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SN1/SN2/E1/E2 mechanisms and reaction conditions
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SN2 is a single-step backside attack with rate law It demands an unhindered (methyl or primary) substrate, a strong nucleophile, and a polar aprotic solvent (DMSO, acetone), and it inverts configuration (Walden inversion). SN1 is a two-step mechanism through a carbocation with It favors tertiary substrates, weak nucleophiles, and polar protic solvents, gives racemization, and can rearrange via hydride or methyl shifts.
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E2 is one concerted step requiring a strong base and an anti-periplanar beta-hydrogen; Zaitsev's more-substituted alkene usually dominates, but bulky bases (t-BuOK) give the Hofmann product. E1 shares the SN1 carbocation, so tertiary substrates in protic solvents with heat give SN1/E1 competition.
Methyl or primary substrate plus strong nucleophile in aprotic solvent: SN2. Tertiary substrate in protic solvent: SN1/E1 mixture, elimination favored by heat. Strong bulky base with any beta-hydrogen: E2. Secondary substrates are the battleground where nucleophile strength and basicity decide.