Reactions between Neutral, Dipolar Molecules

Applying Kirkwood s formula to the transition-state theory for the bimole-cular reaction A + B (AB) — C + D and combining Eq. (5-86) with Eq. (5-75), one obtains an expression for the rate constant of a reaction between two dipolar molecules A and B with moments Ra and r to form an activated complex with dipole moment r [2]  

Based on the same premises, but with some modification of the electrostatic model introduced by Kirkwood, Laidler, and Landskroener obtained another similar 

This equation predicts that a plot of n k/kf) versus 1/fir should give a straight line, and gives an explicit expression for the slope s of this line in terms of the radii and dipole moments as shown by Eq. (5-89)  

If a reaction between neutral, dipolar molecules occurs with the formation of an activated complex with a dipole moment greater than either Rb there will be an increase in the rate constant with increasing Sr according to Eq. (5-88). This is because a medium with higher Sr favours the production of any highly dipolar species as, in this case, the activated complex. In applying Eqs. (5-87) and (5-88) to experimental data, a model for the activated complex has to be constructed in order to evaluate reasonable values for and r. This has been done, for example, for the acid and base hydrolysis of carboxylic esters [11, 242]. 

Another Coulombic energy approach for the calculation of electrostatic solvent effects on reactions between dipolar molecules was made by Amis [12, 21, 244]. He related the rate constant to the energy of activation by the well-known Arrhenius equation k = A exp —E /RT). It is assumed that the effect of the relative permittivity on the rate is given by Eq. (5-90)  

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The concept of cohesive pressure (or internal pressure) is useful only for reactions between neutral, nonpolar molecules in nonpolar solvents, because in these cases other properties of the solvents, such as the solvation capability or solvent polarity, are neglected. For reactions between dipolar molecules or ions, the solvents interact with reactants and activated complex by unspecific and specific solvation so strongly that the contribution of the cohesive pressure terms of Eq. (5-81) to In /r is a minor one. It should be mentioned that cohesive pressure or internal pressure are not measures of solvent polarity. Solvent polarity refiects the ability of a solvent to interact with a solute, whereas cohesive pressure, as a structural parameter, represents the energy required to create a hole in a particular solvent to accommodate a solute molecule. Polarity and cohesive pressure are therefore complementary terms, and rates of reaction will depend... 

Numerous organic reactions are of the ion-dipole type, as, for example, the Sn2 reactions given in Eqs. (5-17) and (5-18) in Section 5.3.1. Considering the reaction between an ion A of charge za and a neutral, dipolar molecule B of dipole moment according to + B (AB) C + D, Laidler and Eyring [2, 251] obtained Eq. (5-92) for the rate constant in a medium of zero ionic strength ... 

The second-order rate constant for the reaction between methoxycarbonyl-acetylene and piperidine increases with increasing solvent polarity. This can be attributed to the increased solvation of the strongly dipolar activated complex, which is formed from neutral molecules [88], Analogous solvent effects have been observed for the nucleophilic addition of aziridine to 3-dimethylaminopropynal [89] and the addition of diethylamine to / -alkoxyvinyl methyl ketones [793],... 

As may be seen from the data of Table 5.4, the interaction of neutral nucleophiles (such as NH3,H20) with the carbonyl reactants corresponds to the repulsion potential. Detailed ab initio (STO-3G and 4-3IG) calculations [94, 102-104] showed that as the water and the formaldehyde molecules draw nearer along the reaction path (of the type depicted in Fig. 5.7), the repulsion between them increases and that the dipolar structures XXXIIa, in contrast to the ions XXXII XXXIIc, do not belong to the minima regions on the PES and do not conform to the bound structures ... 

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