Chapter 7

Chapter 7 - Chapter 7: SN1, E1, E2, not just SN2 SN2, SN1...

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Unformatted text preview: Chapter 7: SN1, E1, E2, not just SN2 SN2, SN1 B: E1, E2 H E1, E2 C C L : Nu Relative "rates" of 4 arrows differ SN2, SN1, E1, E2 "Lingo": position, position; further away: , , and so on. Recall: SN2 slows with weak Nu (and branching). For example: CH3Br + H2O Weak Nu (CH3)3CBr or react! But (CH3)2CHBr Despite being -branched (CH3)3C Br + H OH (CH3)2CH Br + H OCH3 Acetone Hydrolysis (CH3)3C OH + H Br (CH3)2CH OCH3 + H Br DMF Methanolysis Generally: Solvolysis Mechanism: 1. Rate = k[R-L], 1st order unimolecular, only R-L in rate-determining TS: "bottleneck". 2. Stereochemistry: Racemization (extensive, although often not complete). Both observations inconsistent with SN2 mechanism 3. Accelerates with polar (best with protic, in contrast to SN2) solvents: O Hexane < CHCl3 < CH3CCH3 < CH3OH O 4. Accelerates with better L L: Cl < Br < I < OSO2R (CH3)3CCl + CH3OH + Na N3 k3 5. Product determining steps after "bottleneck": Competition between Nu's. + - k1 > k2 > k1 .. CH3OH .. k2 Intermediate + - :N N N :: : : : k3 (CH3)3OCH3 wins (CH3)3CN3 Mechanism 1. Electron deficient! 2. 3. Mechanism explains data: 1st Order rate law Racemization Acceleration in polar solvents Acceleration with better L Fast product determining step Unimolecular nucleophilic substitution: SN 1 "Bottleneck" Bottleneck" Bottleneck: Incomplete Racemization May stay close to form an ion pair Planar and achiral, but both sides not equally accessible in ion pair with BrIdeally; in practice: Slight enantiomer excess The Strong Effect of Polar Solvents on the SN1 Reaction Increasing solvent polarity speeds reaction Increasing solvent polarity retards reaction What makes SN1 possible? 1. SN2 is slow. 2. Branched carbocations are stabilized by hyperconjugation H C + C + CH3 < + CH3CH2 + (CH3)2CH + (CH3)3C Too unstable The only cations feasible in solution Stabilizing (2e)! Better than radical (3e). Bonding MO of neighboring C H bond interacts with empty 2p orbital E 2p Hyperconjugation Summary : Rprim- L : no SN1, only SN2 Rsec- L : both, SN2/SN1 ratios difficult to predict, except in "extreme" cases, such as: (CH3)2CH Cl + CH3S DMF SN2 Rtert- L : only SN1, no SN2 (CH3)2CHSCH3 + Cl (CH3)2CHOH + CF3SO3H (CH3)2CH OSO2CF3 + H2O Solvent SN1 Reactivity of R-X Elimination: E1 and E2 : When Nu :(-)acts as B (-) : Cations are deprotonated Therefore: Elimination E1, a side reaction of SN1. Same first dissociation step to cation: :B + C (CH3)3C L C C LH C Then proton loss to base (solvent) Normally B: acts as Nu: to give SN1. Mechanism of the E1 Reaction We usually omit the base in deprotonations and simply write "-H+ ". Ratios of SN1 to E1 products are independent of L E1 Gives Mixtures All C H at -positions in cation are acidic: CH3OH Cl CH3 Cl + : : : : + CH3OH CH3O : : : : + H H Mixture + + + "Regio-" and stereoisomers ("cis/trans") E1/SN1 ratios are difficult to predict Generally: Increasing amounts of E1 products are formed with: 1. Higher T, because entropy of elimination is positive: (RX is converted to alkene plus HX). Recall: !G = !H T!S, hence positive !S makes !G more negative. 