Alkenes, addition reactions reaction energetics

E. Dovble-Bond Addition Reactions Reactions of recoil tritium atoms in alkenes have been studied and have brought to light an additional reaction type. The energetic tritium atom can add to the double bond. Urch and Wolfgang (1959) have substantiated this hot addition by studying scavenger and moderator effects in alkenes. Decomposition of the hot radical once formed follows a pattern similar to that of the decomposition of thermally excited free radicals, namely... 

The delocalization of the Jt-electrons is energetically favorable, and this affects the reactivity of aromatic compounds There is a tendency towards restoring aromaticity. This is why aromatic compounds, in contrast to regular alkenes (linear chains of carbon atoms containing at least one double bond), do not easily undergo addition reactions, whereby a double bond is replaced by two single bonds. Aromatic compounds show a preference for substitution reactions, which means that atoms are replaced. 

The addition of S-H compounds to alkenes by a radical-chain mechanism is a quite general and efficient reaction. The chain mechanism is analogous to that for hydrogen bromide addition, and the energetics of both the hydrogen abstraction and addition steps are favorable. Thiols and thio acids are both suitable sulfur... 

The addition of hydrogen halides to simple olefins, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule.116 When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 751).137 It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCI only rarely. In the rare cases where free-radical addition of HCI was noted, the orientation was still Markovnikov, presumably because the more stable product was formed.,3B Free-radical addition of HF, HI, and HCI is energetically unfavorable (see the discussions on pp. 683, 693). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides (4-9). Markovnikov addition can be ensured by rigorous purification of the substrate, but in practice this is not easy to achieve, and it is more common to add inhibitors, e.g., phenols or quinones, which suppress the free-radical pathway. The presence of free-radical precursors such as peroxides does not inhibit the ionic mechanism, but the radical reaction, being a chain process, is much more rapid than the electrophilic reaction. In most cases it is possible to control the mechanism (and hence the orientation) by adding peroxides... 

Reaction 8. The energetics of the addition to alkenes of R02 and of H02 will be reasonably similar. For the former reactions (10), AH = —12 kcal. per mole the Semenov-Polanyi equation suggests then that E — 8.5 kcal. per mole. As, however, the activation energy for addition of R02 to styrene (31) is 8.4 kcal. per mole whereas AH = —25 kcal. per mole, it is likely that for monoalkenes E is higher than 8.5. A value of E8 = 11.5 kcal. per mole would seem reasonable. Benson (12) estimates that As — 10"12 8 cc. molecule"1 sec.1. Thus, k8 = 10"17 3 cc. molecule"1 sec.1. Knox (33) has calculated that Ks = 10"20 8 cc. molecule"1, from which k 8 = 103r) per second. 

Neutral aminyl radicals generated from tin hydride-mediated reactions of sulfenamides (Section II,F) have been shown to undergo cyclizations when energetically favored by addition to a strained alkene or by formation of a stabilized intermediate benzylic radical. In both cases, the reverse reaction, cleavage of the /3-amino radical, apparently did not occur (92TL4993). 

Although the Heck reaction may be efficiently employed for synthesis, it has its limits that should not go unmentioned the Heck reaction can not—at least not intermolecularly—couple alkenyl triflates (-bromides, -iodides) or aryl triflates (-bromides, -iodides) with metal-free aromatic compounds in the same way as it is possible with the same substrates and metal-free alkenes. The reason is step 4 of the mechanism in Figure 16.35 (part II). If an aromatic compound instead of an alkene was the coupling partner the aromaticity with this carbopallada-tion of a C=C double bond would have to be sacrificed in step 4. Typically, Heck reactions can only be run at a temperature of 100 °C even if they proceed without any such energetic effort. This is why this additional energetically demanding loss of aromaticity is not feasible. 

Although an energetically less favourable sp2 to sp3 carbanion transformation is involved in these processes, both aryllithium and vinyllithium cyclizations onto alkenes are successful. Moreover, cyclization reactions of vinyllithiums, rather than alkyllithiums, would also incorporate additional functionality (an alkene) into the product, allowing the preparation of alkylidenecycloalkanes with control of the alkene stereochemistry. 

Why do the adduct carbocations 46 not react with a nucleophile to give an adduct of type 48, as happens in electrophilic addition to double bonds The main reason is probably thermodynamic. The hypothetical reaction (5.32) is very exothermic, based on experimental heats of formation. If we attribute this exothermicity to the stabilization of the benzene nucleus, we obtain a value of 150 kJ mol-1 for this stabilization. This stabilization would be lost in reaction (5.31) if the adduct 48 were formed, but is regained if the substituted product 47 is produced. This stabilization does not apply to reaction with alkenes, so addition can take place if the reaction is favourable energetically. 

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