230 likes | 562 Vues
E1. ALCOHOLS WITH SULFURIC ACID. “Acid-assisted” E1. Carbocation Rearrangements. H. C. H. C. 3. 3. C. C. H. C. H. C. C. H. C. H. 3. 3. H. C. H. C. 3. 3. H. O. H. DEHYDRATION OF AN ALCOHOL. An alcohol can be “dehydrated” by treatment with
E N D
E1 ALCOHOLS WITH SULFURIC ACID “Acid-assisted” E1 Carbocation Rearrangements
H C H C 3 3 C C H C H C C H C H 3 3 H C H C 3 3 H O H DEHYDRATION OF AN ALCOHOL An alcohol can be “dehydrated” by treatment with concentrated sulfuric acid. H2SO4 heat + H-O-H b-hydrogen This is a beta-elimination reaction, similar to loss of HCl, but requiring acid conditions, rather than a strong base.
H C 3 C H C H C H 3 H C 3 O H H C 3 C C H C H 3 H C 3 ACID-ASSISTED E1 ELIMINATION LIKE E1 FROM HERE H C fast 3 C H C H C H 3 H C 3 O H + O H H O S O H O slow H C fast 3 C C H C H 3 + H C 3 H O - FOLLOWS ZAITSEV RULE H O S O O (or alcohol or water)
ROLE OF THE ACID ionization “ACID ASSISTED” R+ Alcohols do not ionize because OH- is a strong base (that is, OH- has a high energy). OH- .. .. NO : O-H- R+ R-O-H + .. .. ROH However, if you protonate the OH group, water leaves. + H+ R+ .. .. + YES : H2O R+ O-H R-O-H + + H H ROH2 Water is a stable, low energy, molecule.
EXAMPLES H3PO4 heat H2SO4 heat H3PO4 heat
C H 3 C H C H 3 3 H C C C H C H C C 3 3 C H C H 3 3 C H O H 3 WHY NOT ? b-H here C H 3 H C C C H C H 3 3 C H O H 3 no b-H REARRANGEMENT OF A CARBOCATION SURPRISE! H2SO4 H O + 2 different skeleton !
C H 3 C H C H 3 3 H C C C H C H C C 3 3 C H C H 3 3 C H O H 3 protonation and loss of water loss of H+ C H 3 C H C H 3 3 H C C C H C H C C 3 3 + + C H C H 3 3 H C H 3 OSO3H REARRANGEMENT C C : + methyl shifts with its pair of electrons C H 3 REARRANGEMENT OF A CARBOCATION H2SO4 H O + 2 different skeleton !
TYPES OF CARBOCATION REARRANGEMENTS 1,2-methyl shift methyl migration 1,2-hydride shift hydrogen migration 1,2-phenyl shift phenyl migration groups move with their bonded electrons
REARRANGEMENT - A 1,2-METHYL SHIFT H2SO4 protonation loss of H2O loss of H+ methyl shift
WHY DO THEY REARRANGE ? CARBOCATIONS REARRANGE TO ACHIEVE A LOWER ENERGY energy decrease secondary ion Carbocation Energies 3o < 2o < 1o < CH3+ tertiary ion lowest highest
WHICH GROUP MIGRATES ? BINGO ! Competing Options H or Me or Ph ? tertiary benzylic yes no secondary benzylic no The group that gives the best carbocation will be the one that migrates. secondary
1o 2o RING EXPANSION MeOH = H2SO4 When a carbocation is formed next to a small strained ring (cyclopropane or cyclobutane) the ring will often expand to the next larger size. MeOH This allows the relief of some of the strain. JUST LIKE A METHYL MIGRATION !
ALWAYS STOP STOP - LOOK - THINK R E G N A D E X R E M E EVALUATE FOR A REARRANGEMENT DO YOU HAVE A CARBOCATION? CAN YOU FORM A BETTER CARBOCATION ?
THE E1cb MECHANISM carbanion proton first halogen second This mechanism is rare since it requires special characteristics for the substrate: 1. The proton must be easy to remove (very acidic). This usually requires resonance stabilization in the conjugate base. 2. The leaving group must be hesitant to leave. This usually requires it to be a strong base, or to have a strong bond to carbon.
An Example of an E1cb Substrate acidic because the adjacent carbonyl group provides resonance in the conjugate base methoxide, the leaving group, is a strong base strong bond to carbon
WHY IT WORKS VIA E1cb O C H O O O 3 H H N a O C H H H 3 C H O H 3 O C H O C H O C H 3 3 3 fast step acidic hydrogen easily removed conjugate base stabilized by resonance slow step O + O C H 3 UNIMOLECULAR Slow step does not involve base strong base = poor leaving group
SUMMARY THERE IS A RANGE OF DIFFERENT MECHANISMS FOR b-ELIMINATION REACTIONS
COMPARISON OF b-ELIMINATION MECHANISMS alkyl halides alcohols special E1cb E2 E1 E1 E1 acid assisted strong strong weak base base base acidic neutral stepwise - carbanion concerted stepwise - carbocation “solvolysis” special case - not common stereospecific anti-coplanar not stereospecific requires: acidic H and poor leaving group Zaitsev if stereochem allows Zaitsev Zaitsev Zaitsev carbocation rearrangements
K.I.S.S. alkyl halide + strong base + heat = E2 alkyl halide + solvent + heat (solvolysis) = E1 alcohol + strong acid + heat = E1 (acid assisted) typical situation for E1cb H next to C=O (easy to remove) X = strong base (difficult to break bond) Only E1 reactions have rearrangements (carbocations) Only E2 reactions require anti-coplanar b-hydrogens