Isopropyl carbocation

An isopropyl carbocation cannot experience a beta fission (no C-C bond beta to the carbon with the positive charge).It may either abstract a hydride ion from another hydrocarbon, yielding propane, or revert back to propene by eliminating a proton. This could explain the relatively higher yield of propene from catalytic cracking units than from thermal cracking units. 

Although the geometric relationship suggested by Shiner and by Sunko and their co-workers clearly demonstrates that hyperconjugation is the major contributor to the secondary /3-deuterium KIE in carbocation reactions, Williams (1985) has suggested that there is a significant inductive component to these KIEs. Williams used ab initio MO methods to calculate the geometries of the substrates and the isopropyl carbocation formed in a gas-phase heterolysis (30) of series of isopropyl derivatives at the RHF/4-31G level. 

These structures were then used to generate the force fields and calculate the secondary /3-deuterium-d6 equilibrium isotope effects (EIEs) for the formation of the isopropyl carbocation (Table 30). Because the transition states for formation of the carbocation will be close to the structure of the carbocation, these KIEs should be excellent approximations of the maximum secondary /3-deuterium KIEs expected for the limiting SN1 solvolytic reaction. 

The isopropyl benzene results from a rearrangement of the initially formed propyl carbocation to the more stable isopropyl carbocation. 

Reaction of 1-chloropropane with the Lewis acid AICI3 forms a carbocation. The less stable propyl carbocation undergoes a hydride shift to produce the more stable isopropyl carbocation, which reacts with benzene to give isopropylbenzene. 

Answer The delocalized tertiary carbocation, Ph(CH3)2C, is the most stable. The rcrt-butyl carbocation, (CH3)3C, stabilized by three methyl groups, is of intermediate stability, whereas the isopropyl carbocation, (CH3)2HC, stabilized by only two methyl groups, is the least stable. 

Isopropanol formation is thought to proceed via an intermediate isopropyl carbocation generated by interaction with water and the acid catalyst, which is then hydrated with water. Recovery is by distillation, to the 91% azeotrope, or by ternary distillation of the azeotrope to 100% isopropanol. 

Let us see how reaction rates are related to Markovnikov s Rule. In electrophilic addition reactions, more stable carbocations are formed more rapidly than less stable carbocations. This is because more stable carbocations are lower in energy than less stable carbocations, and it follows that the activation energy for the formation of more stable carbocations is also lower. For example, both isopropyl and propyl cations could be formed from propene and H (eq. 3.21), but the isopropyl cation is more stable (i.e., much lower in energy) than the propyl cation (Figure 3.12). Formation of the isopropyl cation, therefore, has a lower activation energy E and thus, the isopropyl carbocation is formed more rapidly than the propyl cation. Hence, the regioselectivity of electrophilic additions is the result of competing first steps, in which the more stable carbocation is formed at a faster rate. 

Note that the reverse reactions to those shown in Scheme 2.1.1 play a very important role for the formation of new covalent bonds. In addition, radicals or charged species can attack neutral compounds to form different radicals and charged species involving new chemical bonds. Scheme 2.1.2 gives examples of some practical relevance in chemical technology. In transformation (a), a methyl radical attacks a chlorine molecule to form chloromethane and a chlorine radical. This reaction is one of the key steps in technical methane chlorination. In transformation (b), an isopropyl carbocation attacks water to form isopropanol with the release of a proton, the key mechanism in the technical production of isopropanol and all higher secondary and tertiary alcohols. In transformation (c), an anionic methanolate ion acts as starter for an anionic polymerization reaction - one possible starting step in technical anionic polymerization. 

The reaction of propene with HBr is shown below. In this case, two different carbocations are possible If the hydrogen atom had bonded to C-2, a primary, -propyl carbocation would have formed if the hydrogen bonds to C-3 of propene, a secondary, isopropyl carbocation forms. Since a secondary carbocation is more stable than a primary carbocation, only 2-bromoproane forms. 

The isopropyl carbocation has the positive charge on a secondary carbon atom. The -propyl carbocation has the positive charge on a primary carbon atom. The isopropyl carbocation forms, rather than the -propyl cation, because the larger number of alkyl groups attached to a positively charged carbon atom help to stabilize the charge. 

When we examine the mass spectrum of a branched hydrocarbon such as 2,2-dimethylpen-tane, we do not find a parent ion (Figure 14.25). The ion in the spectrum with the largest mass forms when the parent ion fragments to give a tertiary 2-methylpentyl carbocation and a neutral methyl radical. Alternatively, the parent ion can fragment to give a tert-huty carbocation and an -propyl radical. The tert-huxyX carbocation miz 57) is the base peak. It in turn fragments to form an isopropyl carbocation miz 43) and a methyl radical. 

The parent ion can lose a chlorine atom to give an isopropyl carbocation in a process called a-cleav-age. The resulting ion has an mlz of 43. This process produces the base peak. 

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