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SN2 and SN1 mechanisms, substrate/nucleophile effects, solvent effects, and leaving groups
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Study SN2 (backside attack, inversion, bimolecular) and SN1 (carbocation intermediate, racemization, unimolecular) mechanisms. Understand how substrate, nucleophile, solvent, and leaving group determine the pathway.
Predict whether each substrate reacts predominantly by SN1 or SN2 with the indicated nucleophile, and predict the stereochemistry of the product. (a) (R)-2-bromobutane + NaI / acetone. (b) (R)-3-bromo-3-methylhexane + HโO / acetone (heat). (c) Neopentyl bromide ((CHโ)โCCHโBr) + NaOMe / MeOH.
(a) SN2 โ Secondary substrate, strong nucleophile (Iโป is excellent), polar aprotic solvent (acetone). Backside attack inverts the stereocenter โ (S)-2-iodobutane.
(b) SN1 โ Tertiary substrate (no SN2 possible), weak nucleophile (HโO), polar protic solvent. The 3ยฐ carbocation is planar; water attacks both faces โ racemic 3-methylhexan-3-ol (โ 50:50 R:S).
(c) Neither โ extreme steric inhibition. Neopentyl bromide is technically primary but has a quaternary ฮฒ-carbon, which blocks SN2 backside attack catastrophically. SN1 is also disfavored because the resulting primary carbocation is very unstable (and rearrangement to a tertiary cation would change the carbon skeleton). The reaction is ; in practice only trace product forms.
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mol of reacts with mol of . How many grams of water are produced? Which is the limiting reagent? ()
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