Presentation on theme: "OTHER POSSIBLE MECHANISMS"— Presentation transcript:
1 OTHER POSSIBLE MECHANISMS ELIMINATIONOTHER POSSIBLE MECHANISMSDo some elimination reactionsoccur in a different fashion?
2 Three types of elimination reactions are conceivable juststudiedconcertedE2carbocationE1halogenfirstprotonsecondcarbanionE1cbprotonfirsthalogensecond
3 E1 ALKYL HALIDES + WEAK BASE (SOLVOLYSIS) The removal of a b-hydrogen becomes difficult withouta strong base and a different mechanism (ionization)begins to take place….. if the substrate is capable.
4 The E1 Elimination Reaction (two steps) weakbasecarbocationslow:X+step one3o > 2o > 1oalso favoredif a resonancestabilizedcarbocationis formedunimolecularsteptwofastrate = k [RX]Works best in apolar solvent.IONSFORMED
5 E1 ENERGY PROFILE two step reaction carbocation intermediate TS1 E N R startingmaterialstep 1step 2DHslowproduct
7 H H C C C C X C C C C H X H + + THE E1 REACTION IS NOT STEREOSPECIFIC ( THE OPEN CARBOCATION IS PLANAR AND CAN ROTATE)HCarbocation issp2 hybridized( planar )and can reactfrom either side.H+antiCCCCrotationXThese twocarbocationsare equivalentby rotation andby symmetry.synCCCCHXH+rate of C-C rotation= to 1012 / secElimination can be either syn or anti.
11 WITH INCREASING BASE CONCENTRATION BEHAVIOR OF THE RATEWITH INCREASING BASE CONCENTRATIONE2rate = k2 [RX] [B]second orderE1 dominatesat low baseconcentrationE2 dominatesat higher baseconcentrationrate = k1 [RX]first orderRateE1[RX] constant, [B] increasing
12 EFFECT OF BASE CONCENTRATION ON E1/E2 REACTIONS k2 [RX] [B]At high base concentrationE1 never has a chance.E2At low base concentrationE2 is nonexistentk1 [RX]tertiary RX, k’’1secondary RX, k’E1Rate1primary RX, k1[RX] constant, [Base] increasingFor E1 elimination : k’’ (tertiary) > k’ (secondary) > k (primary).
13 EFFECT OF BASE CONCENTRATION ON E1/E2 REACTIONS tertiaryk’’2secondaryk’E22kk2 [RX] [B]2primaryk1 [RX]tertiary RX, k’’1secondary RX, k’E1Rate1primary RX, k[RX] constant, [B]For E2 elimination : line slopes k2 differ for 1o,2o,3o .Different substrates react at different rates,
14 STRUCTURE OF SUBSTRATE R H H R-C-XR-C-XR-C-XRprimaryRsecondaryHtertiaryObviously for E1 which forms a carbocation intermediaterate : tertiary > secondary > primary > methylBut this same order holds for E2 also.tertiary has moreb -hydrogensmore opportunitesfor reactionEtO-
15 WHEN THE E1 MECHANISM OCCURS E1 occurs only1) at zero or low base concentration2) with solvolysis (the solvent is the base)3) with tertiary and resonance capablesubstrates (alkyl halides)If a strong base is present in moderateto high concentration, or the substrateis a primary halide, the E2 reactiondominates.
16 ALKYL HALIDE + BASE E2 mechanism E1 mechanism strong base high base conc.weak baselow base conc.orsolvolysis(solvent is base)E2 mechanismE1 mechanismanti-coplanarrequirementmust be able to make“good” carbocationstereospecificnot stereospecificregioselectiveregioselective
17 rate = k [RBr] [OEt] rate = k [RBr] E2 E1 EXAMPLES .. :O E t C H B r C 3CH2NaOEtEtOHE1CH3+0.01 MCHBr3KOHEtOHrate = k [RBr]
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