Endothermic reaction substitution

The heat of reaction. A//,., is defined to be negative for exothermic reactions and positive for endothermic reactions. Substitution of Equation (6.3.88) into (6.3.87) results in ... 

For cases where AH0 is essentially independent of temperature, plots of in Ka versus 1/T are linear with slope —(AH°/R). For cases where the heat capacity term in equation 2.2.7 is appreciable, this equation must be substituted in either equation 2.5.2 or equation 2.5.3 in order to determine the temperature dependence of the equilibrium constant. For exothermic reactions (AH0 negative) the equilibrium constant decreases with increasing temperature, while for endothermic reactions the equilibrium constant increases with increasing temperature. 

Equation 6-74 is a first order differential equation substituting Equations 6-70 and 6-78 for the temperature, it is possible to simulate the temperature and time for various conversions at AXa = 0.05. Table 6-4 gives the computer results of the program BATCH63, and Figure 6-7 shows profiles of both fractional conversion and temperature against time. The results show that for the endothermic reaction of (+AHR/a) = 15.0 kcal/gmol, the reactor temperature decreases as conversion increases with time. 

Clearly the frontier orbital explanation for reactivity, and for ortho/para and met a selectivity, in the conventional mechanism for aromatic electrophilic substitution is less than compelling—the orbital effects are not opposing the standard pattern, but the original argument based on the stability of the intermediates remains more satisfying, just as it should for an endothermic reaction. 

Because carbocations are key intermediates in many nucleophilic substitution reactions, it is important to develop a grasp of their structural properties and the effect substituents have on stability. The critical step in the ionization mechanism of nucleophilic substitution is the generation of the tricoordinate carbocation intermediate. For this mechanism to operate, it is essential that this species not be prohibitively high in energy. Carbocations are inherently high-energy species. The ionization of r-butyl chloride is endothermic by 153kcal/mol in the gas phase. ... 

Radical substitution reactions by iodine are not practical because the abstraction of hydrogen from hydrocarbons by iodine is endothermic, even for stable radicals. The enthalpy of the overall reaction is also slightly endothermic. Thus, because of both the kinetic problem excluding a chain reaction and an unfavorable equilibrium constant for substitution, iodination cannot proceed by a radical-chain mechanism. 

The reaction of thiyl radicals with silicon hydrides (Reaction 8) is the key step of the so-called polariiy-reversal catalysis in the radical chain reduction. The reaction is strongly endothermic and reversible with alkyl-substituted silanes (Reaction 8). For example, the rate constants fcsH arid fcgiH for the couple triethylsilane/ 1-adamantanethiol are 3.2 x 10 and 5.2xlO M s respectively. 

Benzene addition to Ir(P Pr3)2Cl is an exothermic reaction (22kcal mol-1), while addition to Ir(P Pr3)2(CO)Cl is endothermic (—5kcal mol-1).501 The reaction enthalpies of substitution reactions to complexes containing the Ir(P1Pr3)2Cl fragment are supplied. Reaction of Ir(P1Pr3)2Cl with 2-pyridyl esters gives a p2 (C,0)-bound ketene, (307), where R2 = R1 = aryl or R2 = aryl,... 

As can be seen from the data presented, the high energies of complex formation decrease sharply the endothermicity of the retro-Wittig type decomposition and, moreover, fundamentally change the reaction mechanism. As has been shown for betaines (")X-E14Me2-CH2-E15( + )Me3 (X = S, Se E14 = Si, Ge E14 = P, As), the reaction occurs as bimolecular nucleophilic substitution at the E14 atom. For silicon betaines, the transition states TS-b-pyr with pentacoordinate silicon and nearby them no deep local minima corresponding to the C-b complexes can be localized in the reaction coordinate. 

