ELIMINATION OTHER POSSIBLE MECHANISMS

ELIMINATION OTHER POSSIBLE MECHANISMS Do some elimination reactions occur in a different fashion?

just studied Three types of elimination reactions are conceivable concerted E2 carbocation E1 halogen first proton second carbanion E1cb proton first halogen second

E1 ALKYL HALIDES + WEAK BASE (SOLVOLYSIS) The removal of a b-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) weak base carbocation slow :X + step one 3o > 2o > 1o also favored if a resonance stabilized carbocation is formed step two unimolecular fast rate = k [RX] Works best in a polar solvent. IONS FORMED

E1 ENERGY PROFILE two step reaction carbocation intermediate TS1 E N E R G Y TS2 Ea2 Ea1 step 1 step 2 starting material DH slow product

STEREOSPECIFICITY

THE E1 REACTION IS NOT STEREOSPECIFIC ( THE OPEN CARBOCATION IS PLANAR AND CAN ROTATE) H Carbocation is sp2 hybridized ( planar ) and can react from either side. + H anti C C C C rotation X These two carbocations are equivalent by rotation and by symmetry. syn C C C C H X H + rate of C-C rotation = 1010 to 1012 / sec Elimination can be either syn or anti.

REGIOSELECTIVITY

E1 REACTION IS REGIOSELECTIVE THE ZAITSEV RULE IS FOLLOWED (stereochemistry is not a problem as in E2) very dilute base 0.001 M KOH / EtOH major minor tertiary trisubstituted disubstituted Zaitsev

DIFFERENCES BETWEEN E1 AND E2

BEHAVIOR OF THE RATE WITH INCREASING BASE CONCENTRATION E2 rate = k2 [RX] [B] second order E1 dominates at low base concentration E2 dominates at higher base concentration rate = k1 [RX] first order Rate E1 [RX] constant, [B] increasing

At high base concentration E1 never has a chance. At low base concentration E2 is nonexistent EFFECT OF BASE CONCENTRATION ON E1/E2 REACTIONS k2 [RX] [B] E2 k1 [RX] tertiary RX, k’’ 1 E1 secondary RX, k’ Rate 1 primary RX, k 1 [RX] constant, [Base] increasing For E1 elimination : k’’ (tertiary) > k’ (secondary) > k (primary).

EFFECT OF BASE CONCENTRATION ON E1/E2 REACTIONS tertiary k’’ 2 secondary k’ 2 E2 k k2 [RX] [B] 2 primary k1 [RX] tertiary RX, k’’ 1 E1 secondary RX, k’ Rate 1 primary RX, k [RX] constant, [B] For E2 elimination : line slopes k2 differ for 1o,2o,3o . Different substrates react at different rates,

STRUCTURE OF SUBSTRATE R H H R-C-X R-C-X R-C-X R R H primary secondary tertiary Obviously for E1 which forms a carbocation intermediate rate : tertiary > secondary > primary > methyl But this same order holds for E2 also. tertiary has more b -hydrogens more opportunites for reaction EtO-

WHEN THE E1 MECHANISM OCCURS 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.

ALKYL HALIDE + BASE strong base high base conc. weak base low base conc. or solvolysis (solvent is base) E2 mechanism E1 mechanism anti-coplanar requirement must be able to make “good” carbocation stereospecific not stereospecific regioselective regioselective

.. :O E t .. C H C H C H C H 2 3 B r C H C H C H C H 3 3 + B r EXAMPLES rate = k [RBr] [OEt] E2 6M N a O E t E t O H E1 0.01 M K O H E t O H rate = k [RBr]

SOLVOLYSIS The solvent is the thing !

SOLVOLYSIS MANY E1 REACTIONS ARE SOLVOLYSIS REACTIONS SOLVOLYSIS = THE SOLVENT IS THE REAGENT (BASE) competing product E1 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 major product E2 syn Anti-Zaitsev H NaOEt H stereospecific anti Br H EtOH / D H CH3 H CH3 EtOH / D E1 anti Zaitsev not stereospecific CH3 E1 doesn’t require anti-coplanarity

ELIMINATION OTHER POSSIBLE MECHANISMS