Allyl cation resonance

As shown in Figure 27, an in-phase combination of type-V structures leads to another A] symmetry structures (type-VI), which is expected to be stabilized by allyl cation-type resonance. However, calculation shows that the two shuctures are isoenergetic. The electronic wave function preserves its phase when tr ansported through a complete loop around the degeneracy shown in Figure 25, so that no conical intersection (or an even number of conical intersections) should be enclosed in it. This is obviously in contrast with the Jahn-Teller theorem, that predicts splitting into A and states. 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

Molecular orbitals are useful tools for identifying reactive sites in a molecule. For example, the positive charge in allyl cation is delocalized over the two terminal carbon atoms, and both atoms can act as electron acceptors. This is normally shown using two resonance structures, but a more compact way to see this is to look at the shape of the ion s LUMO (the LUMO is a molecule s electron-acceptor orbital). Allyl cation s LUMO appear s as four surfaces. Two surfaces are positioned near- each of the terminal car bon atoms, and they identify allyl cation s electron-acceptor sites. 

The predicted bond order for a given bond is listed at the intersection of the two atoms of interest in the bond orders table. The illustration at the left shows the predicted bond orders for this molecule (where 1.0 is a traditional single bond, 2.0 is a double bond, and so on). The C-H bonds all have predicted bond orders of about. 9, while the C-C bonds have predicted bond orders of about 1.4. The latter arc consistent with the known resonance structure for allyl cation. ... 

How can we account for the formation of 1,4-addition products The answer is that allylic carbocations are involved as intermediates (recall that allylic means "next to a double bond"). When 1,3-butadiene reacts with an electrophile such as H+, two carbocation intermediates are possible a primary nonal-lylic carbocation and a secondary allylic cation. Because an allylic cation is stabilized by resonance between two forms (Section 11.5), it is more stable and forms faster than a nonallylic carbocation. 

In the present instance, protonation of the C1-C2 double bond gives a carbo-cation that can react further to give the 1,2 adduct 3-chloro-3-methylcyclohexene and the 1,4 adduct 3-chloro-L-methylcyclohexene. Protonation of the C3-C4 double bond gives a symmetrical carbocation, whose two resonance forms are equivalent. Thus, the 1,2 adduct and the 1,4 adduct have the same structure 6-chloro-l-methyl-cyclohexene. Of the two possible modes of protonation, the first is more likely because it yields a tertiary allylic cation rather than a secondary allylic cation. 

Show how resonance can occur in the following organic ions (a) acetate ion, CH,CO, (b) enolate ion, CH,COCH5, which has one resonance structure with a C=C double bond and an —O group on the central carbon atom (c) allyl cation, CH,CHCH,+ (d) amidate ion, CH,CONH (the O and the N atoms are both bonded to the second C atom). 

It has been contended that here too, as with the benzene ring (Ref 6), the geometry is forced upon allylic systems by the a framework, and not the 7t system Shaik, S.S. Hiberty, P.C. Ohanessian, G. Lefour, J. Nouv. J. Chim., 1985, 9, 385. It has also been suggested, on the basis of ab initio calculations, that while the allyl cation has significant resonance stabilization, the allyl anion has little stabilization Wiberg, K.B. Breneman, C.M. LePage, T.J. J. Am. Chem. Soc., 1990, 112, 61. 

Resonance theory depicts the allyl cation as a hybrid of structures D and E ... 

D and E are equivalent resonance structures => the allyl cation should be unusually stable. 

These are resonance structures for the allylic cation formed when 1,3-butadiene accepts a prooton. 

This is not a proper resonance structure for the allylic cation because a hydrogen atom has been moved. 

Structures 4 and 5 resembles 1° carbocations and yet the allyl cation is more stable than a 2° carbocation => resonance stabilization. 

In step 1, a proton adds to one of the terminal carbon atoms of 1,3-butadiene to form the more stable carbocation => a resonance stabilized allylic cation, i) Addition to one of the inner carbon atoms would have produced a much less 1 ° cation, one that could not be stabilized by resonance. 

