Toluenes, bond dissociation energy

The benzyhc position m alkylbenzenes is analogous to the allyhc position m alkenes Thus a benzyhc C—H bond like an allyhc one is weaker than a C—H bond of an alkane as the bond dissociation energies of toluene propene and 2 methylpropane attest... 

From this value and known C—H bond dissociation energies, pK values can be calculated. Early application of these methods gave estimates of the p/Ts of toluene and propene of about 45 and 48, respectively. Methane was estimated to have a pAT in the range of 52-62. Electrochemical measurements in DMF have given the results shown in Table 7.3. These measurements put the pK of methane at about 48, with benzylic and allylic stabilization leading to values of 39 and 38 for toluene and propene, respectively. The electrochemical values overlap with the pATdmso scale for compounds such as diphenyl-methane and triphenylmethane. 

Alkylbenzenes such as toluene (methylbenzene) react with NBS to give products in which bromine substitution has occurred at the position next to the aromatic ring (the benzyiic position). Explain, based on the bond dissociation energies in Table 5.3 on page 156. 

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

Similarly, if we look at the H-C bond-dissociation energies of the hydrocarbons shown in Table 4-6, we would infer that Cl- would remove a hydrogen most rapidly from the carbon forming the weakest C-H bond and, again, this is very much in accord with experience. For example, the chlorination of methylbenzene (toluene) in sunlight leads to the substitution of a methyl hydro-... 

Calculate CH bond dissociation energies in propene and in toluene, leading to allyl and benzyl radicals, respectively. (The energy of hydrogen atom is given at right.) Is bond dissociation easier or more difficult in these systems relative to bond dissociation in 3-ethylpentane (methyl CH) Examine spin density surfaces for allyl and benzyl radicals. Draw Lewis structures that account for the electron distribution in each radical. Does spin delocalization appear to stabilize radicals in the same way charge delocalization stabilizes ions ... 

The bond dissociation energies in Table 1.2 (p. 21) show that only 85 kcal is needed for formation of benzyl radicals from a mole of toluene, as compared with 91 kcal for formation of /crr-butyl radicals and 88 kcal for formation of allyl radicals. Relative to the hydrocarbon from which each is formed, then, a benzyl radical contains less energy and is more stable than a terhbuiyl radical. 

How are we to account for the stability of the benzyl radical Bond dissociation energies indicate that 19 kcal/mole less energy (104 — 85) is needed to form the benzyl radical from toluene than to form the methyl radical from methane. 

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