Allyl system resonance structures

Allyl radical is a conjugated system in which three electrons are delocalized over three carbons The resonance structures indicate that the unpaired electron has an equal probability of being found at C 1 or C 3 C 2 shares none of the unpaired electron... 

The ally carbocation is an example of an intermediate whose structure has been extensively investigated by MO methods. The hybridization/resonance approach discussed earlier readily rationalizes some of the most prominent features of the allyl carbocation. The resonance structures suggest a significant stabilization and imply that the molecule would be planar in order to maximize the overlap of the n system. 

The 1,3-dipolar molecules are isoelectronic with the allyl anion and have four electrons in a n system encompassing the 1,3-dipole. Some typical 1,3-dipolar species are shown in Scheme 11.4. It should be noted that all have one or more resonance structures showing the characteristic 1,3-dipole. The dipolarophiles are typically alkenes or alkynes, but all that is essential is a tc bond. The reactivity of dipolarophiles depends both on the substituents present on the n bond and on the nature of the 1,3-dipole involved in the reaction. Because of the wide range of structures that can serve either as a 1,3-dipole or as a dipolarophile, the 1,3-dipolar cycloaddition is a very useful reaction for the construction of five-membered heterocyclic rings. 

If you see a double bond near the LG and you are not sure if it is a benzylic or allylic system, just draw the carbocation you would get and see if there are any resonance structures. 

In Chapter 10 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for syntheses of heterocyclic compounds and for carbon-carbon bond formation. Table 6.2 lists some of the types of molecules that are capable of dipolar cycloaddition. These molecules, which are called 1,3-dipoles, have it electron systems that are isoelectronic with allyl or propargyl anions, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a 1,3-relationship, and it is this structural feature that leads to the name 1,3-dipolar cycloadditions for this class of reactions.136... 

Although both Sn2 and SnI mechanisms might be formnlated for such reactions, all the available evidence favours an Sn 1 process. This is rationalized in terms of formation of a favourable resonance-stabilized allylic cation by loss of the leaving gronp. In the majority of natnral prodnct structures, the nucleophile has attacked the allylic system on the same carbon that loses the diphosphate, bnt there are certainly examples of nncleophilic attack on the alternative tertiary carbon. 

In allylic systems, favorable overlap of the p orbitals of the n system should require a coplanar arrangement of the three sp2 carbons and their five substituent atoms evidence that such a structure is indeed preferred comes, for example, from proton magnetic resonance observations that demonstrate barriers to bond rotation in the isomeric dimethylallyl ions 21, 22, and 23. These ions form stereo-specifically from the three dimethylcyclopropyl chlorides (Section 12.2), and barriers to rotation about the partial double bonds are sufficiently high to prevent interconversion at low temperature. At — 10°C, 21, the least stable isomer,... 

Let s take a closer look at the electronic structure of allylic systems, using the allyl radical as our example. One resonance form shows the radical electron on Cl, with a pi bond between C2 and C3. The other shows the radical electron on C3 and a pi bond between Cl and C2. These two resonance forms imply that there is half a pi bond between Cl and C2 and half a pi bond between C2 and C3, with the radical electron half on Cl and half on C3. 

Remember that no resonance form has an independent existence A compound has characteristics of all its resonance forms at the same time, but it does not resonate among them. The p orbitals of all three carbon atoms must be parallel to have simultaneous pi bonding overlap between Cl and C2 and between C2 and C3. The geometric structure of the allyl system is shown in Figure 15-10. The allyl cation, the allyl radical, and the allyl anion all have this same geometric structure, differing only in the number of pi electrons. 

Because this is an example of an allyl-type system p<=Y—Z ), a second resonance structure can be drawn that moves the lone pair and the jt bond. To delocalize the lone pair and make the system conjugated, the labeled carbon atom must besp hybridized with the lone pair occupying ap orbital. 

M. Barfield, DFT/FPT studies of the structural dependencies of long-range H/H coupling over four bonds 4/(H,H ) in propanic and allylic systems, Magn. Reson. Chem., 41 (2003) 344-358. 

We should point out that a structure that would indicate an unpaired electron on the central carbon of the allyl system, as shown here, is not a proper resonance structure because resonance theory dictates that all resonance strucmres must have the same number of unpaired electrons (Section 13.4A). This structure shows three unpaired electrons, whereas the other resonance structures for the allyl radical have only one unpaired electron. 

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. 

Allyl (propenyl cation) (Section 13.3) The carbocation formally related to propene by removal of a proton from its methyl group. The two contributing resonance structures of the delocalized carbocation each include a positive charge on a carbon adjacent to the double bond, such that a p orbital on each of the three carbons overlaps to delocalize positive charge to each end of the allyl system. 

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