Preparation of Alcohols from Alkenes

by Hydroboration-oxidation

Background and Mechanism

There are three important methods for converting alkenes to alcohols. The first of these,

and the oldest, is the acid-catalyzed addition of water. The mechanism involves converting

alkenes to alcohols via carbocations, which are subject to rearrangement.

The second method is oxymercuration-demercuration. The major product is the

Markovnikov addition adduct; rearrangements are not observed.

The third method is hydroboration-oxidation. The mechanism involves anti-Markovnikov

and the stereochemistry of addition is “syn.” (Syn means that both the hydrogen and the

hydroxyl are added to the same side of the double bond.) In this experiment, 3,3-dimethyl-

1-butene will undergo hydroboration-oxidation to yield 3,3-dimethyl-1-butanol. The

starting material is a very volatile liquid that can easily be removed along with the solvent,

leaving the product for analysis.

The mechanism of hydroboration is shown here.

Safety

1. Wear eye protection at all times in the laboratory.

2. Wear gloves when handling strong bases. Avoid skin contact with basic solutions.

If base contacts the skin, immediately wash the affected area with copious amounts

of cool water.

3. Work in the hood or in a well-ventilated area during this experiment,

4. Diborane is highly toxic. Dispense it in a hood. Avoid skin contact and inhalation of

vapors.

5. Hydrogen peroxide is an oxidant. Wear gloves when handling.

Microscale experiment

Dry all glassware in an oven prior to the experiment. To a 5-mL short neck flask

add 150 μL of 3,3-dimethyl-1-butene and 1 mL of dry THF. Swirl to mix. Attach a

distillation head fitted with a calcium chloride drying tube on the side neck and a rubber

septum on the center neck. Add dropwise 300 μL of 1 M diborane/THF solution through

the septum in the distillation head using a syringe. The addition should take about 1

minute. Stir the solution for 45 minutes at room temperature.

Remove the distillation head from the flask and add 15 drops of 2 M NaOH. Using

a syringe, add dropwise 0.75 mL of 30% hydrogen peroxide with stirring. Place a water-

jacketed condenser on top of the vial. Heat the mixture for 30 minutes at reflux using a

sand bath. Remove the apparatus from the heat source and remove the condenser. Cool

the reaction mixture to room temperature. Reduce the volume of the solution to about 2.0

mL using a warm sand bath. Cool to room temperature. Cool the flask using ice or cold

water and remove the magnetic stirring bar.

In the hood carefully add 500 μL of 3 M HCl. Add 1.5 mL of methylene chloride.

Cap the flask and shake. Vent by carefully loosening the cap. Draw off the lower (organic)

layer and save this solution in a separate vial. Add 1.5 mL of fresh methylene chloride to

the reaction flask and repeat the extraction. Remove the methylene chloride solution and

combine it with the first extract. Dry the combined methylene chloride solution over

sodium sulfate and transfer the solution to a tared vial containing a boiling chip.

Evaporate the solution to a small volume in the hood using a warm sand bath, leaving an

oily residue. (Be careful not to also boil away the product!) Reweigh the vial containing

the product.

Note:

Some of the solutions are air-sensitive and toxic, especially the diborane in THF

solution. Do this experiment in the hood.

Cleanup & Disposal

Place the aqueous washings in the sink and wash them down the drain. Be sure there are

no residues of diborane solution or peroxide solution left on the lab bench. Place any

recovered methylene chloride solutions in the container labeled “halogenated solvent

waste” in the hood.

Miniscale Experiment

Dry all glassware in an oven prior to the experiment. To a 50-mL round-bottom

flask equipped with stir bar add 2.00 mL of 3,3-dimethyl-1-butene and 7 mL of dry THF.

Swirl to mix. Attach a Claisen adapter fitted with a calcium chloride drying tube on the

side neck and a septum on the center neck. Add dropwise 4.0 mL of 1 M diborane/THF

solution through the septum in the Claisen adapter using a syringe. The addition should

take about 5 minutes. Stir the solution for 45 minutes at room temperature. Do not

expose any of the reagents to air or water vapor. Keep dry.

Remove the Claisen adapter from the flask and add 10.0 mL of 2 M NaOH. Using a

pipette, add dropwise 10.0 mL of 30% hydrogen peroxide with stirring. Attach a water-

jacketed condenser to the flask. Heat the mixture with stirring for 30 minutes at reflux

using a sand bath or heating mantle. Remove the apparatus from the heat source and

remove the condenser. Cool the reaction mixture to room temperature and remove the

stir bar. Cool the flask using ice or cold water.

Carefully add 7.0 mL of 3 M HCl. Transfer the solution to a 125-mL separatory

funnel. Add 5 mL of diethyl ether. Stopper and shake. Vent frequently to relieve

pressure. Draw off the lower (aqueous) layer and save. Pour the ether solution into a

25-mL Erlenmeyer flask. Put the aqueous layer back into the separatory funnel and repeat

the extraction using a second 5-mL portion of diethyl ether. Drain off the aqueous extract

and save for disposal. Combine the second diethyl ether extract with the first ether extract

and dry using anhydrous sodium sulfate. Transfer the solution to a small suction flask and

add a boiling chip. Connect the suction flask to an aspirator with vacuum tubing and draw

a vacuum on the flask for a few minutes or until the diethyl ether has evaporated. Swirl

the flask occasionally as necessary to avoid frothing. Transfer the remaining liquid to

5-mL round bottom flask, add a boiling chip, and do a microscale distillation of the crude

product into a weighed vial. Reweigh the vial containing the product. Run an IR of the

product.