Carbocations, allylic halides

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... 

The carbocations formed as intermediates when allylic halides undergo Stvfl reactions have their positive charge shared by the two end carbons of the allylic system and may be attacked by nucleophiles at either site Products may be formed with the same pattern of bonds as the starting allylic halide or with allylic rearrangement... 

Benzylic halides resemble allylic halides m the readiness with which they form carbocations On comparing the rate of S l hydrolysis m aqueous acetone of the fol lowing two tertiary chlorides we find that the benzylic chloride reacts over 600 times faster than does tert butyl chloride... 

Allylic bromination, 339-340 mechanism of, 339-340 Allylic carbocation, electrostatic potential map of, 377, 489 resonance in, 488-489 SN1 reaction and, 376-377 stability of, 488-489 Allylic halide, S l reaction and. 377 S j2 reaction and, 377-378 Allylic protons, ]H NMR spectroscopy and, 457-458... 

Similar carbocation stabilisation can also occur in the hydrolysis of allyl halides, e.g. 3-chloropropene ... 

Allylic halides can undergo slow dissociation to form stabilized carbocations (SnI reaction). Both 3-bromo-l-butene and l-bromo-2-butene form the same allylic carbocation, pictured above, on dissociation. Addition of bromide ion to the allylic carbocation then occurs to form a mixture of bromobutenes. Since the reaction is run under equilibrium conditions, the thermodynamically more stable l-bromo-2-butene predominates. 

Primary benzylic and allylic halides can undergo SN1 reactions because the carbocations are stabilised by resonance. These carbocations have stability similar to that of secondary alkyl carbocations. 

Benzylic and allylic halides readily undergo SnI reactions because they form relatively stable carbocations. While primary alkyl halides (such as CH3CH2Br and CH3CH2CH2Br) cannot undergo SnI reactions because their carbocations are too... 

We have seen that methyl halides and primary alkyl halides undergo only Sn2 reactions because methyl cations and primary carbocations, which would be formed in an Sisjl reaction, are too unstable to be formed in an Sn2 reaction. Tertiary alkyl halides undergo only S jl reactions because steric hindrance makes them unreactive in an Sn2 reaction. Secondary alkyl halides as well as benzylic and allylic halides (unless they are tertiary) can undergo both S jl and Sn2 reactions because they form relatively stable carbocations and the steric hindrance associated with these alkyl halides is generally not very great. Vinylic and aryl halides do not undergo either S jl or Sn2 reactions. These results are summarized in Table 10.6. 

Nucleophilic substitutions of allylic halides provide much of the experimental evidence on which our understanding of allylic carbocations rests. This evidence includes both rate and product studies. 

Allylic halides are more reactive in nucleophilic substitutions than nonallylic ones, even in reactions that don t involve carbocations. In a typical 8 2 reaction, allyl chloride reacts with potassium iodide in acetone 80 times faster than 1-chloropropane. 

Sections The carbocations formed as intermediates when allylic halides undergo SnI... 

Of the simple alkyl halides that we have studied so far, this means (for all practical purposes) that only tertiary halides react by an S l mechanism. (Later we shall see that certain organic halides, called allylic halides and benzylic halides, can also react by an S l mechanism because they can form relatively stable carbocations see Sections 13.4 and 15.15.)... 

The C - X bond of an allyl halide (e.g., allyl chloride) can also be cleaved readily, as the resulting allyl carbocation is stabilized by resonance. Hence, allyl chloride gives a white precipitate of AgCl readily, when boiled with alcoholic silver nitrate solution. 

Part Three. The benzyl (and allyl) halides are a special case they have resonance. To see how the charge is delocalized in the benzyl carbocation, request two plots the electrostatic potential mapped onto a density surface and the LUMO mapped onto a density surface. Submit these for calculation at the AMI semiempirical level. On a piece of paper, draw the resonance-contributing structures for the benzyl cation. Do the computational results agree with the conclusions you draw from your resonance hybrid ... 

The fast reaction of benzylic and allylic halides is a result of the resonance stabilization that is available to the intermediate carbocations formed. Tertiary halides are more reactive than secondary halides, which are in turn more reactive than primary or methyl halides because alkyl substituents are able to stabilize the intermediate carbocations by an electron-releasing effect. The methyl carbocations have no alkyl groups and are the least stable of all carbocations mentioned thus far. Vinyl and aryl carbocations are extremely unstable because the charge is localized on an sp -hybridized carbon (double-bond carbon) rather than one that is sp -hybridized. 

The majority of grafting from syntheses have been accomplished by converting polymeric halides into macro-initiators. Allyl halides provide suitably stable carbocation derivatives and consequently the partial dehydrohalogenation of poly(vinyl chloride) has been found to enhance the rate of styrene grafting in the presence of aluminium chloride. Similar sites available in chloroprene were activated by silver hexafluorophosphate to induce grafting of IBVE. Nitrosyl and... 

Under the appropriate conditions, tertiary alkyl halides, benzylic halides, and allylic halides, that is, species capable of generating stable carbocations (Table 7.2), are found to undergo reactions that are generally classified as SnI. 

When there is unsaturation a,P to the carbon bearing the halogen undergoing substitution (e.g., in allylic halides) both SnI vide supra) and Sn2 reactions can occur. The normal SnI reaction produces a carbocation or carbocations, which can lead to a set of products different from those found in the Sn2 reaction (see Problem 7.5). 

The mechanism of this reaction is exactly the same as that shown for 3-chloro-3-methyl-l-butene in Mechanism 10.1 only the structure of the starting allylic halide is different. The same carbocation is the key intermediate in both cases. 

Benzylic halides and allylic halides also undergo El reactions, because they form relatively stable carbocations. 

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