Energy from displacement reactions

Figure 13. The Gibbs energy available from a reaction, A B, depends on its displacement from equilibrium when IB)/IA) = K. The AC value is plotted against the mass-action ratio, and this is the value when B1/ A] is maintained constant in the steady state if the rate of substrate supply and substrate removal is constant.

Displacement reactions observed in the gas phase are generally exothermic or thermoneutral as in the case of simple isotope exchange. This requirement is consistent with the limited dynamic range of the experimental techniques which precludes the observation of reactions with sizable activation energies. The relevant thermochemical data for negative ions have become available in recent years through the determination of electron affinities (Janousek and Brauman, 1979), and indirectly from gas-phase acidity scales (Bartmess and Mclver, 1979). Relative gas-phase acidities available at present (Bartmess et al., 1979 Cumming and Kebarle, 1978) are an important consideration in... 

The majority of enzymes that are apt to be mentioned in any discussion of metabolism catalyze nucleophilic displacement reactions (Type 1, Table 10-1). These include most of the reactions by which the energy of ATP cleavage is harnessed and by which polymers are assembled from monomers. They include reactions by which pieces, large or small, are transferred onto or off of polymers as well as the reactions by which polymers are cleaved into pieces.1 3,1... 

A typical example of such reactions is the exothermic Sn2 nucleophilic displacement reaction Cl -I- CH3—Br Cl—CH3 - - Br . Table 5-2 provides a comparison of Arrhenius activation energies and specific rate constants for this Finkelstein reaction in both the gas phase and solution. The new techniques described above cf. Sections 4.2.2 and 5.1) have made it possible to determine the rate constant of this ion-molecule reaction in the absence of any solvent molecules in the gas phase. The result is surprising on going from a protic solvent to a non-HBD solvent and then further to the gas phase, the ratio of the rate constants is approximately 1 10 10 The activation energy of this Sn2 reaction in water is about ten times larger than in the gas phase. The suppression of the Sn2 rate constant in aqueous solution by up to 15 orders of magnitude demonstrates the vital role of the solvent. 

Fig. 5-5. Schematic one-dimensional relative enthalpy diagram for the exothermic bimolecular displacement reaction HO + CH3—Br —> HO—CH3 + Br in the gas phase and at various degrees of hydration of the hydroxide ion [485]. Ordinate standard molar enthalpies of (a) the reactants, (b, d) loose ion-molecule clusters held together by ion-dipole and ion-induced dipole forces, (c) the activated complex, and (e) the products. Abscissa not defined, expresses only the sequence of (a). .. (e) as they occur in the chemical reaction. The barrier heights ascribed to the activated complex at intermediate degrees of hydration were chosen to be qualitatively consistent with the experimental rate measurements cf. Table 5-3 [485]. Possible hydration of the neutral reactant and product molecules, CH3—Br and HO—CH3, is ignored. The barrier height ascribed to the activated complex in aqueous solution corresponds to the measured Arrhenius activation energy. A somewhat different picture of this Sn2 reaction in the gas phase, which calls into question the simultaneous solvent-transfer from HO to Br , is given in reference [487].

The adatom concentration at equilibrium, o,ads [atoms cm ], is determined by the Gibbs energy AGdispi [J mol ] of the displacement reaction of atoms from kink positions to free" adsorption sites on the surface, where it can form an adatom [2.1] ... 

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