Carbocation Hammond postulate

Markovnikov s rule can be restated by saying that, in the addition of HX to an aikene, the more stable carbocation intermediate is formed. This result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an aikene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formation. 

According to the Hammond postulate (Section 6.10), any factor that stabilizes a high-energy intermediate also stabilizes the transition state leading to that inlermediate. Since the rate-limiting step in an S l reaction is the spontaneous, unimolecLilar dissociation of the substrate to yield a carbocation, the reaction is favored whenever a stabilized carbocation intermediate is formed. The more stable the carbocation intermediate, the faster the S l reaction. 

The Hammond postulate says that any factor stabilizing the intermediate carbocation should increase the rate of an S l reaction. Solvation of the carbocation—the interaction of the ion with solvent molecules—has just such an effect. Solvent molecules orient around the carbocation so that the electron-rich ends of the solvent dipoles face the positive charge (Figure 11.14), thereby lowering the energy of the ion and favoring its formation. 

Markovnikov s rule. 191-193 alkene additions and, 191-193 alkyne additions and. 263 carbocation stability and. 192-193 Hammond postulate and, 198-199 hydroboration and. 224-225 oxymercu ration and, 222 Mass number (A), 4 Mass spectrometer, double-focusing, 411... 

Another method for evaluating carbocation stability involves the measurement of solvolysis rates (14,45). Typically, the transition state of the rate-determining step in SN1 reactions is assumed to closely resemble the intermediate ion pair, on the basis of the Hammond postulate (46). Thus, the free energy of activation for this reaction, AG, reflects the relative thermodynamic stabilities of the intermediate carbocations. 

We may ask How does Y know which side will give the more stable carbocation As in the similar case of electrophilic aromatic substitution (p. 508), we invoke the Hammond postulate and say that the lower energy carbocation is preceded by the lower energy transition state. Markovnikov s rule also applies for halogen substituents because the halogen stabilizes... 

A most useful application of the Hammond postulate involves reactions which proceed by the formation of unstable intermediates, such as die carbocations,... 

The Hammond postulate is best applied to reactions widi unstable intermediates, such as the carbocations in die above example. In such cases die transition state is late and die activated complex more resembles the intermediate. Thus... 

In this section, solvolysis reactions are described which are thought to proceed via silyl-substituted carbocations. The reader should be aware of the fact that nearly all effects which are described here are of purely kinetic origin and therefore refer to energy differences between ground states and transition states. Hence they are not strictly applicable to the intermediate silyl-substituted carbocations, although the Hammond postulate suggests a close structural resemblance between the transition state for the ionization and the formed carbocation. 

The resonance structure on the right is much more stable than the one on the left because the octet rule is satisfied for all of the atoms. The cation is actually the conjugate acid of a ketone. Because this cation is so much lower in energy than the usual carbocation, the transition state leading to it is also lower in energy (Hammond postulate). Thus, it is formed readily and the initial addition of the proton is very fast. 

The second step in the electrophilic addition of HC1 to an alkene is exergonic. According to the Hammond postulate, the transition state should resemble the carbocation intermediate. 

How does the Hammond postulate apply to electrophilic addition reactions Tlie foriTUition of a carbocation by protonation of an alkene is an endergonic step. Thus, the transition state for alkene protonation structurally resembles the... 

In the Sj-jl reaction, the rate-determining step is the formation of the carbocation, an endothermic reaction. According to the Hammond postulate, the stability of the carbocation detennines the rate of its formation. 

Like an S l reaction, more substituted alkyl halides yield more substituted (and more stable) car-bocations in the rate-determining step. Increasing the stability of a carbocation, in turn, decreases Eg for the slow step, which increases the rate of the El reaction according to the Hammond postulate. 

Path [ 1 ] forms a highly unstable 1 ° carbocation, whereas Path f2] forms a more stable 2° carbocation. According to the Hammond postulate. Path [2] is faster because formation of the carbocation is an endothermic process, so the transition state to form the more stable 2° carbocation is lower in enei (Figure 10.11). 

In Step [3] two carbocations are possible but only one is formed. Markovnikov addition in Step [3] places the H on the terminal carbon (Cl) to form the more substituted carbocation A, rather than the less substituted carbocation B. Because the more stable carbocation is formed fa.ster— another example of the Hammond postulate—carbocation A must be more stable than carbocation B. 

To understand why some substituents make a benzene ring react faster than benzene itself (activators), whereas others make it react slower (deactivators), we must evaluate the rate-determining step (the first step) of the mechanism. Recall from Section 18.2 that the first step in electrophilic aromatic substitution is the addition of an electrophile (E ) to form a resonance-stabilized carbo-cation. The Hammond postulate (Section 7.15) makes it pos.sible to predict the relative rate of the reaction by looking at the stability of the carbocation intermediate. 

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