Kevin Burgess, December 4, 2016 1 alogenation Of Alkenes from chapter(s) in the recommended text A. Introduction
Kevin Burgess, December 4, 2016 2 B. Mechanism parallel polarized positively is Chlorination and omination intermediates. + - - - 1,2-dibromide + - - - 1,2-dibromide - - - - alogens (X 2 ) are electrophiles Nucleophiles electrophile has a dipole.
Kevin Burgess, December 4, 2016 3 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
Kevin Burgess, December 4, 2016 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. + - - -
Kevin Burgess, December 4, 2016 5 - - - - 2 2 2 2 enantiomers a b + a - - b S,S R,R equal a racemate is not
Kevin Burgess, December 4, 2016 6 equal formation 3 2 - - - + t Bu t Bu attack C 2 from bottom t Bu 1,2-dibromide 3 2 2 attack from top face - attack C 2 from bottom dibromoalkane 3 2 2 attack from bottom face - attack C 2 from top dibromoalkane identical..
Kevin Burgess, December 4, 2016 7 3 2 2 attack from top face - attack C 2 from bottom dibromoalkane 3 2 2 attack from bottom face - attack C 2 from top dibromoalkane diastereomers.
Kevin Burgess, December 4, 2016 8 2 2 2 2 2 2 trans-1,2-dibromophenylethene trans-2,3-dichlorobut-2-ene 2 2 trans- cis-
Kevin Burgess, December 4, 2016 9 Iodination the product is thermodynamically unstable relative to ethene and iodine. I -I - I I carbocation in eclipsed conformation
Kevin Burgess, December 4, 2016 10 I -I - carbocation in anti conformation E-1,2-diphenylethene I 2 t Bu I 2 t Bu
Kevin Burgess, December 4, 2016 11 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.
Kevin Burgess, December 4, 2016 12 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.
Kevin Burgess, December 4, 2016 13 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
Kevin Burgess, December 4, 2016 14 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.
Kevin Burgess, December 4, 2016 15 + 2 O O + 2 O - + - + O + O - O + O - O are