Chapter 2. POLAR ADDITION AND ELIMINATION REACTIONS

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∆∆H # from steric repulsions in transition state cis trans Bromination transition state resembles a three-membered ring ∆∆H from steric repulsions in ground state In general, the difference in enthalpy of the transition states for bromination was greater than the enthalpy difference of the isomeric alkenes, This finding is consistent with a cyclic somewhat closer together than in the alkene. The kinetics of brominations are often complex, with at least three term making contributions under given conditions: Rate = k 1 [alkene][Br 2 ] + k 2 [alkene][Br 2 ] 2 + k 3 [alkene][Br 2 ][Br - ] Br 2 +Br Br 2 Br 2 Br Br - Br C=C C=C C — C C—C— Br 2 Relative reactivity of alkenes toward halogenation Alkene Chlorination Bromination 1.00 1.00 1.15 0.12 63 27 50 17.5 58 57 11,000 13,700 The Hammett correlation for bromination of styrenes is best with σ + substituent constants and, gives ρ = -4.8. The rates of reactions increase with solvent polarity and with additional substitution of electron-releasing alkyl groups at the double bond. All of these features are in accord with an electrophilic mechanism. Chlorination generally exhibits second-order kinetics and usually leads to expulsion of a proton from the chloronium ion intermediate. R R 2 C Cl + R Cl R R C C C C + H + R R 2 C R H Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 4
<strong>2.</strong>4 THE E2, E1, <strong>AND</strong> E1cb MECHANISMS An elimination reaction -the expulsion of a small molecule from an organic substrate- can be classified according to the relative placement of the carbon atoms from which elimination occurs: H .. —C—X C + HX α-elim. H X —C —C C=C + HX β-elim. H X —C—C—C— C C + HX γ-elim. The β-eliminations can be further subdivided by closer examination of the mechanisms involved. E1 B: —C—C + — C=C + B:H + slow H fast X X E2 —C—C— —C — C— BH + + C=C + X - B: H H B E1cb B: X - B:H + + C C C=C + X - Variable-transition-state theory was proposed to cope with the intermediate mechanisms. Increasing C-H breaking in the transition state C X B-H δ+ δ- B-H δ+ δ- B—H δ+ B H δ+ H E1cb X X E1cb-like X Synchronous E2 E1-like δ+ X δ- E1 δ+ X δ- The most important structural features to be considered concerning the mechanistic types are: (1) The nature of the leaving group (2) The nature of the base (3) Steric factors in the substrate (4) Solvent effects Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 5
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Chapter 2. POLAR ADDITION AND ELIMINATION REACTIONS

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