Alkenes, addition reactions activation energy

The second intermediate s identity has been debated since the mid-1980s. In 1984, Liu and Tomioka suggested that it was a carbene-alkenc complex (CAC).17 Similar complexes had been previously postulated to rationalize the negative activation energies observed in certain carbene-alkene addition reactions.11,30 A second intermediate is not limited to the CAC, however. In fact any other intermediate, in addition to the carbene, will satisfy the kinetic observations i.e., that a correlation of addn/rearr vs. [alkene] is curved, whereas the double reciprocal plot is linear.31 Proposed second intermediates include the CAC,17 an excited carbene,31 a diazo compound,23 or an excited diazirine.22,26 We will consider the last three proposals collectively below as rearrangements in the excited state (RIES). 

Initially, the alkyne is converted into the f> alkene. The activation energy for this step is labelled AGi. The -alkene then converts to the Z isomer via an intermediate. The activation energy for this step is AG. Overall, the reaction is the addition of HCl to the alkyne to give the Z-alkene—we could look on the E isomer as just another intermediate. The only difference between the E-alkene and the intermediate in the isomerization reaction is the size of the activation energies it is much easier to isolate the E-alkene because the activation energies to be overcome (AG2 and AG4) are both much larger than those of the intermediate (AG5 and AG3). The activation energy to be overcome to form the E-alkene (AGf) is less than that to be overcome to form the Z-alkene (AGj. ... 

Nature of addition of formaldehyde to alkenes in Prins reaction is studied by MP2(fc)/6-31G(d). The structure of the transition states and intermediates are found. It is shown that the hydrogenated pyrans as 1,3-dioxanes can be formed by addition of formaldehyde oligomers to alkenes. However, the activation energy of this reaction is higher than that of 1,3-diox-anes formation. 

A common feature of these intermediates is that they are of high energy, compared to structures with completely filled valence shells. Their lifetimes are usually very short. Bond formation involving carbocations, carbenes, and radicals often occurs with low activation energies. This is particularly true for addition reactions with alkenes and other systems having it bonds. These reactions replace a tt bond with a ct bond and are usually exothermic. 

Influence of a TT-Bond Adjacent to the Reaction Center on the Activation Energy of the Radical Addition to Alkenes [40-48]... 

The reactivity of a range of alkenes in addition reactions of peroxyl radicals has been reported. Parameters that described the relationship between the activation energy and enthalpy were calculated. An activation energy of 82 kJ moP was determined for the addition of alkylperoxy radicals to isolated C=C bonds, rising by 8.5kJmor when the alkene was conjugated with an aromatic substituent. 

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. 

The diagram above refers to thermodynamic stability. When we discuss addition reactions you will see that the most stable alkene when mixed with an electrophile is the most reactive according to this diagram. This paradox is due to the intermediate, usually a carbocation. Since a tertiary carbocation is more stable, the energy of activation is lowered and a reaction with a tertiary intermediate proceeds more quickly in general, to predict the alkene product, use the above diagram as a reference, but to predict the most reactive alkene to an electrophile, the order is based on cation formation and is nearly reversed. 

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