicate that the primary stabilization in these species comes from the a-substitucnts, and that the stabilization provided by the aromatic ring is secondary.33... 

Only one of the Lewis structures shown is a tertiary carbocation. o-Methylbenzyl cation has tertiary carbocation character m-methylbenzyl cation does not. 

The following should be noted for the sake of completeness. The bromide ion in Figure 2.14 moves far enough away from the a-methylbenzyl cation intermediate that it allows the solvent to react on both sides of the carbenium ion with equal probability. However, the bromide ion does not move away from the carbenium ion to an arbitrary distance. The electrostatic attraction between oppositely charged particles holds the carbenium ion and the bromide ion together at a distance large enough for solvent molecules to fit in between. This is the so-called solvent-separated ion pair. 

These predictions were found to be correct for benzyl and o- and p-methylbenzyl cations, generated by solvolysis of the corresponding tosylates in bicarbonate buffered acetonitrile/water mixtures (Cedheim and Eberson, 1969). With 3% (w/w) water present, essentially all of the product was the alcohol (Fig. 5) from these data it can be estimated that water is 40-50 times more reactive as a nucleophile than acetonitrile towards cations. 

On the other hand, when in an SN1 reaction the nucleophile attacks the carbenium ion after it has separated from the leaving group, the reaction takes place with complete racemization. This is the case with more stable and consequently longer-lived carbenium ions. For example, the a-methylbenzyl cation, which is produced in the ratedetermining step of the solvolysis of R-phenethyl bromide in a water/ethanol mixture is such a cation (Figure 2.14). 

The parent spirocyclopropylbenzenium ion 102 can be prepared by the ionization of -phenylethyl chloride in HF-SbF5-S02ClF at -90 °C followed by warming to -60 °C. At higher temperatures (-27 to -5 C), the ion isomerizes to the a-methylbenzyl cation 103 with an activation energy of 13 kcalmol as shown by the NMR kinetic study (equation 61). The rearrangement probably proceeds through a partially delocalized primary 2-phenylethyl cation 104 ". The NMR spectrum of the ethylenebenzenium ion 102 showed (5 C 68.8 (Cl), 171.8 (C2, C6), 133.4 (C3, C5), 155.4 (C4), 60.7 (CH2). The equivalence of the C3, C5 and C2, C6 carbons as well as the methylene carbons show that the ion has Qv symmetry. The deshielded absorptions for the CH2 carbons are similar to those in other cyclopropylcarbinyl cations. 

The results of analysis based on the gas-phase Y-T equation (27) using gas-phase substituent constants given in Table 15 are summarized for proton-transfer equilibria of ethylenes and acetylenes in Table 16. As an example, the Y-T plot for a-CFa-a-arylethyl cations [5C ] in Fig. 25 may be compared with the plot for the corresponding solvolysis in Fig. 3. The AAG(cc)h for the a-neopentyl-a-r-butyl series [4C" ] has been found to correlate linearly with the gas-phase Y-T equation (27) with ro = 0.81 (Fujio et al., 1997a). a-f-Butyl-a-methylbenzyl cations [18C ] also showed a reduced ro value of 0.89 (Mishima et al., 1992b), a value which is close to r = 0.91... 

We present in Fig. 35 the structure (C symmetry) optimized at the MP2(full)/6-31G(d) level (this work). Recently, a large computational study has been published on benzyl and larger carbocations. It uses both the optimized force field MMP2 extended to carbocations and MP4sdq/6-31G(d)//MP2(full)/6-31G(d) calculations. It provides, inter alia, valuable estimates of standard heats of formation. In this case, the difference in stability between phenethyl and 4-methylbenzyl cations (1.7 by ab initio computations and 2.9 kcal mol by MMP2) is in good agreement with the experimental results in Ref. 51. Our own G2(MP2) results are presented in Table 9. 

Hexamethylbenzene conversions have a complicated character in such media since heating it in cone. H SO results, in the formation of the 2,3,4,5,6-penta-methylbenzyl cation (see also... 

The bridged form of the )3-phenylethyl cation can be observed in superacid media and can be characterized by carbon-13 and proton NMR spectroscopy. The bridged ion subsequently rearranges to the more stable a-methylbenzyl cation, with Ea for rearrangement being 13 kcal/mol. With more substituted cations, the rearrangement to benzyl cations occurs too rapidly for the bridged ion to be observed. ... 

---