Pentadienyl radical, resonance

The activation energy is only 7 kcal/mol less than that for the vinylcyclopropane rearrangement itself, consistent with roughly 20 kcal/mol for the pentadienyl radical resonance energy, and suggests that the reaction is not concerted. 

Draw three resonance forms for the pentadienyl radical. A radical is a substance that contains a single, unpaired electron in one of its orbitals, denoted by a dot ( )-... 

Thus, the three resonance forms for the pentadienyl radical are ... 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

A common biologically active radical is the pentadienyl radical, RCHCHCHCHCHR, where the carbons form a long chain, with R and R, which can be a number of different organic groups, at each end. Draw three resonance structures for this compound that maintain carbon s valence of four. 

In the pentadienyl radical, predict the distribution of the unpaired electron (a) from the resonance model, and (b) from the molecular orbital model. 

Exercise 7.2 The pentadienyl radical is an odd member of the C2 iH2 +i polyenyl radicals (n = 3). It possesses three Kekule-type resonance structures, shown in Fig. 7.Ans.la. One resonance structure has a radical at the central carbon and is labeled Rc, the other two place the radical either on the right- or left-hand carbon atoms and are labeled accordingly as Rr and Ri, respectively. There are other structures with long bonds, but we are going to neglect them in this treatment. This will affect the calculated values of the spin density, and... 

FIGURE 7.Ans.l (a) Resonance structures of pentadienyl radical and their symmetry properties, (b) The VB mixing of the resonance structures, (c) The quasiclassical (spin alternant) determinant that dominates the ground-state wave function, and the corresponding secondary determinants, and the resulting spin density distribution (p) in the ground state, (d) The spin distribution in the covalent excited state. 

The fate ofthe carbon-centred radical L (which is a resonant pentadienyl radical in the case of linoleic acid. Fig. 3) is to lead, in a very rapid reaction with dioxygen, to theformation ofa peroxyl radical, 102 (reaction (2)). 

D.F. McMiUen and D.M. Golden, Cyclohexadienyl radical resonance energy is 7 kcal/ mol greater than that of the pentadienyl radical from bond dissociation energies, Anna. Rev. Phys. Chem., 33, 493 (1982). 

ANSWER The carbon-hydrogen bond in ethane has a BDE of 101 kcal/mol (Table 8.2). But the pentadienyl radical is resonance stabilized and that stabilization will begin to be felt as the bond breaks, further decreasing the BDE. If allylic resonance is worth about 13 kcal/mol (p. 475), we might expect the BDE to be about 101 — 13 = 88 kcal/mol. 

The resonance-stabilized pentadienyl radical combines rapidly with O2 in the first of two propagation steps. Reaction occurs at either C9 or C13 (shown here), giving either of two peroxy radicals containing conjugated diene units. 

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