Carbocation stability, factors affecting

Carbocations are important intermediates in most electrophilic reactions, since the attack of an electrophile on the C=C bond often results in the formation of these species (see Eq. 2). Factors affecting the stability of carbocations are... 

Let s now turn our attention to the transition state for this reaction. What is the structure of the transition state This is an important question because a better understanding of its structure will help in predicting how various factors affect its stability and therefore will aid in predicting how these same factors affect the rate of the reaction. The transition state has a structure that is intermediate between that of the reactant, tot-butyl chloride, and that of the product, the tot-butyl carbocation. It has the bond between the carbon and the chlorine partially broken and can be represented as shown in the following structures ... 

Is the 1,2-addition product also formed more rapidly at higher temperatures even though the 1,4-addition product predominates under these conditions The answer is yes. The factors affecting the structure of a resonance-stabilized allylic carbocation intermediate and the reaction of this intermediate with a nucleophile are not greatly affected by changes in temperature. 

The analysis of pK values to make comparisons of carbanion stability is a standard approach. Yet, as discussed earlier with regard to the use of BDEs and HIAs to analyze radical and carbocation stabilities, other factors can affect the values. This is a more important caveat when evaluating pfCa values than with BDEs or HIAs. As will be discussed in Chapter 5, solvation effects can dramatically change pfCa values, and sometimes in different solvents some pKa values actually reverse in order. Hence, when using pK values to compare carbanion stabilities, it is very important to compare the values under as similar a set of conditions as possible. 

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. 

In the other type of fragmentation, a bond is cleaved so that the positive charge remains with one fragment and the odd electron goes with the other. Only the positive fragment is detected and appears in the mass spectrum. The stability of the product cation and radical determine the favorableness of this type of cleavage. You are already quite familiar with the factors that affect the stability of cations, especially carbocations. Although radicals are inherently more stable than carbocations because they are less electron deficient, they are stabilized by the same factors that stabilize carbocations. Thus, tertiary radicals are more stable than secondary radicals, and secondary radicals are more stable than primary radicals. Resonance stabilization is also important. 

The major carbon centered reaction intermediates in multistep reactions are carbocations (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [CH3], [CHi]", [CHi] , and [ CH2]. 

The question now is, Why is the fert-butyl cation formed faster than the isobutyl cation To answer this, we need to take a look at the factors that affect the stability of carbocations and, therefore, the ease with which they are formed. 

From these studies, it can be concluded that it is not a simple matter to define the general substituent migratory aptitudes in chloiin and bacteriochlorin systems, because distant conjugated peripheral substituents can dramatically affect the stability of carbocation intermediates and hence the products. The facility of the rearrangement of various substituents depends not only on the intrinsic nature of the migratory group but also on the electronic and steric factors present elsewhere on the tetrapyrrolic systems. 

Why is the ferf-butyl cation formed faster To answer this question, we need to look at two things (1) the factors that affect the stability of a carbocation, and (2) how its stability affects the rate at which it is formed. 

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