Canonical forms carbocations

The stability order can be explained by hyperconjugation and by the field effect. In the hyperconjugation explanation, we compare a primary carbocation with a tertiary. It is seen that many more canonical forms are possible for the latter ... 

These observations are explainable by a pathway in which one end of a bromine molecule becomes positively polarised through electron repulsion by the n electrons of the alkene, thereby forming a n complex with it (8 cf. Br2 + benzene, p. 131). This then breaks down to form a cyclic bromonium ion (9)—an alternative canonical form of the carbocation (10). Addition is completed through nucleophilic attack by the residual Br (or added Ye) on either of the original double bond carbon atoms, from the side opposite to the large bromonium ion Br , to yield the meso dibromide (6) ... 

This is because the greater the number of canonical forms, the greater is the stability. Let us see the canonical forms from a tertiary and a primary carbocation. 

This is called the o route to a non-classical carbocation because of the participation of a o bond. If a n bond is involved then it is called a 71 route. Many chemists aruge that the structure written from 7-norbomenyl cation are not non-classical carbocations because they are not canonical forms but real structures and there is rapid EQUILIBRIUM between them. 

Resonance stabilization can also make n -electron donation much more effective by avoiding the formation of a sextet carbocation. Lone-pair donation from tire oxygen of enol derivatives is very important to the good donor ability of these compounds. The resulting oxonium ion has all valence octets (although positively charged) and is thus stabilized over sextet canonical forms. 

The canonical forms that contribute to the structure of the intermediate carbocation are shown in Scheme 2.15. Once again, one con-... 

It is emphasized that none of these individual structures actually exist and that the structure of the intermediate carbocation is a hybrid of the various canonical forms. 

A In benzenesulfonic acid, strongly electronegative oxygen atoms are bonded to the atom attached to the aromatic ring. Therefore electron density is withdrawn from the ring by both mesomeric and inductive effects. The canonical forms for the carbocation resulting from para attack by an electrophile are shown in Scheme 2.13. Application of the principles described in the chapter should enable... 

For instance, structure (5.23) shows one canonical form with the positive charge located on the opposite end of the benzene ring. Note that the cyclopropane based carbocations (5.20) and (5.23) are both examples of non-classical carbocations, i.e. carbocations in which the positive charge is spread across a number of atoms rather than being localised on one. 

The carbocation of Figure 5.19 can be written either as one of the two canonical forms (5.26) and (5.27) or using an electron smear (5.28) as shown in Figure 5.20. The smear is closer to reality since the molecular orbitals of the molecule will be distributed across the three carbons of the allylic cation. However, it will be able to react as if the positive charge were localised at either end. For each individual reaction, the nature of the other reactive species, the reaction conditions and the nature of the product will determine which way round the system will react. 

Like carbocations, carbanions are also stabilized by resonance. Thus, benzyl carbanion and allyl carbanion are more stable than ethyl carbanion. The stabilization by resonance is due to the delocalization of the negative charge, which is distributed over other carbon atoms. The canonical (resonating) forms of benzyl carbanion and allyl carbanion are given below ... 

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