How can I tell whether a rxn will be SN2, SN1, E2, or E1 whenthere are factors t
ID: 683298 • Letter: H
Question
How can I tell whether a rxn will be SN2, SN1, E2, or E1 whenthere are factors that favor two or all of thesemechanisms? For example, in a reaction usingCH3O- Na+ and CH3OHsolvents: the CH3O- is a strong nucleophile,which favors SN2 or E2. Yet the CH3OH is aweak nucleophile and a weak base and also a protic solvent, whichall favor SN1 or E1. Which is the deciding factor that determines SN1, E1, SN2 orE2? Also any clues on how to determine strong/weak bases wouldhelp! Thanks. Will rate LIFESAVER! How can I tell whether a rxn will be SN2, SN1, E2, or E1 whenthere are factors that favor two or all of thesemechanisms? For example, in a reaction usingCH3O- Na+ and CH3OHsolvents: the CH3O- is a strong nucleophile,which favors SN2 or E2. Yet the CH3OH is aweak nucleophile and a weak base and also a protic solvent, whichall favor SN1 or E1. Which is the deciding factor that determines SN1, E1, SN2 orE2? Also any clues on how to determine strong/weak bases wouldhelp! Thanks. Will rate LIFESAVER!Explanation / Answer
Effect of nucleophile/base: Strong nucleophiles promote SN2 because the nucleophile takes placein the slow step, which is the only step for SN2. For SN1, the slowstep is the first step, which is formation of the carbocation, andthe nucleophile does not attack. Weak nucleophiles follow SN1,although nucleophilicity is not of much importance in SN1. E2 willoccur when there is a strong nucleophile or base and the substrateis tertiary, which is too hindered to undergo SN2, although thesereactions may occur simultaneously. Substrate: If the substrate is tertiary or secondary, it will follow SN1. Ifit is a methyl halide, primary, or secondary, it will follow SN2.The order of reactivity for both E1 and E2 is 3o >2o > 1o. Solvents: SN1 requires a very polar ionizing solvent like water or alcohols.SN2 may be weakened by strongly polar solvents, and the reactionmay proceed faster in less polar solvents. This is because thepolarity of the solvent can weaken the strength of the nucleophile.Ionizing solvents are required for E1 but polarity is not veryimportant for E2. SN1 and E1 reactions almost always occur together because theycompete. They are rarely used in synthesis for this reason. As for determining nucleophilicty: A substance with a negative charge is a stronger nucleophile thanthe similar noncharged species. Bases are stronger than theirconjugate acids. Nucleophilicity decreases from left to rightacross the periodic table, following the increase inelectronegativity. This is because more electronegative atoms havetightly bound electrons that are less reactive. Nucleophilicityincreases from top to bottom on the periodic table, along with sizeand polarizability.
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