Halogenation Of Alkenes

Transcription

1 Kevin Burgess, December 4, alogenation Of Alkenes from chapter(s) in the recommended text A. Introduction

2 Kevin Burgess, December 4, B. Mechanism parallel polarized positively is Chlorination and omination intermediates ,2-dibromide ,2-dibromide alogens (X 2 ) are electrophiles Nucleophiles electrophile has a dipole.

3 Kevin Burgess, December 4, electrophilic nucleophilic faster fastest bromination slowest bromination addition A MO View Of alogenations LUMO alkene δ + δ - LUMO π* 2 LUMO σ* OMO π OMO show MOs produced and place electrons stabilizing, primary secondary

4 Kevin Burgess, December 4, alkene δ + δ - LUMO π* 2 OMO σ OMO π show MOs produced and place electrons do not Stereospecificity S N 2 anti endocyclic trans- mostly transopposite stereoselectivelyalways

5 Kevin Burgess, December 4, enantiomers a b + a - - b S,S R,R equal a racemate is not

6 Kevin Burgess, December 4, equal formation t Bu t Bu attack C 2 from bottom t Bu 1,2-dibromide attack from top face - attack C 2 from bottom dibromoalkane attack from bottom face - attack C 2 from top dibromoalkane identical..

7 Kevin Burgess, December 4, attack from top face - attack C 2 from bottom dibromoalkane attack from bottom face - attack C 2 from top dibromoalkane diastereomers.

8 Kevin Burgess, December 4, trans-1,2-dibromophenylethene trans-2,3-dichlorobut-2-ene 2 2 trans- cis-

9 Kevin Burgess, December 4, Iodination the product is thermodynamically unstable relative to ethene and iodine. I -I - I I carbocation in eclipsed conformation

10 Kevin Burgess, December 4, I -I - carbocation in anti conformation E-1,2-diphenylethene I 2 t Bu I 2 t Bu

11 Kevin Burgess, December 4, C. Kinetic And Thermodynamic Control Kinetic Control energy B is more stable than C energy C is more stable than B TS TS A A C B B reaction progress C reaction progress lower cannot is not is dictated 1, and when C is more stable than B it will be 1. energy B is more stable than C energy C is more stable than B A A C B B C reaction progress reaction progress rates of formation, be invariant kinetic one.

12 Kevin Burgess, December 4, Thermodynamic Control reversible >1. will not K B = [ B ] / [ A ] and K C = [ C ] / [ A ] is another K BC = [ B ] / [ C ] independent of coincident activation energy barriers stabilities of the products. These ratios are different Non-coincident Kinetic And Thermodynamic Control energy B forms faster than C but C is more stable than B A B C reaction progress kinetic reversibly. thermodynamic reversibly. be disfavored because it will revert as the reaction proceeds and reversibly forms C.

13 Kevin Burgess, December 4, energy B forms faster than C C is more stable than B only formation of B is reversible A B kinetic product; only B forms reversibly. thermodynamic product; it forms irreversibly. not be observed because it will revert as the reaction proceeds and irreversibly C reaction progress kinetic thermodynamically omination 1,3-Butadiene: Non-coincident Kinetic And Thermodynamic Control 1,4-dibromide more stable 2 2 slow fast 1,2-dibromide less stable kinetic decreases increases. the alkene products: 1,4-dibromide has two groups substituted on the alkene product while 1,2-isomer has only one group. less does not proceed

14 Kevin Burgess, December 4, D. alogenations In Nucleophilic Solvents δ+ δ- + + δ- + δ+ + δ+ δ- δ+ δ- best Regio--selectivity regioisomers. 2, MeO OMe or OMe 2, O O or O halohydrin this is the precursor to the most stable cation.

15 Kevin Burgess, December 4, O O + 2 O O + O - O + O - O are