Using standard references and protocol, we find the three reactions are respectively endothermic by ca 2, 8 and 6 kJmol-1, or ca 2, 4 and 3 kJmol-1 once one remembers to divide by 2 the last two numbers because the allene is dialkylated. So doing, from equations 10 and 11 we find an average ca 3 kJmol-1 (per alkyl group) lessened stability for alkylated allenes than the correspondingly alkylated alkenes. This is a small difference that fits most naturally in the study of substituted cumulenes such as ketenes and ketenimines, i.e. not in this chapter. But it is also a guideline for the understanding of polyenes with more cumulated double bonds. 

An extensive computational analysis expanded the range of the c-d distances for reactive cyclic enediynes to 2.9-3.4 A.38 By comparing unsubstituted enediynes with dialkyl-substituted enediynes, it was found that the activation enthalpy is dependent on factors other than the c-d distance and that reactivity hinges on a subtle interplay of steric and electronic effects that accompany distortion caused by incorporation into a macrocycle. For example, since alkyl substituents stabilize acetylenic bonds to a greater extend than olefinic bonds,39 such substituents stabilize the starting material, thus increasing both the activation barrier and the reaction endothermicity. 

In accordance with the increased charge separation in the transition state and products, the gas-phase EA of 22.6 kcal mol-1 for the reaction reduces to just 4.4 kcal mol-1 with inclusion of semiempirical solvation energies in water while the overall reaction, which is very endothermic in the gas phase becomes exothermic by 5.1 kcal mol-1 with solvation.179 The calculated EA is lower than the experimental values for substitution by A-methylaniline in methanol which fall in the range of 6-15 kcalmol-1 (Table 5).42,43 However, in aqueous solution these barriers would be lower than in methanol. 

The addition and insertion of one molecule of CO into the lithium-hydrogen bond of the (LiFI)4 model compound turned out to be a slightly endothermic partial reaction (23 kJmoU ) (Table 5) , which is hard to believe in light of the successful experiment in LXe. Using (LiMe)4 as substitute it was shown that the reaction is actually exothermic... 

Abstraction of a hydride from carbon is almost invariably an endothermic process. The rate of the reaction depends on the stability of the transition structure which closely resembles the product carbocation and is expected to be stabilized by the same factors, among them, substitution by X and C substituents. Nevertheless, initial interactions set the trajectory for the hydride abstraction reaction. The interaction of a C—H bond with a C substituent is shown in Figure 10.7. The feature relevant to the present discussion is that the HOMO which involves some admixture of the C—H bond has been raised in energy. Therefore, attack by electrophiles, while most likely at the n bond of the C substituent, is also possible at the C—H bond. The interaction of an X substituent with a CH bond is shown in Figure 10.7a. In general a single X or C substituent is not sufficient to activate the C—H bond toward hydride abstraction. 

The intrinsic stability of the aromatic n system has two major consequences for the course of reactions involving it directly. First, the aromatic ring is less susceptible to electrophilic, nucleophilic, and free-radical attack compared to molecules containing acyclic conjugated n systems. Thus, reaction conditions are usually more severe than would normally be required for parallel reactions of simple olefins. Second, there is a propensity to eject a substituent from the tetrahedral center of the intermediate in such a way as to reestablish the neutral (An + 2)-electron system. Thus, the reaction is two step, an endothermic first step resulting in a four-coordinate carbon atom and an exothermic second step, mechanistically the reverse of the first, in which a group is ejected. The dominant course is therefore a substitution reaction rather than an addition. 

In a reaction with a moderately long chain, much more of the product will be produced by abstraction (4) than by coupling (5). Cleavage steps like (2) have been called SHl (H for homolytic), and abstraction steps like (3) and (4) have been called Sh2 reactions can be classified as ShI or Sh2 on the basis of whether RX is converted to R by (2) or (3).9 Most chain substitution mechanisms follow the pattern (3), (4), (3), (4). . . . Chains are long and reactions go well where both (3) and (4) are energetically favored (no worse that slightly endothermic, see pp. 683, 693). The IUPAC designation of a chain reaction that follows the pattern (3), (4). . . is ArDR + ARDr (R stands for radical). 

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