Presentation on theme: "ELIMINATION ELIMINATION OTHER POSSIBLE MECHANISMS Do some elimination reactions occur in a different fashion?"— Presentation transcript:
ELIMINATION ELIMINATION OTHER POSSIBLE MECHANISMS Do some elimination reactions occur in a different fashion?
Three types of elimination reactions are conceivable E2 E1 E1cb concerted halogen first proton second proton first halogen second carbocation carbanion just studied
ALKYL HALIDES + WEAK BASE (SOLVOLYSIS) (SOLVOLYSIS) E1 The removal of a -hydrogen becomes difficult without a strong base and a different mechanism (ionization) begins to take place ….. if the substrate is capable.
The E1 Elimination Reaction ( two steps ) + :X:X slow fast rate = k [RX] carbocation 3 o > 2 o > 1 o Works best in a polar solvent. IONS FORMED unimolecular also favored if a resonance stabilized carbocation is formed step one step two weak base
starting material product Ea 1 Ea 2 HH intermediate TS 2 TS 1 ENERGY PROFILE two step reaction ENERGYENERGY step 1step 2 carbocation E1 slow
C X C H C X C H CC H CC H + + Carbocation is sp 2 hybridized ( planar ) and can react from either side. These two carbocations are equivalent by rotation and by symmetry. anti syn THE E1 REACTION IS NOT STEREOSPECIFIC rotation rate of C-C rotation = to / sec ( THE OPEN CARBOCATION IS PLANAR AND CAN ROTATE) Elimination can be either syn or anti.
E1 REACTION IS REGIOSELECTIVE THE ZAITSEV RULE IS FOLLOWED majorminor M KOH / EtOH tertiarytrisubstituted (stereochemistry is not a problem as in E2) disubstituted very dilute base Zaitsev
DIFFERENCES BETWEEN E1 AND E2 E1 AND E2
[RX] constant, [B] increasing Rate rate = k 1 [RX] E1 rate = k 2 [RX] [B] E2 BEHAVIOR OF THE RATE WITH INCREASING BASE CONCENTRATION second order first order E1 dominates at low base concentration E2 dominates at higher base concentration
[RX] constant, [Base] increasing Rate EFFECT OF BASE CONCENTRATION ON E1/E2 REACTIONS secondary RX, k’ tertiary RX, k’’ primary RX, k k 1 [RX] E1 For E1 elimination : k’’ (tertiary) > k’ (secondary) > k (primary). k 2 [RX] [B] E2 At high base concentration E1 never has a chance. At low base concentration E2 is nonexistent 1 1 1
secondary RX, k’ tertiary RX, k’’ primary RX, k [RX] constant, [B] Rate k 1 [RX] E1 EFFECT OF BASE CONCENTRATION ON E1/E2 REACTIONS k 2 [RX] [B] E2 For E2 elimination : line slopes k 2 differ for 1 o,2 o,3 o. Different substrates react at different rates, primary secondary tertiary k 2 k’ k’’ 2
Obviously for E1 which forms a carbocation intermediate rate : tertiary > secondary > primary > methyl But this same order holds for E2 also. STRUCTURE OF SUBSTRATE R-C-X R R R H H H primarysecondarytertiary tertiary has more -hydrogens EtO - more opportunites for reaction
E1 occurs only 1) at zero or low base concentration 2) with solvolysis (the solvent is the base) 3) with tertiary and resonance capable substrates (alkyl halides) If a strong base is present in moderate to high concentration, or the substrate is a primary halide, the E2 reaction dominates. WHEN THE E1 MECHANISM OCCURS
E2 mechanismE1 mechanism strong base high base conc. weak base low base conc. ALKYL HALIDE + BASE solvolysis must be able to make “good” carbocation or anti-coplanar requirement stereospecificnot stereospecific (solvent is base) regioselective
MANY E1 REACTIONS ARE SOLVOLYSIS REACTIONS SOLVOLYSIS SOLVOLYSIS = THE SOLVENT IS THE REAGENT (BASE) E1 competing product EtOH adds to the carbocation EtOH solvent acts as base - no other base is present
SOMETIMES E1 AND E2 RESULTS DIFFER
A COMPARISON OF E1 AND E2 CH 3 Br H H H H H Zaitsev Anti-Zaitsev NaOEt EtOH / E2 stereospecific anti not stereospecific E1 EtOH / anti syn major product E1 doesn’t require anti-coplanarity