Allylic free radical

Like an allylic carbocation, an allylic radical is resonance stabilized. However, in contrast to an allylic carbocation, which has two electrons in a delocalized 7i system, an allylic radical has three 7t electrons, one of which is unpaired. The unpaired electron is delocalized with an equal probability of being at either C-1 or C-3. The central carbon atom has no radical character. Lewis structures for the allylic 

We can compare the stability of the allylic radical versus a primary radical by comparing the bond dissociation energies of the C—H bond of the primary carbon atom of propane to the bond dissociation energy of the allyl C—H bond of propene, as shown below. 

The 58 kJ mole difference between these two bond dissociation energies is a direct measure of the resonance energy of the allylic radical. 

Typical 8 2 displacements occur when primary allylic halides react with good nucleophiles. 

With secondary and tertiary allylic halides or under solvolysis conditions, 8 1 reactions can compete with 8 2, and a mixture of direct displacement and allylic rearrangement products results. 

3-Chloro-l-pentene was treated with phenoxide ion as shown for l-chloro-2-pentene in the preceding equation. Two C11H14O constitutional isomers were obtained. Suggest a reasonable structure for each isomer. 

Phenoxide ion is about 6 pK units less basic than ethoxide. Unlike ethoxide, phenoxide reacts with secondary alkyl halides by substitution, not elimination. 

Just as allyl cation is stabilized by electron delocalization, so is allyl radical  

Allyl radical is a conjugated system in which three electrons are delocalized over three carbons. The unpaired electron has an equal probability of being found at C-1 or C-3. 

Reactions that generate allylic radicals occur more readily than those involving simple alkyl radicals. Compare the bond dissociation energies of the primary C—H bonds of propane and propene  

It requires less energy, by 42 kJ/mol (10 kcal/mol), to break a bond to a primary hydrogen atom in propene than in propane. The free radical produced from propene is allylic and stabilized by electron delocalization the one from propane is not. 

SAMPLE SOLUTION (a) Allylic hydrogens are bonded to an allylic carbon. An allylic carbon is an sp -hybridized carbon that is attached directly to an sp -hybridized carbon of an alkene. Cyclohexene has four allylic hydrogens. 

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The allyl free radical with 3 electrons, 2 in the bunding orbital and I in the nonbonding orbital, has... 

Not all the properties of alkenes are revealed by focusing exclusively on the func tional group behavior of the double bond A double bond can affect the proper ties of a second functional unit to which it is directly attached It can be a sub stituent for example on a positively charged carbon in an allylic carbocation, or on a carbon that bears an unpaired electron in an allylic free radical, or it can be a substituent on a second double bond in a conjugated diene... 

Conjugare is a Latin verb meaning to link or yoke together and allylic carbocations allylic free radicals and conjugated dienes are all examples of conjugated systems In this chapter we 11 see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone... 

Substitution. In free-radical substitution, the olefin reacts with a free-radical source to form the allyl free radical, which in turn reacts with available reagent to produce both the final product and a new free radical. 

Reversible electron addition to the enone forms the radical anion. Rate determining protonation of the radical anion occurs on oxygen to afford an allylic free radical [Eq. (4b) which undergoes rapid reduction to an allylic carbanion [Eq. (4c)]. Rapid protonation of this ion is followed by proton removal from the oxygen of the neutral enol to afford the enolate ion [Eq. (4c)]. 

The allylic position of olefins is subject to attack by free radicals with the consequent formation of stable allylic free radicals. This fact is utilized in many substitution reactions at the allylic position (cf. Chapter 6, Section III). The procedure given here employs f-butyl perbenzoate, which reacts with cuprous ion to liberate /-butoxy radical, the chain reaction initiator. The outcome of the reaction, which has general applicability, is the introduction of a benzoyloxy group in the allylic position. 

The reaction of the diene with a free radical produces an allyl radical having unpaired electron delocalised over more than one carbon atom. The allyl free radical can undergo 1, 2, or 1, 4 addition. 

Thus in the rearrangement, in the transition state, there will be an overlap between the orbital of the atom or free radical (R) of the group which is migrating and the orbital of an allylic free radical. So there will be an overlap between the HOMO of one component and the HOMO of the other. 

Free radical allylation free radical reduction (homogeneous)... 

The question is, why doesn t the X in Figure 4-7 add to the double bond to form the following radical shown in Figure 4-8 The answer is that this would give a secondary free radical, which is less stable than the allylic free radical because it doesn t have resonance stabilization. 

Although not in the top 50, it is an important monomer for making epoxy adhesives as well as glycerine (HO-CH2-CHOH-CH2-OH). Propylene is first chlorinated free radically at the allyl position at 500°C to give allyl chloride, which undergoes chlorohydrin chemistry as discussed previously to give epichlorohydrin. The student should review the mechanism of allyl free radical substitution from a basic organic chemistry course and also work out the mechanism for this example of a chlorohydrin reaction. 

Problem 8.43 CH3CH==CH2 is subjected to allylic free-radical bromination. Will the reaction product be exclusively labeled H C = CH CHjBr Explain. ... 

Allyl Free Radicals. Ayscough and Evans (3) have recently studied, by ESR measurements, the types of allylic free radicals produced by gamma-irradiation of several monomeric olefins. In irradiated polyethylene the allyl free radical is quite stable, persisting for several months at room temperature (31). The presence of these allyl free radicals is most noticeable in the case of high density polyethylene, and this type of free radical is undoubtedly the cause of the slow oxidation of polyethylene at room temperature, which lasts for 40 or more days after irradiation (10). Williams and Dole (40) could observe little if any oxidation of low density polyethylene when it was exposed to air after irradiation. By oxidation we mean formation of carbonyl groups as detected by infrared absorption studies at 1725 cm"1. Parenthetically, it should be noted that adding an oxygen. molecule to a free radical produces initially another type of free radical, a peroxy free radical, but in this paper we shall not discuss free radicals of this or any other types except those of hydrocarbons. 

As other workers have shown (6, 27, 32), the concentration of the allyl free radical decreases on exposure to ultraviolet light at liquid... 

The exact mechanism by which ultraviolet light eliminates the allyl free radical and heating restores it is not known, but the following... 

On heating, migration of a hydrogen atom, alpha to a polyene group, to an alkyl free radical site would also convert a polyene to a polyenyl free radical, a transformation similar to transformation of the allyl free radical described above ... 

One of the most pressing problems in the radiation chemistry of high polymers is to obtain definite proof of one or the other of the mechanisms proposed for the interoonversion of alkyl and allyl free radicals. Such information might also be of great help in understanding other effects produced by irradiating high polymers. 

Allylic carbocation Allylic free radical Conjugated diene... 

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