Radical addition Hammond postulate

Radical additions are typically highly exothermic and activation energies are small for carbon30-31 and oxygen centered32,33 radicals of the types most often encountered in radical polymerization, Thus, according to the Hammond postulate, these reactions are expected to have early reactant-like transition states in which there is little localization of the free spin on C(J. However, for steric factors to be important at all, there must be significant bond deformation and movement towards. sp hybridization at Cn. 

Free-radical addition to a double bond is a strongly exothermic process, because a TT-bond is broken and a o-bond is formed and so, according to the Hammond postulate, early transition states are involved where there is limited bond breaking or making. Consequently, the influence of polar and steric effects becomes significant, in competition with the stability of the developing intermediate radical formed, in determining the orientation and relative rates of addition. 

Few examples exist in the literature concerning the stereoselective addition of acyl radicals to a radical acceptor in an acyclic manner. Equation (13.1) shows the efficient 1,2-asymmetric induction in the addition of aliphatic or aromatic acyl radicals to chiral acyclic alkenes 1 [7]. The corresponding a-hydroxy ketones 3 were produced with high syn selectivity (Table 13-1). This acyl radical addition is very exothermic, and it is hypothesized that Hammond s postulate can be invoked to predict a transition state that is very close in energy to the starting alkene 1. The X-ray structure of 1 was then used to rationalize the stereochemical outcome of this radical addition by determination of the least sterically hindered path for the approaching radical. 

The first step in the mechanism is endothermic and rate determining. The 3° radical produced in anti-Markovnikov attack (A) of bromine radical is several kJ/mole more stable than the 1° radical generated by Markovnikov attack (B). The Hammond Postulate tells us that it is reasonable to assume that the activation energy for anti-Markovnikov addition is lower than for Markovnikov addition. This defines the first half of the energy diagram. 

We can see that the polar effects found for electrophilic aromatic additions are much less pronoimced with radical additions. This is in part due to the fact that the influence on radical stability of electron donating or electron withdrawing groups is lower than the effect on car-benium ions. However, the lower sensitivity is also due to an early transition state for radical addition because the addition is exothermic, and a late transition state for electrophilic addition because this addition is endothermic. By the Hammond postulate, the early transition state for radical addition would have little radical character on the aromatic ring, and there-... 

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