Unsaturated system allylic radical

The new p orbital of the allyl radical is conjugated with those of the double bond => the allyl radical is a conjugated unsaturated system. 

Figure 13.3 The n molecular orbitals of the allyl cation. The allyl cation, like the allyl radical, is a conjugated unsaturated system. The shapes of molecular orbitals for the allyl cation calculated using quantum mechanical principles are shown alongside the schematic orbitals. 

Brown and Suzuki have shown that treatment of trialkylboranes with ethenyl-(Scheme 42, Eq. 42a) and ethynyloxiranes (Scheme 42, Eq. 42b) in the presence of a catalytic amount of oxygen, affords the corresponding allylic or allenic alcohols. The mechanism may involve the addition of alkyl radicals to the unsaturated system leading to l-(oxiranyl)alkyl and l-(oxiranyl)alkenyl radicals followed by rapid fragmentation to give alkoxyl radicals that finally complete the chain process by reacting with the trialkylborane [104-106]. 

In general, radical cations of alkenes or cyclopropanes produce nonconjugated radicals, while those of dienes give rise to allyl radicals, and those of vinylcyclopropane or vinylcyclobutane systems generate either allylic or nonconjugated radicals with an additional element of unsaturation. In contrast to the most thoroughly characterized nucleophilic substitution of appropriate neutral molecules. 

The interesting behavior own mainly by (V)-EPTM was not confirmed by terpolymers containing 5,6-dimethylene-24ioibomene (X) or 5- 2 -vinyl-3 -butenyl)-2-norbomene (XI), both being model compounds of triene (V). In fact, the first EPTM contains two conju ted double bonds in obligatory dsoid conformation which were found very reactive toward methyl free radicals. The second EPTM has one bis-allylic hydrogen atom in the unsaturated system ... 

A complete analysis of the products reported in Fig. 1 requires some more comments on cyclopentadiene and benzene. Both are typical secondary products, and are mainly the result of successive addition and condensation reactions of alkenes and unsaturated radicals. Once a significant amount of ethylene and propylene is formed, vinyl and allyl radicals are present in the reacting system and form butadiene, via butenyl radicals. Successive addition reactions of vinyl and allyl-like radicals on alkenes and dialkenes sequentially explain the formation of cyclopentadiene and benzene. These reactions are discussed in-depth in the literature and will be also analysed in the coming paragraphs (Dente et al., 1979). It seems worthwhile mentioning that these successive reactions and interactions of small unsaturated radicals and species constitute the critical sub-mechanism for the correct evaluation of ethylene selectivity. In fact, once the primary decomposition of the hydrocarbon feed has largely completed, the primary products and mainly small alkenes can be... 

We shall use the allyl radical to illustrate how one can treat planar unsaturated organic molecules using multiconfigurational methods. Some properties of the groimd state will be studied and, in addition, the electronic spectrum. The system has acmally been used as a example in a course given at the Department of Theoretical Chemistry in Lund called Quantum Chemistry at Work and a number of students have performed the calculations. We shall use their results. Some of them were recently published [69]. 

Mechanism a) does not seem very likely. Of the various pathways available to the radical pair, bonding to the central atom of an allylic system or rearrangement of allyl radical to cyclopropyl radical are energetically the least favorable. Closure to either starting material or isomeric ketone would be expected to take precedence. Neither isomeric p,Y Unsaturated ketone nor scrambling of the substituents on the methane carbon atom in residual starting material is found. 

Benzylic radicals and benzylic cations are conjugated unsaturated systems and hoth are unusually stable. They have approximately the same stabilities as allybc radicals (Section 10.8) and allylic cations (Section 13-4). This exceptional stability of benzylic radicals and cations can be explained by resonance theory. In the case of each entity, resonance structures can be written that place either the unpaired electron (in the case of the radical) or the positive charge (in the case of the cation) on an ortho or para carbon of the ring (see the following structures). Thus resonance delocalizes the unpaired electron or the charge, and this delocabzation causes the radical or cation to be highly stabibzed. 

Vinylmethylene cis- or trans-) (11) is the prototypical a, P-unsaturated carbene. The n system of vinylmethylene allows delocalization of the single electron occupying the p orbital in the triplet and open shell (op) singlet state, resulting in an allyl radical-like tt system. The biradical character of such wavefunctions demands a multi-configuration treatment as artificial symmetry breaking is observed with HF theory. ... 

The chemistries of the benzyiic and allylic positions are very similar. Intermediate carbocations, free radicals and carbanions formed at these positions are stabilized by delocalization with the adjacent ir system, the benzene ring in the case of the benzyiic position. Another aspect of arene chemistry is the enhanced stability of unsaturated arenes having double bonds conjugated with the benzene ring. This property is akin to the stability of conjugated di- and polyenes. 

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