Page - (000036) - in Biomedical Chemistry: Current Trends and Developments

intermediate can be formed. The double bond is formed in a step involving the two reagents, thus the elimination mechanism is bimolecular. It is noteworthy that the E2 mechanism competes with the SN2 mechanism, as both reactions involve a base and an electronegative leaving group. More steric hindrance and stronger bases favour an E2 mechanism over SN2. 1.1.4.4.3 E1cB ‒ Unimolecular Elimination through Conjugate Base Scheme 1.1.18: General E1cB reaction mechanism. E1cB mechanism is the symmetric version of the E1 mechanism. First, the proton is removed from the main molecule to form its conjugate base, a carbanion, which promotes the elimination of the electrophilic group. This mechanism is preferred whenever a carbanion intermediate is stabilized, in most of the cases by resonance (Scheme 1.1.18). This mechanism is particularly relevant in biological transformation, as it is by far the most frequent elimination mechanism because of the high occurrence of carbonyl compounds which form relatively stable carbanions. 1.1.4.5 Nucleophilic Carbonyl Addition Reactions This type of reaction happens between a nucleophile and a carbonyl group where a pair of electrons from the nucleophile is transferred to the carbonyl carbon (Scheme 1.1.19). Scheme 1.1.19: General simplified scheme for a nucleophilic addition reaction If the nucleophile is neutral and gives a pair of electrons then it will acquire a positive charge, which in turn is compensated by a