Allyl cation resonance forms

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. 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

The vinyl cation analog of an allylic carbonium ion is an allenyl cation 242, where the empty p orbital on the unsaturated carbon overlaps with the perpendicular n bond of the allenyl system. Allenyl cation 242 is of course a resonance form of the well known alkynylcarbonium ion,... 

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

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. 

Estimating stability it is possible to apply criteria commonly used in organic chemistry. Tertiary alkyl carbocation is more stable than the secondary one which is in its turn more stable than the primary one. For the carbon ions of this type the row of the stability is reversed. Allyl and benzyl cations are stable due to the resonance stabilization. The latter having four resonance structures may rearrange to be energetically favorable in the gas phase tropilium cation possessing seven resonance forms (Scheme 5.3). 

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. 

The electrophile (H ) adds to form an allylic carbocation with positive charge delocalized at C and C (resonance forms II and III). This cation adds the nucleophile at C to form the 1,2-addition product or at C" to form the 1,4-addition product. 

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 best approach is to work through this reaction mechanistically. Addition of hydrogen halides always proceeds by protonation of one of the terminal carbons of the diene system. Protonation of C-l gives an allylic cation for which the most stable resonance form is a tertiary carbocation. Protonation of C-4 would give a less stable allylic carbocation for which the most stable resonance form is a secondary carbocation. 

The seven resonance forms for tropylium cation (cycloheptatrienyl cation) may be generated by moving tt electrons in pairs toward the positive charge. The resonance forms are simply a succession of allylic carbocations. 

Finally, it is important to recognize that an SN1 reaction that forms an allylic carbo-cation often provides more than one site at which the nucleophile can bond. The nucleophile may bond to either of the carbons that bear the positive charges in the resonance structures. If the allylic cation is not symmetrical, this will result in the formation of two products one normal and one rearranged. An example of such an allylic rearrangement is... 

Draw another resonance form for each of the substituted allylic cations shown in the preceding figure, showing how the positive charge is shared by another carbon atom. In each case, state whether your second resonance form is a more important or less important resonance contributor than the first structure. (Which structure places the positive charge on the more-substituted carbon atom )... 

We can represent a delocalized ion such as the allyl cation either by resonance forms, as shown on the left in the following figure, or by a combined structure, as shown on the right. Although the combined structure is more concise, it is sometimes confusing because it attempts to convey all the information implied by two or more resonance forms. 

The key to formation of these two products is the presence of a double bond in position to form a stabilized allylic cation. Molecules having such double bonds are likely to react via resonance-stabilized intermediates. 

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. 

Spinner, 1954 Vernon, 1954). The opposing destabilization is probably due to the greater (electron-withdrawing) inductive effect of the triple bond than that of the double bond in allyl cations. As a matter of fact, it has been evaluated from the solvolysis rate of derivatives 174 and 175 that the resonance contributions of the limiting forms (172a) and (173a) to their respective hybrids are similar (Richey and Richey, 1970). 

The polarized Br molecule is then attacked by the rr electron systeu of the nucleophilic benzene ring in a slow, ru te limiting step to yield nenaromallc carbocation intermediate. This carbocotion is doubly all frecall the allyl cation. Section 11.9) and has three resonance forms ... 

Hydration follows Markovnikov s rule, and a new similar chiral C is formed. The orignal chiral C remains R, but the new chiral C may be R or S. The two diastereomers are not formed in equal amounts [see Problem 5.20(6)]. (c) (E) is rac-CH3CH2CHBrCH=CH2 (F) is trans- and (G) c -CH3CH2CH=CHCH2Br. The intermediate R in this Sn 1 reaction is a resonance-stabilized (charge-delocalized) allylic cation... 

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