Cyclohexadienyl cations

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... 

When a molecular orbital method was used to calculate the charge distribution in cyclohexadienyl cation it gave the results indicated How does the charge at each carbon compare with that deduced by examining the res onance structures for cyclohexadienyl cation ... 

Most of the resonance stabilization of benzene is lost when it is converted to the cyclohexadienyl cation intermediate In spite of being allylic a cyclohexadienyl cation IS not aromatic and possesses only a fraction of the resonance stabilization of benzene... 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Figure 12 3 adapts the general mechanism of electrophilic aromatic substitution to the nitration of benzene The first step is rate determining m it benzene reacts with nitro mum ion to give the cyclohexadienyl cation intermediate In the second step the aro maticity of the ring is restored by loss of a proton from the cyclohexadienyl cation... 

Complexation of bromine with iron(III) bromide makes bromine more elec trophilic and it attacks benzene to give a cyclohexadienyl intermediate as shown m step 1 of the mechanism (Figure 12 6) In step 2 as m nitration and sulfonation loss of a proton from the cyclohexadienyl cation is rapid and gives the product of electrophilic aromatic substitution... 

Two of the n electrons of benzene are used to form a bond to bromine and give a cyclohexadienyl cation intermediate... 

Step 2 Loss of a proton from the cyclohexadienyl cation yields bromobenzene... 

Figure 12 7 illustrates attack on the benzene ring by tert butyl cation (step 1) and subsequent formation of tert butylbenzene by loss of a proton from the cyclohexadienyl cation intermediate (step 2)... 

FIGURE 12 7 The mechanism of Friedel-Crafts alkylation The molecular model depicts the cyclohexadienyl cation intermediate formed in step 1... 

Why IS there such a marked difference between methyl and trifluoromethyl substituents m their influence on electrophilic aromatic substitution s Methyl is activating and ortho para directing trifluoromethyl is deactivating and meta directing The first point to remember is that the regioselectivity of substitution is set once the cyclohexadienyl cation intermediate is formed If we can explain why... 

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... 

When we examine the cyclohexadienyl cation intermediates involved m the nitra tion of (trifluoromethyl)benzene we And that those leading to ortho and para substitu tion are strongly destabilized... 

Sections How substituents control rate and regioselectivity m electrophilic aro 12 10-12 14 matic substitution results from their effect on carbocation stability An electron releasing substituent stabilizes the cyclohexadienyl cation inter mediates corresponding to ortho and para attack more than meta... 

Wnte a structural formula for the most stable cyclohexadienyl cation intermediate formed in each of the following reactions Is this intermediate more or less stable than the one formed by electrophilic attack on benzene" ... 

Cyclohexadienyl cation intermediate nitro group IS destabilizing... 

Arenium ion (Section 12 2) The carbocation intermediate formed by attack of an electrophile on an aromatic substrate in electrophilic aromatic substitution See cyclohexadienyl cation... 

Cyclohexadienyl cation (Section 12 2) The key intermediate in electrophilic aromatic substitution reactions It is repre sented by the general structure... 

Substituted aromatics, eg, aLkylbenzenes, sometimes experience attack at the substituent position by NO/ (7). A cyclohexadienyl cation is formed it is unstable and the nitro group migrates on the ring to a carbon atom that is attached to a hydrogen. Loss of the proton results in a stable nitroaromatic. 

In order for a substitution to occur, a n-complex must be formed. The term a-complex is used to describe an intermediate in which the carbon at the site of substitution is bonded to both the electrophile and the hydrogen that is displaced. As the term implies, a a bond is formed at the site of substitution. The intermediate is a cyclohexadienyl cation. Its fundamental structural characteristics can be described in simple MO terms. The a-complex is a four-7t-electron delocalized system that is electronically equivalent to a pentadienyl cation (Fig. 10.1). There is no longer cyclic conjugation. The LUMO has nodes at C-2 and C-4 of the pentadienyl structure, and these positions correspond to the positions meta to the site of substitution on the aromatic ring. As a result, the positive chargex)f the cation is located at the positions ortho and para to the site of substitution. 

If the transition state resembles the intermediate n-complex, the structure involved is a substituted cyclohexadienyl cation. The electrophile has localized one pair of electrons to form the new a bond. The Hiickel orbitals are those shown for the pentadienyl system in Fig. 10.1. A substituent can stabilize the cation by electron donation. The LUMO is 1/13. This orbital has its highest coefficients at carbons 1, 3, and 5 of the pentadienyl system. These are the positions which are ortho and para to the position occupied by the electrophile. Electron-donor substituents at the 2- and 4-positions will stabilize the system much less because of the nodes at these carbons in the LUMO. 

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