Bond dissociation enthalpies propene

Gryzbowska et al. [106] compared the reaction products formed when pulses of allyl iodide or propene were passed over bismuth oxide or molybdenum oxide. A clear limitation of these experiments is that even the simplest bismuth molybdate catalysts contain neither bismuth oxide nor molybdenum oxide, but instead are made up of a binary oxide of bismuth and molybdenum, whose structure is different to that of bismuth oxide and molybdenum oxide. Gryzbowska et al. selected allyl iodide because of the very low bond dissociation enthalpy associated with the C-I bond, implying that a surface allyl species would readily form from this starting material. In addition, a lower reaction temperature was required for the reaction of allyl iodide than for propene reflecting the greater inherent reactivity of the former. 

The benzylic position in alkylbenzenes is analogous to the allylic position in alkenes. Thus a benzylic C—H bond, like an allylic one, is weaker than a C—H bond of an alkane, as the bond dissociation enthalpies of toluene, propene, and 2-methylpropane attest ... 

We attributed the decreased bond dissociation enthalpy in propene to stabilization of allyl radical by electron delocalization. Similarly, electron delocalization stabilizes benzyl radical and weakens the benzylic C—H bond. 

To predict which of the various C—H bonds in propene is most likely to break when a mixture of propene and bromine or chlorine is heated, we need to look at bond dissociation enthalpies. We find that the bond dissociation enthalpy of an allylic C—H bond in propene (Table 8.7) is approximately 92 kj (22 kcal)/mol less than that of a vinylic C—H bond and 50 kJ (12 kcal)/mol less than a C—H bond of ethane. The allyl radical is even more stable than a 3° radical this unusual stability also applies to carbocations. The reason the allylic C—H bond is so weak is discussed in Section 8.6B. Note from Table 8.7 that the benzyl radical CgH5CH2- is stabilized in exactly the same way as the allyl radical and for the same reason benzylic compounds undergo many of the same reactions as allylic compounds (Section 21.5). 

Delocalization of the unpaired electron stabilizes allylic radicals and causes reactions that generate them to proceed more readily than those that give simple alkyl radicals. Compare, for example, the bond dissociation enthalpies of the primary C—H bonds of propane and propene ... 

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