Slide 1 : DE HYDRATION OF AN ALCOHOL ELIMINATION REACTION
Slide 2 : DEHYDRATION OF AN ALCOHOL An alcohol can be “dehydrated” by treatment with
concentrated sulfuric acid. + H-O-H H2SO4 heat b-hydrogen This is a beta-elimination reaction, similar to loss
of HCl, but requiring acid conditions, rather than
a strong base.
Slide 3 : H O S O O O H C H C H 3 O C H H H 3 C H 3 C H C H C H 3 C H 3 C H 3 C H H O S O O O + + ACID-ASSISTED E1 ELIMINATION FOLLOWS
ZAITSEV RULE slow fast fast - (or alcohol or water) LIKE E1
FROM HERE
Slide 4 : ROLE OF THE ACID Alcohols do not ionize because OH- is a strong
base (that is, OH- has a high energy). R-O-H R+ + O-H- However, if you protonate
the OH group, water leaves. R-O-H R+ + O-H H H + Water is a stable, low energy, molecule. .. .. .. .. : : .. .. + H+ NO YES “ACID ASSISTED” R+ R+ OH- H2O ROH ROH2 + ionization
Slide 5 : EXAMPLES H3PO4 heat H2SO4 heat H3PO4 heat
Slide 6 : CARBOCATION
REARRANGEMENTS
Slide 7 : + H 2 O REARRANGEMENT OF A CARBOCATION H2SO4 SURPRISE! different
skeleton !
Slide 8 : + H 2 O REARRANGEMENT OF A CARBOCATION H2SO4 different
skeleton !
Slide 9 : TYPES OF CARBOCATION REARRANGEMENTS 1,2-methyl shift 1,2-hydride shift 1,2-phenyl shift methyl migration hydrogen migration phenyl migration groups move with their bonded electrons
Slide 10 : methyl shift protonation
loss of H2O loss of H+ REARRANGEMENT - A 1,2-METHYL SHIFT H2SO4
Slide 11 : WHY DO THEY REARRANGE ? secondary ion tertiary ion CARBOCATIONS
REARRANGE
TO ACHIEVE A
LOWER ENERGY Carbocation Energies 3o < 2o < 1o < CH3+ energy
decrease lowest highest
Slide 12 : WHICH GROUP MIGRATES ? tertiary
benzylic The group that gives the
best carbocation will be
the one that migrates. yes secondary
benzylic secondary no no Competing Options H or Me or Ph ? BINGO !
Slide 13 : RING EXPANSION 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 H2SO4 MeOH This allows the relief of some of the strain. = JUST LIKE A METHYL MIGRATION !