Dehydration of an Alcohol
Dehydration is reversible
When performing the acid-catalyzed dehydration of an alcohol, the yield of product can be increased by distilling off the product during the reaction.The dehydration is reversible, and removing the product from the reaction flask reduces the hydration reaction.
Where should the tip of the thermometer be placed in a microscale distillation set-up?
At or slightly below the side arm of the distillation head.
Cyclohexanol vs 1-hexanol
In the presence of acid and heat, cyclohexanol will undergo a more effective dehydration reaction because it is a secondary alcohol and the cation in the intermediate of the mechanism will be more stable.
Describe the complete role of the acid catalyst in the dehydration of an alcohol.
The acid protonates the hydroxyl group and then the conjugate base deprotonates an adjacent carbon.
Why does the dehydration of an alcohol more often use concentrated sulfuric acid, H2SO4, as the acid catalyst rather than dilute hydrochloric acid, HCl?
The additional water solvent from a dilute solution could reverse the dehydration reaction. The presence of the chloride ion could result in a competing substitution reaction.
How can a catalyst be recognized in a mechanism?
The catalyst is used and then regenerated in a later step.
Mechanism
The dehydration of a secondary alcohol, like cyclohexanol, is a mechanism that occurs in two steps. First, the alcohol is protonated to leave as a water molecule, creating a cation intermediate. Then, a hydrogen ion is removed, moving the electrons from that bond to make a carbon-carbon double bond.
If the dehydration reaction of an alcohol is successful, what changes would be seen in the IR spectrum for the product compared to the starting material?
The disappearance of an O-H and C-O band from the starting material. The addition of a C-C double bond band in the product.
What are the concerns presented by overheating a distillation to a dry flask?
The remaining solid residue might contain explosive peroxides. The empty glassware might heat quickly, igniting vapors from the distillation.
