Chapter 10.6: Acid-Catalyzed Dehydration of Alcohols

o Step 1: Add a Proton  Proton transfer from the hydronium ion to the hydroxyl group gives an oxonium ion o Step 2: Take a Proton Away and Simultaneously Break a Bond to Give Stable Molecules or Ions  Simultaneous proton transfer to solvent and loss of water gives the carbon-carbon double bond of the terminal alkene o Step 3: 1,2 Shift and Simultaneously Break a Bond to Give Stable Molecules of Ions  Simultaneous shift of a hydride ion from the β-carbon to the α-carbon and loss of water gives a carbocation intermediate o Step 4: Take a Proton Away  Transfer of a proton from a carbon adjacent to the carbocation to solvent gives the rearranged alkenes.

H2SO4 (catalytic)

dehydration

Principle of Microscopic Reversibility

this principle states that the sequence of transition states and reactive intermediates in the mechanism of any reversible reaction must be the same, but in reverse order, for the reverse reaction as for the forward reaction

o 1° alcohol < 2° alcohol < 3° alcohol (easiest to hardest)

• Large amount of water (use of dilute aqueous acid) favor alcohol formation, whereas scarcity of water (use of concentrated acid) or experimental conditions where water is removed (heating the reaction mixture above 100°C) favor alkene formation

• When isomeric alkenes are obtained in acid-catalyzed dehydration of an alcohol, the alkene having the greater number of substituents on the double bond (the more stable alkene) generally predominates (Zaitsev’s rule)

elimination of water

o Step 1: Add a Proton  Proton transfer from the hydronium ion to the hydroxyl group of the alcohol gives an oxonium ion, which converts –OH, a poor leaving group, into –OH2, a better leaving group o Step 2: Break a Bond to Give Stable Molecules of Ions  Breaking of the C—O bond and loss of H2O gives a 2° carbocation intermediate o Step 3: Take a Proton Away  Proton transfer from a carbon adjacent to the positively charged carbon to H2O gives the alkene. In this step, the electrons of the C—H sigma bond become the electrons of the pi bond.

Zaitsev