The carbocations so far studied are called classical carbocations in which the positive charge is localized on one carbon atom or delocalized by resonance involving an unshared pair of electrons or a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non-classical carbocation the positive charged is delocalized by double or triple bond that is not in the allylic position or by a single bond. These carbocations are cyclic, bridged ions and possess a three centre bond in which three atoms share two electrons. The examples are 7-norbomenyl cation, norbomyl cation and cyclopropylmethyl cation. 

Fig. 8. (a) Homoaromatic lit-electron conjugation in silyl cation 3. (b) Allyl-type resonance in Sit-electron radical 26. ... 

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. 

At first glance, this appears to be a secondary carbocation, bnt on further exanfination one can see that it is also an allylic cation. Allylic carbocations are stabilized by resonance, reselling in dispersal of the positive charge (see Section 6.2.1). From these two resonance forms, we can predict that both carbons 2 and 4 will be electron deficient. Now this has particnlar consequences when we consider subseqnent attack of the nucleophile on to the carbocation. There are two possible centres that may be attacked, resnlting in two different prodncts. The prodncts are the result of either 1,2-addition or 1,4-addition. The addition across the fonr-carbon... 

You may think that is the end of the problem but, since we have an unsymmetrical diene, it is also necessary to consider protonation of the other double bond. Protonation on C-4 also gives a favourable resonance-stabilized allylic carbocation, this time with primary and secondary limiting structures. Protonation on C-3 gives an unfavourable primary carbocation with no resonance stabilization. Since the products formed are related to initial protonation at C-1, it is apparent that, despite the stability associated with an allylic cation, a tertiary limiting structure is formed in preference to that with a secondary limiting structure. 

E) is rac-CH,CH,CHBrCH=CH2 (F) is trans- and (G) c -CH,CHjCH=CHCH2Br. The intermediate R in this 8, 1 reaction is a resonance-stabilized (charge-delocalized) allylic cation... 

Unfortunately, while it is clear that the allyl cation, radical, and anion all enjoy some degree of resonance stabilization, neither experiment, in the form of measured rotational barriers, nor higher levels of theory support the notion that in all three cases the magnitude is the same (see, for instance, Gobbi and Frenking 1994 Mo el al. 1996). So, what aspects of Hiickel theory render it incapable of accurately distinguishing between these three allyl systems ... 

The allyl cation (9) is the simplest member of the class of resonance-stabilized cations that includes the alkyl-substituted cyclopentenyl cations. But one could also say that the carbenium ion (CH3) is the simplest member of a class of cations that includes the trityl cation. In each case, 10 or so orders of magnitude of acidity separate the primitive member from its more elaborate derivatives. 

In early 1993, Haw and co-workers (107) reported in situ studies of allyl alcohol-/-13C on HZSM-5 and CsHX. No persistent carbenium ions were observed, but 1,3 label exchange was observed for the alcohol on the weakly acidic zeolite. We interpreted this as support for a transient allyl cation. The low stability of this cation was invoked to explain the failure to observe this species as a persistent species. Downfield signals observed in that study were attributed to the formation of propanal. Later in 1993, Biaglow, Gorte, and White (BGW) (108) reported similar studies conducted at different loadings and assigned a downfield resonance (variously reported at 216 and 218 ppm by BGW) to the allyl cation in HZSM-5. 

We may perform the same analysis for the allyl radical and the allyl anion, respectively, by adding the energy of 4>2 to the cation with each successive addition of an electron, i.e., H (allyl radical) = 2(a + V2/3) + a and Hn allyl anion) = 2(a + s/2f) + 2a. In the hypothetical fully 7T-localized non-interacting system, each new electron would go into the non-interacting p orbital, also contributing each time a factor of a to the energy (by definition of o ). Thus, the Hiickel resonance energies of the allyl radical and the allyl anion are the same as for the allyl cation, namely, 0.83/1. 

---