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A QUICK REVIEW OF TWO SUBSTITUTION REACTIONS

A QUICK REVIEW OF TWO SUBSTITUTION REACTIONS. S N 1 / S N 2. RBr + NaOH ROH + NaBr. rate = k 2 [RBr] [NaOH]. S N 2. RECALL FROM LAST LECTURE. Two similar reactions, different kinetics. 1). First Case. 55 o C. primary. S N 1. rate = k 1 [RBr].

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A QUICK REVIEW OF TWO SUBSTITUTION REACTIONS

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  1. A QUICK REVIEW OF TWO SUBSTITUTION REACTIONS SN1 / SN2

  2. RBr + NaOH ROH + NaBr rate = k2 [RBr] [NaOH] SN2 RECALL FROM LAST LECTURE Two similar reactions, different kinetics. 1) First Case 55oC primary

  3. SN1 rate = k1 [RBr] RECALL FROM LAST LECTURE 2) Second Case 55oC tertiary

  4. rate = k1 [RBr] QUESTION SN1 SN2 rate = k2 [RBr] [NaOH] Is this difference significant, or is it just a “mathematical illusion” of some kind?

  5. THE WORK OF HUGHES AND INGOLD

  6. HUGHES AND INGOLD ca. 1940 It was found experimentally that : Most primary compounds undergo SN2 reactions with second order kinetics. Most tertiary compounds undergo SN1 reactions with first order kinetics. Most nucleophilic substitution reactions fit one or the other of these two patterns. Is this significant, or is it just a mathematical illusion of some kind?

  7. 2-BROMOOCTANE A STUDY IN STEREOCHEMISTRY Let’s use an optically-active compound.

  8. R = C6H13 chiral can react both SN1 and SN2 2-BROMOOCTANE * abbreviation * R CH CH3 Br secondary

  9. DIFFERENT KINETICS AT DIFFERENT BASE CONCENTRATIONS k1[RBr] SN1 low conc [ OH- ] < 0.1 M high conc [ OH- ] > 1 M SN2 1) H2O 2) -H+ The two mechanisms compete. k2[RBr][OH]

  10. SN1 SN2 Total Rate = k1 [RBr] + k2 [RBr] [OH-] TOTAL SN2 dominates at low [OH-] k2 [RBr] [OH-] dominates at high [OH-] Rate SN1 k1 [RBr] [ OH- ] [RBr] constant

  11. STEREOCHEMICAL RESULTS * 2-Bromooctane is chiral ..... What if we start with an enantiomerically pure compound (one enantiomer only)? Is the stereohemical result the same in each case : SN1andSN2 ?

  12. STEREOCHEMISTRY CHANGES R-(-)-2-bromooctane [a]D = -36.0o complete inversion SN2 conditions high [OH-] S R S-(+)-2-octanol SN1 conditions [a]D = +10.3o low [OH-] R S + complete racemization enantiomers [a]D = 0o racemic mixture

  13. THE MECHANISMS Can we explain the stereochemical results with our proposed mechanisms for SN1 and SN2 reactions? Let’s Try .......

  14. SN1

  15. SN1 MECHANISM 50% sp2 + - planar carbocation attacks top and bottom equally (R) 50% RACEMIZATION (S) enantiomers (R) racemic mixture

  16. SN2

  17. R .. .. : H .. : : C Br .. CH3 H SN2 MECHANISM nucleophilic attack (R)-config O R attacks back lobe .. : C H O .. CH3 INVERSION (S)-config H

  18. CONCEPTUAL ANALOGY 1 LIKE POOL OR BILLIARDS 1) two balls at rest and touching X 2) forceful shot directly on axis Similar in concept to an attack from the back forcing the nucleophile to leave. Nu X 3) straight-on collision Nu X 4) momentum transfer Nu X

  19. CONCEPTUAL ANALOGY 2 INVERSION OF AN UMBRELLA IN THE WIND Inversion of the umbrella is similar in concept to the inversion of an SN2 atom.

  20. .. R : H O R .. : C Br : HO C CH3 H CH3 H R THE INVERSION PROCESS 2p HO C B sp2 partial bonding HO Br C activated complex is trigonal planar (sp2 ) CH3 H configuration is inverted sp3 sp3 Ea (R)-configuration (S)-configuration

  21. ACTIVATED COMPLEX FOR SN2 MIDPOINT OF THE REACTION trigonal planar (sp2) R forming breaking C HO Br 5 bonds to Carbon CH3 … but the bonds to Br and OH are only half-formed and are not full bonds. H NOT A STABLE SPECIES

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