Question

Sodium Borohydride Reduction Reaction

1. Dissolve 600 mg of your carbonyl compound in 3 mL of methanol. Cover the flask with aluminum foil when not adding reagents or taking TLC samples to keep any water from getting in. 2. Cool your flask to 0° C using ice/water.
3. Allow to stir in the ice for 2 minutes then slowly add 0.4g of NaBH4 to your solution, then remove from ice bath and stir at room temperature.
4. After 10 min, TLC the reaction against the starting unknown carbonyl compound. TLC every 10 minutes until the starting material is no longer present. NOTE: See “General Technique for Monitor Reaction by TLC” in this module and the Resource module for directions on monitoring the reaction via TLC. Please note you will have to acidify the aliquot and add ether to the test tube. After stirring TLC from the organic layer.
5. Once reaction is complete cool again with an ice bath while stirring and slowly add 10mL of water.
6. Then add dilute HCl solution dropwise until solution is slightly acidic.
7. Remove from ice bath and pour into separatory funnel.
8. Extract with 20mL of ether three times, each time combing the collect ether. After three extractions discard aqueous layer in acidic waste.
9. Wash your ether layers twice with 5 mL of saturated sodium bicarbonate solution, (NaHCO3 (aq). Discard aqueous layer into basicwaste.
10. Wash separatory funnel (with soap) then wash the organic layer with 10mL of water. Discard water in basic waste and place ether in dry flask.
11. Dry ether with sodium sulfate or magnesium sulfate.
12. Use Buchner funnel clamped to bars to filter and remove drying agent.
13. Pour the dry ether into a clean flask which you have previously weighed and recorded the mass.
14. Allow ether to evaporate till the following week. Weigh the flask to find the yield. If you have a solid take a melting point. Take an NMR of the product and deduce the structure.

Source: Roseburg, R. E. Journal of Chemical Education 2007, 80 (9), 1474–1476.

Given the methods above, can you explain the following:

a. The solvent is methanol. What role does it play?

b. Dicuss the number of equivalents of hydride per molecule of sodium borohyride. To that end, how many hyride equivalents were added in this reaction?

c. What was the role of the HCl?

Answer 1

(a) Methanol is used as a solvent in the reaction. It donates a proton to the alkoxide ion formed during the reaction

(b) One molecule of NaBH₄ gives four hydride ions or in other words one mole of NaBH₄ gives four moles of H^- ions

Moles of NaBH₄ = given mass / molar mass = 0.4 / 37.8 = 10.5mmol

10.5 mmol = 10.5 X 4 = 42.0 mmol of hydride.

The moles of carbonyl compound can not be calculated as the molar mass of carbonyl is not given.

Let the molar mass of carbonyl = x

Moles of carbonyl = 0.6g / x

Equivalents of hydride = moles of hydride / moles of carbonyl

(c) HCl is used to neutralise the reaction mixture, as it contains the base.