2. Very poorly nucleophilic medium (slows SN1), e.g: Acetone solvent. Why not base? Base (unless very weak) changes the mechanism once again. Bimolecular Elimination E2 With strong base: Mechanism changes, base attacks R-L directly at -H: E2 (faster than SN1/E1) Mechanism: 1. Rate = k [R-L][:B] 2nd order 2. L leaves in TS: RCl + bimolecular TS RBr RI L 3. H removed in TS: Deuterium isotope effect kH/kD~ 7 4. Stereochemistry: Anti-TS B: C H * C * The E2 Reaction is Stereospecific One diastereomer of RX (e.g. R,R/S,S below) gives only one stereoisomer of alkene product: Or the S,R-R,S pair: Mechanism of the E2 Reaction TS: staggered, best overlap, least e-repulsion E2 in Cyclic Systems Hindered Base Ensures E2 Summary Factors that Affect the Competition between SN and E Factor 1: Base strength of the nucleophile Weak Bases Substitution more likely H2O , ROH , PR3 , halides , RS , N3 , NC , RCOO Strong Bases Likelihood of elimination increased HO , RO , H2N , R2N Factor 2: Steric hindrance around the reacting carbon Sterically unhindered Primary haloalkanes Substitution more likely Sterically hindered Branched prim, or sec and tert haloalkanes Likelihood of elimination increased Factor 3: Steric hindrance in nucleophile (strong base) Sterically unhindered HO , CH3O , CH3CH2O , H2N Substitution may occur Sterically hindered (CH3)3CO , [(CH3)3CH]2N Elimination strongly favored Reactivity of Prim Haloalkanes R-X with Nucleophiles (Bases) For unhindered primary R X : SN2 with good nucleophiles that are not strongly basic CH3CH2CH2Br + CN Acetone CH3CH2CH2CN + Br- SN2 with good nucleophiles that are also strong bases CH3CH2CH2Br + CH3O CH3OH CH3CH2CH2OCH3 + Br- But E2 with strong, hindered bases CH3 - (CH3)3COH CH3CH2CH2Br + CH3CO CH3 HBr CH3CH CH2 No (or exceedingly slow) reaction with poor nucleophiles (CH3OH) For branched primary R X : SN2 with good nucleophiles (although slow compared with unhindered RX) CH3 CH3CCH2Br + I H Acetone CH3CCH2I + BrH CH3 E2 with strong base (not necessarily hindered) CH3CCH2Br + CH3CH2O H CH3 CH3CH2OH HBr CH3 CH3C CH2 No (or exceedingly slow) reaction with poor nucleophiles or neopentyl systems (in which E is not possible) Reactivity of Sec Haloalkanes R-X with Nucleophiles (Bases) SN1 and E1, when X is a good leaving group in a highly polar medium with weak nucleophiles CH3 CH3CBr H CH3 CH3COCH2CH3 + CH3CH H Major CH2 Minor (more on increasing T) CH3CH2OH HBr SN2 with high concentrations of good, weakly basic nucleophiles CH3 CH3CBr H + CH3S CH3CH2OH CH3CSCH3 + BrH CH3 E2 with high concentrations of strong base (for example, HO or RO in alcohol solvent) CH3 CH3CBr H + CH3CH2O CH3CH2OH HBr CH3CH CH2 Reactivity of Tert Haloalkanes R-X with Nucleophiles (Bases) SN1 and E1 in polar solvents when X is a good leaving group and only dilute or no base is present CH3 CH3 CH3CH2CBr CH3 HOH, acetone HBr CH3CH2COH + alkenes CH3 E2 with high concentrations of strong base H3C CH2 CH3CH2CCl H3C CH2 CH3O, CH3OH HCl H3C CH2 CH3CH2C CHCH3 ...
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This note was uploaded on 03/21/2012 for the course CHEM 140A taught by Professor Whiteshell during the Fall '04 term at UCSD.

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