Page - (000035) - in Biomedical Chemistry: Current Trends and Developments

various steps having its variations similarly to the nucleophilic substitutions. In all of the following cases, the reactions shown are [1,2]- eliminations, meaning that the reaction implicates electron movement between two vicinal carbons. Other types of eliminations may occur involving more distant carbon atoms and heteroatoms ‒ [1,3] in decarboxylations or [1,4] in aldol condensations (Chapter 1.1.4.6). 1.1.4.4.1 E1 ‒ Unimolecular Elimination An E1 reaction mechanism occurs when a relatively stable carbocation may be formed by elimination of a stable anion (X-), such as a chloride ion (Cl-) for a good leaving group. The only rate determining (slow) step is the dissociation of the leaving group to form a carbocation (hence a unimolecular reaction). A base (B) captures the proton released from one carbon away of the carbocation formed in the E1 reaction mechanism. It is worthy to note that the E1 mechanism competes with the SN1 mechanism since the nucleophile (B) may react directly at the halogenated carbon (substitution) or at the neighbouring hydrogen atom (elimination). Steric hindrance both at the base and at the carbocation, as well as stronger bases promote elimination. Scheme 1.1.16: General E1 reaction mechanism. 1.1.4.4.2 E2 ‒ Bimolecular Elimination Scheme 1.1.17: General E2 reaction mechanism. E2 mechanisms occur through a concerted transfer of a set of electron pairs from the base to the more electronegative group (X), the latter leaving as its anion and a vicinal proton is transferred to the base (Scheme 1.1.17). This mechanism is preferred if no stable ionic