Delocalised allylic cation

This clearly reflects formation of the same, delocalised allylic cation (23, cf. p. 105) as an ion pair intermediate from each halide, capable of undergoing subsequent rapid nucleophilic attack by EtOH at either C, or C3 ... 

The deamination of the allylic 3-aminocholest-4-enes is rather remarkable in that the products are not derived from an intermediate delocalised allylic cation 14) [20]. Other cholest 4-en-3-yl derivatives solvolyse to give the allylic cation (14) irrespective of the original C(3)"Configuration, and the cation affords a characteristic mixture of cholesta-3,5-diene and cholest-4-en-3a yl and -SjS-yl derivatives (p. 381). The 3(S-amine (10), however, gave only cholest-4-en 3 -ol (ii) on deamination, whereas the 3a-amine (12) gave cholesta-2,4-diene (13) [20], Both amines therefore react in the same... 

Very few allylic derivatives are stable only H, C, O, N, Si, and F are secure from a [1,3] shift. Halogens may move from one end to the other of the allylic system by an ionic pathway via the delocalised allylic cation 15 or by a radical chain pathway via the dibromoalkyl radical 19. These are both equilibrium reactions and the dominant component is the one with the more heavily substituted double bond 16 or 20. 

C, and the cation 2.111 into the W-shaped cation 2.110 with the same half-life at 35 °C. These correspond to enthalpies of activation of 74 and 101 kJ mol 1 (18 and 24 kcal mol-1), respectively. This measurement only sets lower limits to the rotation barrier of an allyl cation, because it is not known whether rotation takes place in the cations themselves or in the corresponding allyl chlorides with which they could be in equilibrium.141 The barrier in cations is also much affected by solvation and by the degree of substitution at the termini, since the transition structure for rotation draws on such stabilisation more strongly than the delocalised allyl cation does. 

The selectivity of this reaction is rationalised by preferential formation of a delocalised allylic cation rather than a non-allylic sec C-3 cation. 

With unsymmetrical dienes (74a and 74b) and unsymmetrical adducts, the problem of orientation of addition (cf. p. 184) arises. Initial attack will still be on a terminal carbon atom of the conjugated system so that a delocalised allylic intermediate is obtained, but preferential attack will be on the terminal carbon that will yield the more stable of the two possible cations i.e. (75) rather than (76), and (77) rather than (78) ... 

A positively charged ion is electron deficient and acts as an electrophile. The atom that bears the positive charge is the electrophilic centre. In case of a carbocation (Following fig.), this is the carbon atom. Some molecules (e.g. the allylic cation) are able to delocalise their positive charge between two or more atoms in which case all the atoms capable of sharing the charge are electrophilic centres ... 

In this case, the ethanoic acid is a solvent with a high dielectric constant, but it is also a weak nucleophile, and so it provides ideal conditions for a long lived carbonium ion intermediate. This is then statistically attacked at either carbon atom at the ends of the delocalised 1,3-dimethyl allyl cation. As a result this gives rise to the 1 1 ratio of substituted products, i.e. equal amounts of the products that result from the SN1 and SN1 reactions. 

The derived 2,6a-diols (667) rearrange on treatment with acid, apparently through the delocalised homo-allylic cation (668), to give the A -compounds (669) and (670). 

There are more interesting rearrangement possibilities inherent in delocalised cations, e.g. allylic rearrangements. 

Electrochemical oxidation of alkenes results in the removal on one electron from the alkene function to give a 7t-radical-cation where the electron deficiency is delocalised over tire conjugated system. The majority of alkene radical-cations cannot be characterised because they readily lose an allylic proton in aprotic sol-... 

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