Resonance 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... 

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... 

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... 

Wheland intermediates, a complexes, or arenium ions In the case of benzenoid systems they are cyclohexadienyl cations. It is easily seen that the great stability associated with an aromatic sextet is no longer present in 1, though the ion is stabilized by resonance of its own. The arenium ion is generally a highly reactive intermediate and must stabilize itself by a further reaction, although it has been isolated (see p. 504). 

SAMPLE SOLUTION (a) There are the customary three resonance structures for the cyclohexadienyl cation plus a resonance structure (the most stable one) derived by delocalization of the nitrogen lone pair into the ring. 

The three most stable resonance structures for cyclohexadienyl cation are... 

The positive charge is shared equally by the three carbons indicated. Thus the two carbons ortho to the. s/Z-hybridizcd carbon and the one para to it each bear one third of a positive charge (+0.33). None of the other carbons is charged. The resonance picture and the simple MO treatment agree with respect to the distribution of charge in cyclohexadienyl cation. 

A-l. Write the three most stable resonance contributors to the cyclohexadienyl cation found in the ortho bromination of toluene. 

B-2. Which of the following resonance structures is not a contributor to the cyclohexadienyl cation intermediate in the nitration of benzene ... 

In the first step of the actual Ar-SE reaction, a substituted cyclohexadienyl cation is formed from the electrophile and the aromatic compound. This cation and its derivatives are generally referred to as a sigma or Wheland complex. Sigma complexes are described by at least three carbenium ion resonance forms (Figure 5.1). There is an additional resonance form for each substituent, which can stabilize the positive charge of the Wheland complex by a pi electron-donating (+M) effect (see Section 5.1.3). This resonance form is an all-octet formula. 

The dearomatization of benzene requires that a resonance energy estimated at 36 kcal mol-1 be overcome [14]. For this reason, when benzene attacks an electrophile, the end result is typically a substitution product rather than an addition product. The initial cyclohexadienyl cation is unstable to deprotonation by even extremely poor bases, and it therefore loses a proton and returns to an aromatic state. 

PROBLEM 12.1 In the simplest molecular orbital treatment of conjugated systems, it is assumed that the it system does not interact with the framework of charge distribution in cyclohexadienyl cation, it gave the results indicated. How does the charge at each carbon compare with that deduced by examining the most stable resonance structures for cyclohexadienyl cation ... 

The carbocation formed in this step is an arenium ion or cyclohexadienyl cation, also known as a a-complex. It is an allylic carbocation and is stabilized by the electron delocalization represented by resonance among the contributing structures ... 

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