Question

HYDROBORATION-OXIDATION OF 1-HEXENE 1. In a screw cap V-vial, prepare a solution of 0.33 g of iodine in 3 mL of dry THF. Firmly tighten the cap; keep the vial closed until ready to use. 2. In a 25 mL round-bottom flask, weigh 0.12 g of sodium borohydride. Add 8 mL of dry THF; stopper the flask with a septum, and stir the mixture for 10 min at room temperature 3. Prepare a drying tube from a short Pasteur pipet packed with anhydrous CaCl2 between two cotton plugs 4. Add 0.885 mL of 1-hexene to the borohydride solution Assemble the apparatus as shown; place the reaction flask in an ice-water bath 5. Using a syringe, draw the iodine solution from the Vv-vial Pierce the septum with the syringe needle and add the iodine solution dropwise, a few drops at a time. Add each new portion when the solution becomes colorless or light yellow The addition should be complete within 20-30 min. Remove the water bath and stir the resulting mixture for at least 40 min at room temperature 6. Remove the rubber septum and the drying tube, and add dropwise 3 mL of water. Stir the mixture for 5 min and add 1.5 mL of 3N N2OH solution. 7. Place the apparatus in an ice-water bath. Add 1.5 mL of hydrogen peroxide solution dropwise with vigorous stirring. Once the addition is complete, stir the resulting mixture for 15 min at room temperature 8. Pour the reaction mixture into a 50 mL Erlenmeyer flask containing 6 g of potassium carbonate. Swirl the flask until alll carbonate is dissolved. 9. Transfer the mixture to a 60 mL separatory funnel. Add 5 mL of water and 5 mL of diethyl ether, and separate the layers. Extract the aqueous layer one more time with 7 mL portion of diethyl ether. Combine the organic layers and return the ethereal solution to the separatory funnel 10. Wash the combined organic extract successively with 10 mL portions of 1) sodium thiosulfate, 2) water, and 3) brine. After each wash, drain and discard the aqueous (bottom) layer. 11. Transfer the ethereal extract into a clean, dry 50 mL Erlenmeyer flask. Dry the ethereal extract with several spatula-tips full of anhydrous magnesium sulfate. Filter the solution into a dry, tared 100 mL round bottom flask

____ What was the purpose of hydrogen peroxide and why was it added slowly with cooling?

Answer 1

Solution:

1-hexene is a terminal alkene.

Normal conversion of 1-hexene to alcohol by addition of water will give 2-hexanol

But via hydroboration oxidation we will get 1-hexanol.Also it is a syn-addition. That is the H and OH will be added on the same side of the double bond.

The mechanism is a two step process. In the first step, the borane forms an intermediate with 1-hexene in which the boron attached to the terminal carbon (can be explained by the electrophilic character of Boron and the stability of carbocation) . In the second step the terminal boron is replaced by -OH. The Hydrogen added to the double bond is from Borane and the OH added to the terminal Carbon is from H₂O₂

Thus, the purpose of H₂O₂ is to replace the Boron in the internediate by OH.

The reason why is to be added slowly, dropwise is that to avoid excess concentration of H2O2 in the reaction mixture at any time other wise it will further oxidise the product alcohol into aldehyde or acid. The reason why the condition to be ice cold is that H₂O₂ is highly unstable under hot condition as it may decompose to oxygen and water and also the alcohol formation is an exothermic reaction which may increase the temperature during the reaction. So it is necessary to add slowly with vigorous stirring and under ice cold condition.