Energetics, hydrogen abstraction

The addition of S—H compounds to alkenes by a radical-chain mechanism is a quite general and efficient reaction. The mechanism is analogous to that for hydrogen bromide addition. The energetics of both the hydrogen abstraction and addition steps are favorable. Entries 16 and 17 in Scheme 12.5 are examples. 

The initiation is by Br, as hydrogen abstraction by RO from HBr (as above) is energetically much more favourable than the alternative of bromine abstraction to form ROBr + H. The alternative addition of Br to (63) to form MeCH(Br)CH2 (66) does not occur, as secondary radicals, e.g. (65), are more stable (cf. p. 310) than primary, e.g. (66). 

As indicated in Table 1, free radicals can be formed in the gas phase by the collision of energetic free electrons with monomer molecules. And in a related process, hydrogen atoms are produced together with a hydrogen depleted monomer molecule. The ease with which this process occurs appears to increase with increasing saturation of the monomer . Once formed, the hydrogen atoms can produce further free radicals by either hydrogen abstraction or addition to an olefin. 

First, hydroperoxy radicals must react predominantly via Reaction 8 and not via Reaction 7. It is difficult to assess this competition because of the uncertain energetics of these reactions. Assuming that k7 — k3, Reaction 8 is faster than Reaction 7 when the hydrogen abstracted in 7 is primary, the rate constants are approximately equal when it is secondary, and 7 predominates when it is tertiary. Only under conditions where the yields of alkenes are considerable and the alkane has no tertiary C—H bonds will Reaction 8 be important. Even then, abstraction of allylic hydrogen from the alkene by HO2 will compete strongly with Reaction 8. 

As with chlorination, the rate-limiting step in bromination is the first propagation step abstraction of a hydrogen atom by a bromine radical. The energetics of the two possible hydrogen abstractions are shown below. Compare these numbers with the energetics... 

The access to silyl radicals is different from that to similar carbon compounds. Hydrogen abstraction is energetically unfavourable, while the pyrolysis which organosilicon compounds undergo differs from that of hydrocarbons due to the inability of silicon to form pirpn bonds. Pyrolysis of organosilicon compounds proceeds usually via non-chain sequences from which the rate of initial bond rupture into silyl radicals may be obtained 91 ... 

---