Acid-base reactions activation energy

In order to understand why the activation energies differ between the two pathways, Mui et al. examined the transition state geometries [279]. They found that as electron density is donated from the amine lone pair to the down silicon atom upon adsorption into the precursor state, the up Si atom in the dimer becomes electron rich. At this stage, the dative bonded precursor can be described as a quaternary ammonium ion. The N—H dissociation pathway can thus be interpreted as the transfer of a proton from the ammonium ion to the electron-rich up Si atom through a Lewis acid-base reaction. In the transition state for this proton transfer, the N—H and Si—H... 

The first step in this mechanism is a relatively slow reaction. (The activation energy for this step is roughly 80 kJ/mol.) If this reaction is done in water, the next step is extremely fast. The (CH3)3C+ ion is a Lewis acid because it has an empty orbital that can be used to accept a pair of electrons. Water, on the other hand, is a reasonably good Lewis base. A Lewis acid-base reaction therefore rapidly occurs in which a pair of nonbonding electrons on a water molecule are donated to the carbocation to form a covalent C—O bond. 

Because the activation energies are small and the stereoelectronic requirements are not difficult to meet, most acid-base reactions are very fast in comparison to other types of organic reactions. Therefore, it is usually not necessary to be concerned with the rates of acid-base reactions. In organic reactions that have mechanisms involving several steps, including an acid-base step, one of the other steps in the mechanism usually controls the rate. 

In a reversible acid-base reaction + B2 H-A2, the equilibrium constant is given by the ratio of the forward and reverse rate constants, K = /Cf/7cr. If the equilibrium is very far on the left-hand side k( ky, /Cf may in this case be quite small, since the activation energy of the forward reaction must be at least equal to its endothermicity. It might be thought, therefore, that a reaction involving a very weak acid or a very weak base (or both) would always be slow enough to be followed by conventional means. However, the time taken to reach equilibrium will still... 

The approximate pA values and the energy conventions laid out in this activity are the keys to understanding acid-base reactions and all other polar reactions in this course. A moderate time investment at this point will save you vast amoimts of frustration farther down the line. These memorized bits of information will serve as a crutch while you develop your deeper conceptual understanding of organic chemistry. 

Proton, by definition, is called specific acid, and if the overall energy barrier (activation energy) of a reaction is reduced in the presence of proton as a catalyst, then the reaction is said to involve specific acid catalysis. Generally, catalyst proton reacts with reactant (substrate) in a so-caUed acid-base reaction process, which, in turn, activates the reaction system (by either the preferential destabilization of reactant state or stabilization of transition state in the rate-determining step) for the product formation. The products do not contain any molecular site, which has enough basicity to trap the proton catalyst irreversibly or even reversibly. Thus, for a detectable S A catalysis, the basicity (measured by the magnitude of basicity constant, KJ of the basic site of electrophilic reactant, products, and solvent should vary in the order K, (for electrophilic reactant) > (for solvent) > Kb (for products). 

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