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halidesg - Copyright Arizona State University Substitution...

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Alkyl Halides 1 Copyright, Arizona State University Copyright, Arizona State University Alkyl Halides Substitution and Elimination (more...) • alkenes do mainly addition reactions X–Y X Y addition reaction • halides mainly do two other reactions, each illustrates many generally useful principles of organic reactivity... C X LB C LB + X LA "leaving group" LB (nucleophile) C C X LB + X elimination reaction "leaving group" LB/BB H LA/BA C C LB–H + "S N 2" "E2" substitution reaction 1 Nomenclature • Look for the longest chain that CONTAINS the functional group, in this case a halogen Br (1R)-bromo-1-cyclohexylethane named as a substiuted alkyl halide therefore, cyclohexane is a substituent! 2 Second Order Nucleophilic Substitution (SN2) Reaction Substitution by making a new bond AT THE SAME TIME as breaking the old bond Example H C Br Me Et H–O H Me Et Br HO C sp 2 H C HO Me Et (R)– (S)– LB LA + Br partial bonds "backside attack!" * * • reaction proceeds with inversion (i.e. BACKSIDE ATTACK, think about an umbrella turning inside out in the wind!), called a Walden inversion. Often this will lead to a change in absolute configuration, i.e. R to S or vica versa, but not necessarily! reaction coordinate Energy C HO H Me Et Br HO C Br H Me Et E a H Me Et Br HO C partial bonds are LONG partial bonds are WEAK
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Alkyl Halides 2 Copyright, Arizona State University • even though the transition state apparently has one more bond than either the reactants or the products, the partial bonds are very LONG and thus WEAK, which is why the transition state is higher in energy than either reactants or products (2 weak partial bonds add up to less than one real bond) • The reaction is CONCERTED (all bonds are made and broken at the same time) v Visualize the Walden Inversion • HO– is Lewis Base and Nucleophile, Br– is "Leaving Group" • reaction "goes" because 1) weaker bond is converted into a stronger bond C–Br: B.D.E. ~ 65 kcal/mol (weaker bond) HO–C: B.D.E. ~ 90 kcal/mol (stronger bond) 2) stronger base (–OH) is converted into a weaker base (Br–) 3) in this way HIGHER energy electrons are converted into LOWER energy electrons Why the Name Second Order Nucleophilic Substitution (S N 2)?? S - Substitution reaction N - Nucleophile does the substitution (like a Lewis base, but see below) 2 - kinetically 2nd order, because both the nucleophile and the halide are involved in the rate determining step , in this case the only step! rate = k [nucleophile] [halide] rate constant • increase the concentration of EITHER or BOTH the nucleophile and halide and reaction rate increases proportionally What is a Nucleophile? Definition of a BASE H 3 C–O O H H H LB/BB LA/BA K eq H 3 C–O + H 2 O H new bond + Lewis/Brønsted base strength measured by size of K eq ( thermodynamic definition ) • stronger base means stronger new bond means larger Keq • weaker base means weaker new bond means smaller KEq Definition of a NUCLEOPHILE H 3 C–O X H 3 C Nucleophile (and LB) Electrophile (and LA) k H 3 C–O –CH 3 + X + Nucleophile
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