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

____ Reaction workup details (role of K₂CO₃,Na₂S₂O₃, brine)?

Answer 1

Answer :-

1] Role of K₂CO₃ :-

The hydroboration–oxidation reaction is a two-step hydration reaction that converts an alkene into an alcohol. Hydroboration–oxidation is an anti-Markovnikov reaction, with the hydroxyl group attaching to the less-substituted carbon.

The role of potassium carbonate (K₂CO₃) in the separation step was to get rid of all of the water. Because Potassium carbonate is soluble in water, a saturated solution was formed with it. It appears to display the snowball effect; that is, all potassium carbonate will bind with all the water, and now excess potassium carbonate will remain in the solution. Also, because it is insoluble in alcohol, none of the product was taken away; only the water was taken away. This is because the Potassium carbonate acted as a drying agent, for drying the organic phase.

2] Role of Na₂S₂O₃ :-

If any colour is caused by excess halogen reagent from your reaction mixture, then it is washed with sodium thiosulfate (Na₂S₂O₃). The solution will become clear, as Na₂S₂O₃ will dehalogenate the solution or mixture and neutralize the solution. Here, due to iodine, there is generation of yellow or pale yellow colour, so to remove it or to discolour it and to remove the iodine, sodium thiosulfate is used.

3] Role of brine :-

The process of synthesizing and isolating an organic compound often results in an organic compound or solution contaminated with traces of water. For instance, in aqueous extractions some water will be transferred into the organic phase because of the partial miscibility of the organic phase and water. Here, 1-hexanol, which will be formed is slightly miscible in water. Also, many reactions themselves are performed in an aqueous solution. This water must be removed before the required compound can be properly characterized.

The bulk of the water can often be removed by shaking or "washing" the organic layer with saturated aqueous sodium chloride (otherwise known as brine). The salt water works to pull the water from the organic layer to the water layer. This is because the concentrated salt solution wants to become more dilute and because salts have a stronger attraction to water than to organic solvents.

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