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25 Cards in this Set
- Front
- Back
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SN2
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substitution nucleophilic bimolecular
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SN2
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CH3-I + OH- ----> CH3-OH + I-
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SN2
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occurs with inversion of stereochemistry in a single step
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SN2
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second order kinetics, rate = k [R-X][nucleophile]
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SN2
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reaction coordinate vs E profile has single hump; the transition state is 5-coordinate with an sp2-hybridized central C; rxn is called "bimolecular" because 2 species (the R-X and nucleophile) are involved in the rate determining step
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SN2
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R-X reactivity shows effect of steric hindrance to approach of nucleophile; most reactive R in R-X = 1o > 2o > neopentyl ~ 3o; rnx effectively does not occur with neopentyl and 3o R-X’s
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SN2
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rate increases with good nucleophiles; nucleophilicity increases as go down in PT; eg, Cl- < Br- < I-; RO- < RS-; R3N < R3P
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SN2
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rate of reaction largest in polar aproptic solvents; protic solvents can H-bond to - nucleophile which effectively reduces the nucleophilicity (availability of e-pair)
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SN1
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substitution nucleophilic unimolecular
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SN1
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(CH3)3C-I + H2O ----> (CH3)3C-OH + H-I; in this reaction, H2O is both the nucleophile and the solvent (called a solvolysis reaction)
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SN1
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occurs with partial racemization (usually more inversion product than retention product)
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SN1
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first order kinetics, rate = k [R-X]; rate does not
depend on concn of nucleophile |
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SN1
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reaction coordinate vs E profile has two humps (transition states) and two steps; step # 1 (rate-controlling step) is ionization of R-X to form a carbocation; step # 2 is fast and involves rxn of the carbocation with the nucleophile
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SN1
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R-X reactivity parallels that of carbocation stability; 3o most reactive, 1o and methyl least
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SN1
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rate of reaction largest in polar protic solvents (high dielectric constant) capable of cation and anion stabilization
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E2
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elimination bimolecular
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E2
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R-X + base ----> alkene + "H-X"
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E2
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2nd order kinetics; base and R-X are involved in the
single reaction step |
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E2
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need a strong base, such as RO-
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E2
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optimal conformation of R-X for loss of H and X is
anti coplanar; in cyclohexanes, H and X preferentially involved are both axial |
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E2
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Zaitsev’s rule applies; major organic product will be most stable (highly substituted) alkene
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E1
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elimination unimolecular
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E1
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similar to SN1 in that carbocation intermediate is formed in the first slow step; carbocation loses H+ rapidly in the second fast step
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E1
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important mechanism for 3o R-X’s in the presence of any type of base
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E1
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with 3o R-X’s and strong base, both E1 and E2 pathways occur simultaneously
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