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Wednesday, May 1, 2013

Sodium Borohydride Reduction of Acetophenone - Lu Le Laboratory



Sodium borohydride is an odorless white to gray-white microcrystalline powder which often forms lumps. It is soluble in water, without decomposition, however it reacts vigorously with acid solutions. The salt can be recrystallized by dissolving in warm (50 °C) diglyme followed by cooling the solution.    (Wikipedia)




The reduction of an aldehyde or ketone with sodium borohydride is straight forward and usually affords a high yield of the alcohol. The usual procedure (and the one employed in thi sex periment) involves dissolving the borohydride in 95% ethanol and adding the carbonyl compound to this solution. To ensure complete reaction, an excess of sodium borohydride is used.







The reaction between sodium borohydride and acetophenone is exothermic. Therefore, it is important to add the acetophenone drop-wise and to control the reaction temperature with an ice bath. After the reaction has been completed, the excess borohydride and the ethoxyborohydridesare destroyed with aqueous acid. Because hydrogen gas is evolved, this treatment with acid must be carried out in a fume hood or a very well-ventilated room.





Because ethanol, the reaction solvent, is water-soluble, a clean separation of organic and inorganic products cannot be achieved by a simple extraction with water and diethyl ether at this point. (Too much product would be lost in the aqueous ethanol layer.) To circumvent thisproblem, the first step in the work-up is to boil off much of the ethanol. In a larger-scale reaction, the ethanol would be distilled and collected. In a small-scale reaction such as in this experiment, the ethanol can be boiled away in the fume hood. When most of the ethanol has been removed, the product 1-phenylethanol oils out.



Water and diethyl ether are then added to the residue for the extraction of the organic compounds from the inorganic salts. The ether extract is dried with either sodium sulfate or magnesium sulfate. The crude product is obtained by distilling the ether.



Because of its high boiling point, 1-phenylethanol cannot be distilled at atmospheric pressure. Although it could be vacuum-distilled, distillation could not separate it from unreacted starting material (if any) because the two compounds have boiling points only 1E apart. (An infrared spectrum can be used to determine if any ketone is present in the product.)





Chemicals



1.      Acetonphenone: 4.000g

2.      Anhydrous sodium sulfate

3.      Diethly ether: ~15mL

4.      95% ethanol: 10mL

5.      3M hydrochloric acid: 3~4mL

6.      Sodium borohydride: 0.500g





Procedure



1.      Place 0.5 g sodium borohydride in a beaker.


2.      Add 10mL of 95% ethanol and stir until the solid is dissolved.


3.      Weight 4.0g of acetophenone into a conic flask and prepare an ice bath.


4.      Add the acetophenone dropwise to the borohydride solution while stirring the mixture continuously. Keep the temperature of the reaction mixture between 30~50 by controlling the rate of addition and by cooling the beaker in an ice bath.



 

5.      After the addition is completed, allow the mixture to stand at room temperature for 15 minutes with occasional stirring.


6.      Add about 4mL of 3M hydrochloric acid to the reaction mixture.






7.      Heat the mixture to boil on a hot plate until the mixture separates into two layers.




8.      Cool the reaction mixture in an ice bath.

9.      Transfer the mixture in a separatory funnel and add 7mL diethyl ether and add some water to dissolve the precipitate.


10.  Extract aqueous layer with the diethyl ether and do it again with another portion of diethyl ether.

11.  Combine the ether extracts and wash them with an equal volume of water.

12.  Dry them with anhydrous sodium sulfate.



13.  Filter the dried solution into a round-bottom flask.


14.  Use a Rotary to dry the solvent ether.


15.  Measure the yield and refractive rate.



Experiment Record

Weight of 1-phenylethanol
2.850g
Theoretical weight
4.066g
Yield
70.07%
Reflective Index in 30.1
1.5023






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