Exothermic processes bond formation

The addition of an ion to butadiene is clearly an exothermic process in the gas phase due to the formation of aa-bond substituting a rc-bond. The agreement of the reaction enthalpies of the reactions (11) and (12) with equal R (except R = H) is surprising (Table 11). 

The solubilization of water in lecithin-reversed micelles has been found to be an exothermic process. This finding confirms that water interacts with the zwitterionic head group of lecithin, promoting the formation of strong intermolecular H bonds [104]. 

This reaction is very exothermic (A// —180 to —200kJ mol-1) and, therefore, seems to be very probable from the thermochemical point of estimation. The pre-exponential factor is expected to be low due to the concentration of the energy on three bonds at the moment of TS formation (see Chapter 3). To demonstrate that this reaction is responsible for the oxidative destruction of polymers, PP and PE were oxidized in chlorobenzene with an initiator and analyzed for the rates of oxidation, destruction (viscosimetrically), and double bond formation (by the reaction with ozone) [131]. It was found that (i) polymer degradation and formation of double bonds occur concurrently with oxidation (ii) the rates of all three processes are proportional to v 1/2, (iii) independent of p02, and (iv) vs = vdbf in PE and vs = 1.6vdbf in PP (vdbf is the rate of double bond formation). Thus, the rates of destruction and formation of double bonds, as well as the kinetic parameters of these reactions, are close, which corroborates with the proposed mechanism of polymer destruction. Therefore, the rate of peroxyl macromolecules degradation obeys the kinetic equation ... 

Nitrosoimines can undergo thermal reaction, a unimolecular, two-step mechanism has been proposed, as shown in Scheme 3.22 [193]. In this mechanism, a concerted electrocyclization is envisioned to form the strained four-membered ring in 41, followed by a presumably forbidden, but highly exothermic, deazetization to give 41. The electrocyclic ring closure is, at first glance, a 4-electron process, analogous to the cyclization of butadiene [194] or acrolein [194, 195]. This would be expected to involve rotation around the C=N bond coupled with C-O bond formation. 

Several computational studies have addressed whether the dipolar cycloaddition of nitronates is a concerted or stepwise process (93,100). Natural population analysis reveals that their is very little zwitterionic character in the transition state. The formation of the C C bond marginally precedes the C—O bond on the basis of calculated bond lengths and orders in the transition structure. These calculations also show that the reaction is a concerted process that is shghtly asynchronous. In addition, the cycloaddition likely proceeds through an early transition state and is overall an exothermic process. 

Water molecules in the vicinity of hydrophobic polymer chains are highly hydrogen bonded and form ordered structures, called ice-bergs, which are similar to the structure of water molecules in ice [96]. Since the formation of ice-bergs lowers both enthalpy and entropy of mixing, this formation is an exothermic process. This is called hydrophobic interaction. Although the energy of the hydrophobic interaction is on the order of sub kcal/mol to a few... 

The mixing of rubbing alcohol and water is an exothermic process, as evidenced by the warmth you feel upon combining the two. At the molecular level, hydrogen bonds are being formed between alcohol molecules and water molecules. Recall from Section 7.1 that the hydrogen bond is a molecule-to-molecule attraction. It is the formation of these intermolecular attractions between alcohol and water molecules that results in the release of heat. 

Therefore, mixing is favored if heat is liberated (i.e., exothermic process) during bond formation. In other words,... 

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