Carbocations tertiary

Carbocations are classified according to their degree of substitution at the positively charged carbon The positive charge is on a primary carbon m CH3CH2" a secondary car bon m (CH3)2CH" and a tertiary carbon m (CH3)3C Ethyl cahon is a primary carbocation isopropyl cation a secondary carbocation and tert butyl cation a tertiary carbocation... 

Why do carbocations rearrange The answer is straightforward once we recall that tertiary carbocations are more stable than secondary carbocations (Section 4 10) Thus rearrangement of a secondary to a tertiary carbocation is energetically favorable As... 

FIGURE 5 7 The first formed carbocation from 3 3 dimethyl 2 butanol is secondary and rearranges to a more stable tertiary carbocation by a methyl migration The major portion of the alkene products is formed by way of the tertiary carbocation... 

FIGURE 5 8 Methyl migration in 1 2 2 tnmethylpropyl cation Structure (a) is the initial second ary carbocation structure (b) is the transition state for methyl migration and structure (c) is the final tertiary carbocation... 

The product of the reaction is derived from the more stable carbocation—in this case it IS a tertiary carbocation that is formed more rapidly than a secondary one... 

Addition begins m the usual way by protonation of the double bond to give m this case a secondary carbocation This carbocation can be captured by chloride to give 2 chloro 3 methylbutane (40%) or it can rearrange by way of a hydride shift to give a tertiary carbocation The tertiary carbocation reacts with chloride ion to give 2 chloro 2 methylbutane (60%)... 

The notion that carbocation formation is rate determining follows from our previous experience and by observing how the reaction rate is affected by the shucture of the aUcene Table 6 2 gives some data showing that alkenes that yield relatively stable carbocations react faster than those that yield less stable carbocations Protonation of ethylene the least reactive aUcene m the table yields a primary carbocation protonation of 2 methylpropene the most reactive m the table yields a tertiary carbocation As we have seen on other occa sions the more stable the carbocation the faster is its rate of formation... 

More stable transition state has some of the character of a tertiary carbocation... 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

Bromo 3 methylbutane The tertiary carbocation then reacts with water to yield the observed product... 

Unbranched primary alcohols and tertiary alcohols tend to react with hydrogen halides without rearrangement The alkyloxonmm ions from primary alcohols react rap idly with bromide ion for example m an Sn2 process Tertiary alcohols give tertiary alkyl halides because tertiary carbocations are stable and show little tendency to rearrange... 

Write the principal resonance structures of o methylbenzyl cation and rn methylbenzyl cation Which one has a tertiary carbocation as a contnbuting resonance form ... 

One way to assess the relative stabilities of these various intermediates is to exam me electron delocalization m them using a resonance description The cyclohexadienyl cations leading to o and p mtrotoluene have tertiary carbocation character Each has a resonance form m which the positive charge resides on the carbon that bears the methyl group... 

Oxygen stabilized carbocations of this type are far more stable than tertiary carbocations They are best represented by structures m which the positive charge is on oxygen because all the atoms have octets of electrons m such a structure Their stability permits them to be formed rapidly resulting m rates of electrophilic aromatic substitution that are much faster than that of benzene... 

The tertiary carbocation formed m this step can react according to any of the various reaction pathways available to carbocations One of these is loss of a proton to give a double bond... 

Loss of a proton from the tertiary carbocation formed m this step gives limonene an abundant natural product found m many citrus fruits Capture of the carbocation by water gives a terpmeol also a known natural product... 

The same tertiary carbocation serves as the precursor to numerous bicyclic monoterpenes A carbocation having a bicychc skeleton is formed by intramolecular attack of the rr electrons of the double bond on the positively charged carbon... 

C =CH2 IS more reactive because it gives a tertiary carbocation when... 

A hydride shift produces a tertiary carbocation a methyl shift produces a secondary carbocation... 

Cycloahphatics capable of tertiary carbocation formation are candidates for nucleophilic addition of nitriles. HCN in strong sulfuric acid transforms 1-methyl-1-cyclohexanol to 1-methyl-1-cyclohexylamine through the formamide (47). The terpenes pinene (14) [2437-95-8] and limonene [5989-27-5] (15) each undergo a double addition of HCN to provide, after hydrolysis, the cycloahphatic diamine 1,8-menthanediamine (16) (48). 

This reanangement is shown in orbital terms in Figure 5.8. The relevant orbitals of the secondary car bocation are shown in structure (a), those of the transition state for reanangement in (b), and those of the tertiary carbocation in (c). Delocalization of the electrons of the C—CH3 a bond into the vacant p orbital of the positively charged car bon by hyperconjugation is present in both (a) and (c), requires no activation energy, and... 

Secondary carbocation Tertiary carbocation (G is a migrating group it may be either a hydrogen or an alkyl group)... 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that fonn tertiary carbocations. In some cases reaction conditions can be developed that favor the formation of higher molecular-weight polymers. Because these reactions proceed by way of carbocation intermediates, the process is refened to as cationic polymerization. 

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