Carbocations, allylic

Allylic carbocations are carbocations in which the positive charge is on an allyhc carbon. Allyl cation is the simplest aUyhc carbocation. 

A substantial body of evidence indicates that aUyhc carbocations are more stable than simple alkyl cations. For example, the rate of solvolysis of a chloride that is both tertiary and allylic is much faster than that of a typical tertiary alkyl chloride. 

The first-order rate constant for ethanolysis of the allylic chloride 3-chloro-3-methyl-l-butene is over 100 times greater than that of fert-butyl chloride at the same temperature. 

Both compounds react by an SnI mechanism, and their relative rates reflect their activation energies for carbocation formation. Since the allylic chloride is more reactive, we reason that it ionizes more rapidly because it forms a more stable carbocation. Structurally, the two carbocations differ in that the allylic carbocation has a vinyl substiment on its positively charged carbon in place of one of the methyl groups of tert-butyl cation. 

A vinyl group stabilizes a carbocation more than does a methyl group. Why  

Allylic carbocations are more stable than simple alkyl cations because the C=C group acts as an electron-donating substituent to the positively charged carbon. We can represent this electron donation in resonance terms as  

The double bond is conjugated to the positively charged carbon. Instead of being localized on a single carbon, the positive charge is shared by the carbons at each end of the three-carbon allyl unit. Likewise, the electrons in the tt bond are delocalized over three carbons instead of two. The two resonance forms of allyl cation are equivalent, and the positive charge is shared equally by the carbons at each end. 

Another way to show charge dispersal and rr-electron delocalization in allylic systems is by a dashed-line representation in the resonance hybrid  

It is important to recognize that the center carbon does not bear a positive charge and the + sign above the middle of the dashed line is meant only to signify that the allylic unit as a group is positively charged. 

In our discussion of reactions, we saw that two Lewis structures represent the resonance stabihza-tion of an allylic carbocation (Section 10.3). The positive charge is located on C-1 in one resonance form and on C-3 in the other. 

Dashed lines show the delocahzation of two charges in the allylic carbocation. The allylic carbocation C-1 and C-3 each have a partial positive charge of+1/2 C-2 has a charge of zero. 

The allyl carbocation is much more stable than a primary alkyl carbocation. We estimate that primary allylic carbocations and secondary carbocations have about the same stabihty since they form in reactions at comparable rates. 

For example, 3-chloro-3-methyl-l-butene that reacts with solvents such as water or ethanol is 100 times faster than rerr-butyl chloride. 

3-chloro-3-methyl-l-butene even more reactive in Sj,jl reactions 

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Not all the properties of alkenes are revealed by focusing exclusively on the func tional group behavior of the double bond A double bond can affect the proper ties of a second functional unit to which it is directly attached It can be a sub stituent for example on a positively charged carbon in an allylic carbocation, or on a carbon that bears an unpaired electron in an allylic free radical, or it can be a substituent on a second double bond in a conjugated diene... 

Conjugare is a Latin verb meaning to link or yoke together and allylic carbocations allylic free radicals and conjugated dienes are all examples of conjugated systems In this chapter we 11 see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone... 

Electron delocalization m allylic carbocations can be indicated using a dashed line to show the sharing of a pair of rr electrons by the three carbons The structural formula IS completed by placing a positive charge above the dashed line or by adding partial pos itive charges to the carbons at the end of the allylic system... 

Because the positive charge m an allylic carbocation is shared by two carbons there are two potential sites for attack by a nucleophile Thus hydrolysis of 3 chloro 3 methyl 1 butene gives a mixture of two allylic alcohols... 

The carbocation formed on ionization of 1 chloro 3 methyl 2 butene is the same allylic carbocation as the one formed on ionization of 3 chloro 3 methyl 1 butene and gives the same mixture of products... 

Later m this chapter we 11 see how allylic carbocations are involved m elec trophihc addition to dienes and how the principles developed m this section apply there as well... 

Allylic carbocations and allylic radicals are conjugated systems involved as reactive intermediates m chemical reactions The third type of conjugated system that we will examine conjugated dienes, consists of stable molecules... 

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

Use Learning By Modeling to view the charge distribu tion in the allylic carbocation shown in the equation... 

This chapter focused on the effect of a carbon-carbon double bond as a stabilizing substituent on a positively charged carbon m an allylic carbocation, on a carbon bearing... 

Section 10 10 Protonation at the terminal carbon of a conjugated diene system gives an allylic carbocation that can be captured by the halide nucleophile at either of the two sites that share the positive charge Nucleophilic attack at the carbon adjacent to the one that is protonated gives the product of direct addition (1 2 addition) Capture at the other site gives the product of conjugate addition (1 4 addition)... 

The carbocation formed m this step is a cyclohexadienyl cation Other commonly used terms include arenium ion and a complex It is an allylic carbocation and is stabilized by electron delocalization which can be represented by resonance... 

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 radical is much more stable if both stmctures exist. Quantum mechanical theory implies that the radical exists in both states separated by a small potential. Moreover, both molecular orbital theory and resonance theory show that the allyl carbocation is relatively stable. 

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 allylic carbocation resulting from protonation of the center carbon might seem the obvious choice, but, in fact, the kinetically favored protonation leads to the vinyl cation... 

Vinyl and phenyl mfluoromethyl groups are reactive in the presence of aluminum chloride [10] Replacement of fluorine by chlorine often occurs Polyfluori-nated trifluoromethylbenzenes form reactive a,a-difluorobenzyl cations in antimony pentafluoride [11] 1 Phenylperfluoropropene cyclizes in aluminum chloride to afford 1,1,3-trichloro 2 fluoroindene [10] (equation 10) The reaction IS hypothesized to proceed via an allylic carbocation, whose fluoride atoms undergo halogen exchange... 

FIGURE 10.2 Electron delocalization in an allylic carbocation. (a) The tt orbital of the double bond, and the vacant 2p orbital of the positively charged carbon, (b) Overlap of the tt orbital and the 2p orbital gives an extended TT orbital that encompasses all three carbons. The two electrons in the tt bond are delocalized over two carbons in part (a) and over three carbons in part (b). 

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