Question

Sodium Borohydride Reduction of a Ketone

1. How will the dibenzalacetone product be isolated from the reaction mixture and how will it then be further purified? How will the product be characterized?

2. What apparatus and conditions (solvent, temp., reaction time, etc) will be used for the reaction?

3. Identify which is the reducing agent and which is the oxidizing agent in the reaction of benzophenone with sodium borohydride and explain your reasoning, ie. show the changes in oxidation states (best to give a drawing as per the introduction)?  

Answer 1

1) Dibenzalacetone is obtained by aldol condensation of benzaldehyde with acetone. It will not be formed in the reduction of ketone by NaBH4.

So the reaction will be in ethanolic sodium hydroxide solution.

The product separates out as precipitate in the reaction (it will be yellow in colour).

The product will be isolated by the use of suction filteration. the product will be separted then, and will be dried over. We may wash the product with ethanol at low temperature.

The product can be characterized by NMR spectroscopy, IR spectroscopy and as the product is coloured (yellow) it will also show distinct absorption peak in UV-Visible spectrum.

2) If we are talking about the above mentioned reaction (Sodium Borohydride Reduction of a Ketone)

Then we will take following conditions for the reaction

a) Apparatus: Beaker , Thermostat , Stirrer

Solvent: we will use methanol / ethanol as solvent

Reaction Time: The time will be nearly 30 minutes.

For the reaction we will use Sodium borohydride for reduction of ketone.

By each mole of NaBH4 four moles of ketones get reduced. We will take excess of NaBH4 as required from calculation. The Excess reagent ensures complete reduction of ketone also it increases the rate of reaction .

(i) Dissolve the ketone in methanol (solubility increases with increase in temperature slightly)

(ii) Then we will add NaBH4 to the solution , the reaction mixture will be stirred to complete the reaction (the reaction will occur at room temperature initially)

(iii) After 15-20 minutes we will add some water and will warm the reaction mixture

(iv) cooled slowly, then on ice bath

(v) Product will precipitate out, filtered with Buckner funnel.

3) Reducing agent : NaBH4

Oxidizing agent : The ketone

Reaction will be

so as we can see the oxidation number of carbonyl carbon is decreased from +2 to +1 [so it reduced]

while the oxidation number of boron will increase so it is oxidized