Classical carbocation

The differentiation of bridged nonclassical from rapidly equilibrating classical carbocations based on NMR spectroscopy was difficult because NMR is a relatively slow physical method. We addressed this question in our work using estimated NMR shifts of the two structurally differing ions in comparison with model systems. Later, this task... 

Proton affinities of ethene (684 121) and 680129) kJ mol-1) measured experimentally correspond with results from ab initio calculations (698 kJ mol-1 130)). MINDO/3 calculations (with AHf(H+) = 1528 kJ mol-1 91)) also deliver a result of comparable value (714 kJ mol 1) when the formation of a classical carbocation during the protonation is assumed. 

The carbocations so far studied are called classical carbocations in which the positive charge is localized on one carbon atom or delocalized by resonance involving an unshared pair of electrons or a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non-classical carbocation the positive charged is delocalized by double or triple bond that is not in the allylic position or by a single bond. These carbocations are cyclic, bridged ions and possess a three centre bond in which three atoms share two electrons. The examples are 7-norbomenyl cation, norbomyl cation and cyclopropylmethyl cation. 

It we accept the existence of bridged ions, the question to be answered is why should such ions be formed in preference to classical carbocations in any particular reaction. One reasonable answer is that when several intermediates are possible, the most stable one is the one likely to be formed. Since charge is most diffuse in the bridged ion than in the classical ion, the bridged structure would be expected to be more stable than the classical structure. 

The non-classical carbocations can be generated if proper substrate is chosen. For example the norbomyl carbocation can be generated by the departure of a leaving group from an exosubstitued substrate... 

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. 

Distinction between neighbouring group participation and non-classical carbocation... 

It is important to distinguish between neighbouring group participation and non-classical carbo-cation. A non-classical carbocation can be formed due to participation of several species as a neigh-bouring group. 

Here we have a c = c group attached to a carbon atom which is adjacent to be carbon atom where nucleophilic substitution can occur and during the course of the reaction becomes bonded of partially bonded to the reaction centre to form a non-classical or bridged ion (Fig. 1 to 1(c)). Thus the rate and/or the stereochemistry may be affected. This explains why the acetolysis of 5 is 1011 times faster than that of 5(a), because it involves the formation of a non-classical carbocation... 

Similarly the addition of bromine to maleic acid giving a racemic mixture of dibromosuccinic acid is again a trans addition. Here also there is first the formation of a bridged (or a non-classical) carbocation followed by the attack of the bromide ion. The various steps are as follows ... 

The addition of halogens and halogen acids to alkenes has been shown to be predominantly trans and where the results do not agree, explanations have been given in terms of steric factors. Dewar has proposed that in all electrophilic addition reactions where a classical carbocation is formed, cis addition is the rule and where there is the preponderance of the trans product, the effect is due to steric factors. 

Therefore summarizing, the problem of addition reactions to alkenes is not so simple and is more complicated than what it looks. It depends on so many factors, e.g., the nature of alkene, the addendum and on the reaction conditions. If the addition proceeds through the formation of bridged ions, then trans addition is the rule. But if it involves a classical carbocation, then cis addition is the mechanism. 

One factor that increases the stability of the bridged ion is the nature of the addendum and in this respect the order is I > Br > Cl > F. This is why with iodine we always get the trans addition and the order has been established experimentally. But where the addendum is not an iodine atom and the classical carbocation is stabilized by resonance, then cis addition takes up which may later on by rearrangement give the trans isomer. It has also been formed that the nature of the solvent also affects the amounts of cis and trans products. 

The "classical" - "non-classical" carbocation controversy concerned the Wagner-Meerwein rearrangement of norbomyl systems ... 

The tertiary 5-methyl and 5-phenyl-2,4-dehydro-5-homoadamantyl cations, 26a, were prepared57 by two different routes either from the corresponding alcohols or the corresponding 2-enclassical carbocations with varying degrees of charge delocalization. 

The side-on (r)2) bonding in M r 2-H2 and other a-complexes has been termed non-classical, on analogy to the 3-center, 2-electron bonding in non-classical carbocations and boranes (Fig. 2). One of the first questions raised when H2 complexes were discovered is whether they would be important in catalytic reactions. As will be shown below the answer is an emphatic yes, as exemplified by the elegant asymmetric catalytic hydrogenation systems of Nobel-laureate Ryoji Noyori. Also, the mechanism of catalytic silane alcoholysis directly involves two different a complexes M(r 2-Si-H) and M(r 2-H2). In both of these systems, the crucial step is heterolytic cleavage of the H H and/or Si-H bond, the primary subject of this review. 

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