Bonding allyl cation

The carbocation is stabilized by delocalization of the tt electrons of the double bond and the positive charge is shared by the two CH2 groups Substituted analogs of allyl cation are called allylic carbocations Allyl group (Sections 5 1 10 1) The group... 

The positively charged allyl cation would be expected to be the electron acceptor in any initial interaction with ethylene. Therefore, to consider this reaction in terms of frontier orbital theory, the question we need to answer is, do the ethylene HOMO and allyl cation LUMO interact favorably as the reactants approach one another The orbitals that are involved are shown in Fig. 1.27. If we analyze a symmetrical approach, which would be necessary for the simultaneous formation of the two new bonds, we see that the symmetries of the two orbitals do not match. Any bonding interaction developing at one end would be canceled by an antibonding interaction at the other end. The conclusion that is drawn from this analysis is that this particular reaction process is not favorable. We would need to consider other modes of approach to analyze the problem more thoroughly, but this analysis indicates that simultaneous (concerted) bond formation between ethylene and an allyl cation to form a cyclopentyl cation is not possible. 

It is worth noting that in the case of the reactions of ediylene and butadiene with the allyl cation, the MO description has provided a prediction that would not have been recognized by a pictorial application of valence bond terminology. Thus, we can write an apparently satisfactory description of both reactions. 

Any structural effect which reduces the electron deficiency at the tricoordinate carbon will have flie effect of stabilizing the caibocation. Allyl cations are stabilized by delocalization involving the adjacent double bond. 

The 7t-electron delocalization requires proper orbital alignment. As a result, there is a significant barrier to rotation about the carbon-carbon bonds in the allyl cation. The results of 6-31G/MP2 calculations show the perpendicular allyl cation to be 37.8 kcal/mol higher than the planar ion. Related calculations indicate that rotation does not occur but that... 

Run an AIM=BondOrders calculation for allyl cation at the MP2/6-31G(d) model chemistry. What are the predicted atomic charges and bond orders for this molecule ... 

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. ... 

Are the carbon-carbon bond distances in allyl cation, allyl radical allyl anion all similar, or are they significantly... 

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). 

The bonding n molecular orbital of the allyl cation contains two spin-paired electrons. 

The hydrogenation of acyclic homoallylic alcohols with a 1,1-disubstituted ole-fmic bond by cationic [Rh(diphos-4)]+ catalyst proceeds in modest to moderate stereoselectivity, generally forming 1,3-anti compounds (Table 21.10, entries 1, 4 and 5), and the effect of the stereogenic center at the allylic position overrides the directivity of hydroxyl group. The 1,3-syn product is then observed though in poor selectivity (entry 3) [19, 57, 58]. Inspection of the hydrogenation prod-... 

The fact that the anodic oxidation of allylsilanes usually gives a mixture of two regioisomers suggests a mechanism involving the allyl cation intermediate (Scheme 3). The initial one-electron transfer from the allylsilane produces the cation radical intermediate [9], Although in the case of anodic oxidation of simple olefins the carbon-allylic hydrogen bond is cleaved [28], in this case the... 

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