Covalent catalysis definition

In electrophilic covalent catalysis, the enzyme withdraws electrons from the reaction center, thereby activating the substrate. The distinction between electrophilic and nucleophilic catalysis is not always easy or profitable, since electrophilic attack is often preceded by a step in which the catalyst acts as a nucleophile to bind to the reactive center on the substrate. Further, electrophilic catalysis in one direction is likely to be nucleophilic in the other. Often, the definition depends upon whether the step presumed to be decisive is nucleophilic or electrophilic. 

Another definition of acids and bases is due to G. N. Lewis (1938). From the experimental point of view Lewis regarded all substances which exhibit typical acid-base properties (neutralisation, replacement, effect on indicators, catalysis), irrespective of their chemical nature and mode of action, as acids or bases. He related the properties of acids to the acceptance of electron pairs, and bases as donors of electron pairs, to form covalent bonds regardless of whether protons are involved. On the experimental side Lewis definition brings together a wide range of qualitative phenomena, e.g. solutions of BF3, BC13,... 

By definition, hydroxide ion is called a specific base and, hence, SB catalysis involves hydroxide ion as a catalyst. SB-catalyzed reactions must be only inter-molecular, because in the intramolecular catalytic reaction, the catalyst (hydroxide ion) is required to be covalently or electrovalently attached with the substrate, and under such circumstances, substrate-bound O or OH group becomes the GB catalyst. There are two possible modes of SB catalysis (1) hydroxide ion acts as a base in an acid-base reaction of overall catalytic process as shown in Scheme 2.28, and (2) hydroxide ion acts as a nucleophile in the overall catalytic process as shown in Scheme 2.29. It is evident from Scheme 2.28 that Y must be a stronger base than HO" so that hydroxide ion could not be consumed during the... 

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