Predict whether the following gas phase reactions will be...

Predict whether the following gas phase reactions will be thermodynamically favorable or unfavorable. Explain your answer. a. $\mathrm{CH}_{4}+-<\mathrm{CH}_{3}{ }^{-}+\cdots<$ b. $\mathrm{CH}_{4}+\mathrm{N} \equiv \mathrm{CCH}_{2}^{+} \rightleftharpoons \mathrm{CH}_{3}^{+}+\mathrm{N} \equiv \mathrm{CCH}_{3}$ c. $\mathrm{CHF}_{3}+\mathrm{CH}_{3}{ }^{-} \rightleftharpoons \mathrm{CF}_{3}{ }^{-}+\mathrm{CH}_{4}$

Predict whether the following gas phase reactions will be thermodynamically favorable or unfavorable. Explain your answer.
a. $\mathrm{CH}_{4}+-<\mathrm{CH}_{3}{ }^{-}+\cdots<$
b. $\mathrm{CH}_{4}+\mathrm{N} \equiv \mathrm{CCH}_{2}^{+} \rightleftharpoons \mathrm{CH}_{3}^{+}+\mathrm{N} \equiv \mathrm{CCH}_{3}$
c. $\mathrm{CHF}_{3}+\mathrm{CH}_{3}{ }^{-} \rightleftharpoons \mathrm{CF}_{3}{ }^{-}+\mathrm{CH}_{4}$

Key Concepts

Thermodynamic Favorability

Thermodynamic favorability in chemical reactions is determined by the change in Gibbs free energy (?G), which in turn is influenced by the reaction’s enthalpy and entropy changes. A reaction with a negative ?G is favorable, meaning it can occur spontaneously under the conditions considered. In gas-phase reactions, where solvent effects are absent, the intrinsic energetic differences between reactants and products—including bond strengths and ionic stabilities—play a crucial role in determining ?G.

Gas-Phase Acidities and Basicities

In the gas phase, the inherent acid and base properties of molecules are characterized by their proton affinities and gas-phase acidities rather than by their solution-phase behavior. Since there is no solvent to stabilize charges, the intrinsic ability of a molecule to donate or accept a proton (or electron) is determined solely by its structure. This concept is essential when assessing reactions that involve proton or electron transfers, as the relative gas-phase acidities and basicities can decisively influence the thermodynamic outcome.

Bond Energies and Reaction Enthalpies

Bond energies or bond dissociation energies offer direct insight into the energetic costs and gains associated with breaking existing bonds and forming new ones in a reaction. The net reaction enthalpy, calculated from these individual bond energies, is a key factor in determining the overall enthalpic contribution to the reaction’s free energy change. Accurate accounting of these energy changes is fundamental in evaluating whether a gas-phase reaction is thermodynamically favorable.

Ion Stability and Charge Distribution

The stability of ionic species in the gas phase depends largely on how well a molecule can accommodate or disperse a charge. Factors like the electronegativity of atoms, polarizability, and possible resonance stabilization are critical in determining ion stability. In reactions involving charged species, the relative stabilization of carbocations, carbanions, or other ionic intermediates significantly affects the reaction energetics and thus the thermodynamic favorability.

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