Carbocations classical

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... 

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

Alkyl halides and sulfonates are the most frequently used alkylating acceptor synthons. The carbonyl group is used as the classical a -synthon. O-Silylated hemithioacetals (T.H. Chan, 1976) and fomic acid orthoesters are examples for less common a -synthons. In most synthetic reactions carbon atoms with a partial positive charge (= positively polarized carbon) are involved. More reactive, "free carbocations as occurring in Friedel-Crafts type alkylations and acylations are of comparably limited synthetic value, because they tend to react non-selectively. 

Up to this point in our discussion, we have considered only carbocations in which the cationic carbon can be 5p -hybridized and planar. When this hybridization cannot be achieved, die carbocations are of higher energy. In a classic experiment, Bartlett and Knox demonstrated that the tertiary chloride 1-chloroapocamphane was inert to nucleophilic substitution. Starting material was recovered unchanged even after refluxing for 48 h in ethanolic silver nitrate. The umeactivity of this compound is attributed to the structure of... 

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. 

When one chooses a radical as the reference system, the stability of a carbocation or carbanion can be defined by the free-energy change for discharging the ion in vacuum, and the change can be approximately described by the classical Born equation (3) (Bom, 1920), provided that the ion is represented by a conducting sphere on which the charge is located. 

The structure of nonclassical carbocations, such as norbomenyl 3, has been the subject of debate since the 1950s when Saul Winstein published his milestone studies on the solvolysis of tosylated norbomenyl compounds. It was proposed that the norbomenyl cation should be represented as the nonclassical structure 4+, with a 3-center, 2-electron cyclic system (3c-2e), rather than as the classical equilibrium... 

The hydration of an alkene double bond under strongly acidic conditions is again a classical reaction that involves a carbocation intermediate, which often leads to various competing reaction products.27 The regiochemistry of the water addition follows the Markovnikov rule.28... 

Organic halides play a fundamental role in organic chemistry. These compounds are important precursors for carbocations, carbanions, radicals, and carbenes and thus serve as an important platform for organic functional group transformations. Many classical reactions involve the reactions of organic halides. Examples of these reactions include the nucleophilic substitution reactions, elimination reactions, Grignard-type reactions, various transition-metal catalyzed coupling reactions, carbene-related cyclopropanations reactions, and radical cyclization reactions. All these reactions can be carried out in aqueous media. 

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. 

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