29E2 Is Stereoselective The conformers that have the bulkiest groups on oppositesides will be the major product.
30Stereochemistry of E2Note: E isomer is major product because bulky groups, methyl and t-butyl, are anti.
31Stereochemistry of E2Important note: use models! E2 for 2-bromo-3-phenylbutaneonlyE isomer(2S, 3S)onlyZ isomer(2S, 3R)
32bulkiest groups on opposite sides E1 Eliminationsstereoselectiveplanar, carbocation intermediatesyn and anti elimination can occurbulkiest groups on opposite sides(Major product is the one with bulkiest groups on opposite sides.)
33Elimination from Cyclics - E2 In an E2 reaction, both groups must be periplanar and anti to each other.This is NOT the most stable conformer.
35Elimination from Cyclics - E2 More stable conformer has groups equatorial.Occurs 200 x faster!
36Elimination from Cyclics - E2 In an E2 reaction, the H is removed from the most substituted b-carbon unless that hydrogen is not in the axial
37Elimination from Cyclics - E1 In an E1 reaction, the elimination is not concerted.Intermediate is a carbocationGroups do not need to be in the axial position
38Elimination from Cyclics - E1 Carbocation intermediates may rearrange.(Zaitsev’s rule is followed, but first watch out for a rearrangement.)
39Stereochemistry Review mechanismproductscommentsSN1Both enantiomers formed (more inverted than retained)Racemization w/ some inversionE1Both E and Z formed (more of the stereoisomer with bulkiest groups trans)Also C+ intermediateSN2Only the inverted product is formedInversionE2Both enantiomers formed (more of the stereoisomer with bulkiest groups trans)
40Using Isotopes In Chemistry Look at mechanismLook at research handoutLook at medical questions handout
41Substitution vs. Elimination E1 vs SN1orE2 vs SN2
42Steps To Deciding Decide if it favors Sn2/E2 or Sn1/E1 conditions Decide if it favors Substitution or Elemination…….. That’s all there is to it From Yahoo Images
43Deciding Between First or Second order Substrate – 1st order – primary and secondary 2nd order – secondary and tertiaryNucleophile 1st order – weaker nuclephile2nd order – strong nucleophileSolvent 1st order – polar protic solvent2nd order – polar aprotic or neutral2 out of three ain’t bad………… or is that just a long lost song
44Deciding Between Sn2 and E2 SubstrateBaseTemperatureFrom Yahoo Images
45Deciding Between Sn2 and E2 Substrate Sn2 – primary alkyl halide E2 – secondary alkyl halide
46Deciding Between Sn2 and E2 Nuclophile Sn2 – Weaker and Smaller E2 – stronger and Bulkier
47Deciding Between Sn2 and E2 Temperature Sn2 – Lower Temperature E2 – Higher Temperatue
48SN1 / E1 ConditionsAll alkyl halides have same order of reactivity for SN1 or E1Substitution is favored at lower temperatures
49Practice Activity Each person create a reaction and conditions Have your neighbor decide which mechanism it will undergoSwitch and do it againFrom Yahoo Images
50Williamson Ether Synthesis Alkyl halide with alkoxide (SN2 reaction)Alexander Williamson (1850!)React alcohol with sodium metal or sodium hydrideCH3OH Na CH3O Na /2 H2NaH
51Williamson Ether Synthesis If you want to prepare n-propyl n-butyl ether, there are two methods.
52Williamson Ether Synthesis If you want to prepare ethyl t-butyl ether, there are two methods… HOWEVER, if you use t-butyl halide, you will obtain primarily the elimination product.
53(note: use a very strong base NH2 , not OH-, or Alkyne SynthesesElimination Reactions involving geminal dihalides(note: use a very strong base NH2 , not OH-, orit stops at vicinyl dihalide!)
54Synthesis ProblemsSynthesize 1,3-cylcohexadiene from cyclohexane.