Resonance stabilization energies benzyl

A corresponding correlation is obtained for the rate constants of a,a -phenyl substituted alkanes 26 (R1 = C6H5, R2 = H, R3 = alkyl) (see Fig. 1 )41). It has, however, a different slope and a different axis intercept. When both correlations are extrapolated to ESp = 0, a difference of about 16 kcal/mol in AG is found. This value is not unexpected because in the decomposition of a,a -phenyl substituted ethanes (Table 5, no. 22—27) resonance stabilized secondary benzyl radicals are formed. From Fig. 1 therefore a resonance energy of about 8 kcal/mol for a secondary benzyl radical is deduced. This is of the expected order of magnitude54. ... 

These resonance stabilization energies of free radicals can be quite large, e.g. 50 kj mol 1 for benzyl-, and 70kjmol 1 for methyl-Np. These must be included in the overall energy balance of the reaction, and can make all the difference between a fast, highly exergonic process, and an endergonic process which in practice does no take place at all. 

As mentioned with benzyl groups, an allylic center is also quite susceptible to autoxidation chemistry (Fig. 109). The allylic hydrogen has a weak C-H bond dissociation energy due to the resonance stabilization energy of the resulting allylic radical (157). 

Allyl and benzyl radical are substantially stabilized, as anticipated from the resonance structures (see Section 1.3.6). Comparing the BDEs of propene and toluene to an appropriate reference such as ethane suggests resonance stabilization energies of 12.4 and 14.1 kcal / mol, respectively. An alternative way to estimate allyl stabilization is to consider allyl rotation barriers (Eq. 2.12). Rotating a terminal CH2 90° out-of-plane completely destroys allyl resonance, and so the transition state for rotation is a good model for an allylic structure lacking resonance. For allyl radical the rotation barrier has been determined to be 15.7 kcal / mol, in acceptable agreement with the direct thermochemical number. 

The energy balance of photodissociation the importance of stabilization of the free radicals. When chlorobenzene or chloro-Np loses the halogen atom, a phenyl or a naphthyl radical is formed with the odd electron localized in an sp2 orbital which is orthogonal to the aromatic zr orbitals such a radical is not stabilized through resonance, unlike the benzyl- or the methyl-Np radicals for which several resonance structures can be drawn (Figure 4.32). 

Of this group only benzyl chloride is not an aryl halide its halogen is not attached to the aromatic ring but to an. v/r -hybridized carbon. Benzyl chloride has the weakest carbon-halogen bond, its measured carbon-chlorine bond dissociation energy being only 293 kJ/mol (70 kcal/mol). Homolytic cleavage of this bond produces a resonance-stabilized benzyl radical. 

All /1-eliminations from the benzyl derivative in Figure 4.5 exhibit a certain stereoselectivity, in this case -stcreoselectivity. This is true regardless of whether the elimination is syn- or awft -selective or neither. The reason for the preferred formation of the /. -product is again product-development control. This comes about because there is a significant energy difference between the isomeric elimination products due to the presence (E isomers) or absence (Z isomers) of styrene resonance stabilization. 

Alkyl and aryl ethers are relatively stable to acids and bases due to the high C-0 bond energy and it is difficult to recover the parent alcohols from them therefore, most useful ether-type protections utilize resonance stabilization (by delocalization) of the benzylic-type cation or... 

The 9-phenylxanthyl radical is a resonance-stabilized triphenylmethyl analog. The corresponding carbonium ion and carbanion are also stabilized and can be prepared in sulfolane, so that A//het can be directly measured.The data for benzyl and t-butyl are obtained by measuring the reduction and oxidation potentials of the radicals in acetonitrile. The results show that C6H5CH2 and (CH3)3C are much harder than the 9-phenylxanthyl radical (the latter is just one of several studied with similar properties ). The solution hardnesses are then responsible for the difficulty in forming the ions in the benzyl and t-butyl cases, and the stability of the ions in the resonance-stabilized cases. The effect of the small hardness in the latter cases also is evident in the small bond energy for homolytic dissociation. 

HuckeFs rule leads one to expect resonance stabilization in the cyclopentadienate and cycloheptatrienyl (tropyllum) ions. With a view to determining the resonance energy of the latter, Turner has measured the heat of hydrogenation of tropylium chloride to cycloheptaiie and hj drogen chloride, in acetic acid solution, ag — 86 23 rt 0 08 kcal/mole. An energetic comparison of the relative stabilities of tropylium chloride and the isomeric benzyl chloride... 

The population of the highest accessible level, v, is very sensitive to slight variations in i.e., a small decrease in because of an Increase in Dj(R-H) markedly lowers P (v ). Surprisal plots (with all models) confirm this sensitivity, in that reduction of the energy causes the I(f )point to rise above the line formed by the I(fv) points, vtoluene reaction, using the full thermochemical energy, showed a very serious deviation of this sort. Since the benzyl radical is resonance stabilized, we reasoned that F + toluene might obey the three-body model if the available energy... 

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