Exothermic reactions transition state

The structural requirements of the mesomeric betaines described in Section III endow these molecules with reactive -electron systems whose orbital symmetries are suitable for participation in a variety of pericyclic reactions. In particular, many betaines undergo 1,3-dipolar cycloaddition reactions giving stable adducts. Since these reactions are moderately exothermic, the transition state can be expected to occur early in the reaction and the magnitude of the frontier orbital interactions, as 1,3-dipole and 1,3-dipolarophile approach, can be expected to influence the energy of the transition state—and therefore the reaction rate and the structure of the product. This is the essence of frontier molecular orbital (EMO) theory, several accounts of which have been published. 16.317 application of the FMO method to the pericyclic reactions of mesomeric betaines has met with considerable success. The following section describes how the reactivity, electroselectivity, and regioselectivity of these molecules have been rationalized. 

One of the focal points of mechanistic interest has been into the nature of the transition state. A postulate which bears heavily on this topic and which is now most commonly referred to as the Hammond postulate (Hammond, 1955) has become central in the study of transition state structure. Hammond s postulate may be stated as follows the interconversion of two states of similar energy on a reaction pathway will involve only a small amount of structural reorganization. A precise interpretation of this postulate leads only to the limited conclusion that transition states of highly exothermic reactions are similar in structure and energy to reactants, while for strongly endothermic reactions transition states resemble products. 

The structural requirements of the mesomeric betaines described in Section III endow these molecules with reactive -electron systems whose orbital symmetries are suitable for participation in a variety of pericyclic reactions. In particular, many betaines undergo 1,3-dipolar cycloaddition reactions giving stable adducts. Since these reactions are moderately exothermic, the transition state can be expected to occur early in the reaction and the magnitude of the frontier orbital interactions, as 1,3-dipole and... 

Because the reaction of a chlorine radical with an alkane to form a primary, secondary, or tertiary radical is exothermic, the transition states resemble the reactants more than they resemble the products (see the Hammond postulate, Section 4.3). The... 

In the alkylation of the exocyclic enolates of cyclohexane rings, the reaction occurs predominately to place the electrophile in the equatorial position (Eq. 11.11, path b). The electrophile avoids the hydrogens placed 1,3 to the nucleophilic carbon. However, because these alkylations are exothermic, the transition states are early, and the stereochemistry is not very sensitive to the alkylating agent. 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

Plot energy (vertical axis) vs. carbon-carbon distance (horizontal axis). Is this reaction endothermic or exothermic Is there a point on the diagram that can be identified as a transition state If so, what is the barrier for this reaction ... 

Obtain the energies of propene, dimethylborane, and 1-propyldimethyl borane, and calculate AH n for dimethylborane addition. Is this reaction exothermic or endothermic Use this result and the Hammond Postulate to predict whether the transition state will be more reactant like or more product like . Compare the geometry of the transition state to that of the reactants and products. Does the Hammond Postulate correctly anticipate the structure of the transition state Explain. 

Radical additions are typically highly exothermic and activation energies are small for carbon30-31 and oxygen centered32,33 radicals of the types most often encountered in radical polymerization, Thus, according to the Hammond postulate, these reactions are expected to have early reactant-like transition states in which there is little localization of the free spin on C(J. However, for steric factors to be important at all, there must be significant bond deformation and movement towards. sp hybridization at Cn. 

A highly exothermic (low activation energy) reaction will generally have a transition state that resembles the reactants 1... 

One notes that the proportionality constant, a, depends on the reaction energy, AEy. Therefore, Eq. (1.3) is not strictly a linear relation between activation energy change and reaction energy. In the extreme limit of high exothermicity of the reaction energy a = 0, and the crossing point of the two curves is at the minimum of curve Vj. In this case the transition state is called early. Its structure is close to that of the reactant state. 

The stabilisation of the intermediate cation explains why the activation energy of the reaction is reduced this will increase its speed considerably. The transition state resembles the cation. There are numerous consequences so far as risk is concerned the speed at which heat evolves increases due to the exothermicity of the reaction and there is an easy polysubstitution, which leads to trisubstitutions that can give rise to very unstable compounds. 

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