Electrophilic addition reaction transition state

As in the case of electrophilic addition, the transition state for formation of the intermediate [e.g., (86)] contains no bonds that are not present either on it or the reactants. The same is true for the transition state [e.g., (88)] for the elimination reaction, which indeed is the converse of a reverse substitution in which displaces, e.g., NO2 from the ring. Both reactions should therefore be of BEP type. Moreover, since electrophilic substitution must be exothermic overall in order to occur, the conversion of the intermediate to the product must be more exothermic than its reversion to the reactants. The reaction path for the system should then have the form indicated in Fig. 5.30, the first maximum being higher than the second. Consequently the first step in such reactions, i.e., addition of the electrophile, should normally be rate determining. This indeed is usually the case, replacement of deuterium taking place as easily as that of hydrogen. The absence of a deuterium isotope effect shows that the CH or CD bond must be intact in the transition state. 

Although the enamine (30) underwent addition reaction with ethyl azido-dicarboxylate, it failed to add another mole of jS-nitrostyrene. In a similar manner the morpholine enamine of 2-methylcyclohexanone also failed to react with this olefin, i.e., jS-nitrostyrene, which is undoubtedly due to the 1,3-diaxial interaction between the methyl group and the incoming electrophile in the transition state. 

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

In an electrophilic addition reaction, the transition state for alkene protonation resembles the carbocation intermediate. 

Electrophilic addition reactions are some of the most important processes (21) involving a cationic transition state or intermediate. The acid-catalysed hydration of arylalkenes such as styrene [32] is typical of such processes... 

To und niiand the reasono for the Markovnikov oriofitation of electrophilic addition reactions, we need to learn more about the structure and stability of carbocations and about the general nature of reactions and transition states. The fu-ac point to explore involves structure. 

We have seen that the rate of a reaction is determined by the free energy of activation, which is the difference between the free energy of the transition state and the free energy of the reactant (Section 3.7). The more stable the transition state, the smaller is the free energy of activation, and therefore, the faster is the reaction. Because the free energy of activation for the formation of the ferf-butyl cation is less than that for the formation of the isobutyl cation, the ferf-butyl cation will be formed faster. Thus, in an electrophilic addition reaction, the more stable carbocation will be the one that is formed more rapidly. 

Therefore, the more stable transition state is achieved by adding the nucleophile to the most substituted sp carbon—the one bonded to the lesser number of hydrogens— because the partial positive charge will be on a secondary carbon rather than on a primary carbon. Thus, this reaction also follows the general rule for electrophilic addition reactions The electrophile adds to the sp carbon that is bonded to the greater number of hydrogens. In this case, the electrophile is Br. ... 

An alkyne is less reactive than an alkene. This might at first seem surprising because an alkyne is less stable than an alkene (Figure 6.2). However, reactivity depends on AG, which in turn depends on the stability of the reactant and the stability of the transition state (Section 3.7). For an alkyne to be both less stable and less reactive than an alkene, two conditions must hold The transition state for the first step (the rate-limiting step) of an electrophilic addition reaction for an alkyne must be less stable than the transition state for the first step of an electrophilic addition reaction for an alkene, and the difference in the stabilities of the transition states must be greater than the difference in the stabilities of the reactants so that AGli yne > alkene (Fi UTC 6.2). 

Why is the transition state for the first step of an electrophilic addition reaction for an alkyne less stable than that for an alkene The Hammond postulate predicts that the structure of the transition state will resemble the structure of the intermediate (Section 4.3). The intermediate formed when a proton adds to an alkyne is a vinylic... 

Like an 8 1 reaction, the first step in the electrophilic addition reaction is the rate determining step. According to the Hammond postulate, the transition state for this step resembles the carbocation intermediate. Structural features that stabilize the carbocation intermediate also stabilize the transition state and accelerate the electrophilic addition reaction. 

The transition state structure for the addition of molecular fluorine to ethylene VIIc was found by the 3-2IG calculations [17] to closely resemble that for the addition of HCl (IV). Of special interest are the computational results obtained on transition state structures Vlld, Vile for gas phase electrophilic addition reactions of molecular chlorine and bromine to ethylene. Whereas the four-centered structure VIIc for the fluorination of ethylene indicates concerted c/s-addition, which is in accord with experimental finding for this reaction [18], the transition state geometries for chlorination and bromination may be regarded as cyclic halonium ions backed by halogenide counter-ions. Noteworthy is that the calculations [17] predict the heterolysis to occur intrinsically without any assistance from polar solvents. The three-centered structures Vlld, Vile help to clarify the reason for trans-stereoselectivity of the chlorination and bromination reactions of ethylene [1, 19]. 

Water attacks the more substituted ring carbon because it leads to the more stable transition state since the partial positive charge is on a secondary rather than a primary carbon. Therefore, this reaction, too, follows the general mle for electrophilic addition reactions the electrophile (here, Br ) adds to the sp carbon that is bonded to the most hydrogens and the nucleophile (H2O) adds to the other sp carbon. 

Yamabe, S., Minato, T., 8c Inagaki, S. (1988). Ab initio structures of transition-states in electrophilic addition-reactions of molecular halogens with ethene. Journal of the Chemical Society Chemical Communications, 532, 35. 

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