Chapter 1.

Background

Hydride based reducing agents LiAlH₄ (lithium aluminum hydride) and NaBH₄ (sodium borohydride) react with ketones and aldehydes to produce a 1° or 2° alcohol product. Both reagents were discovered by Schlesinger in the 1940s and are routinely used in organic synthesis. NaBH₄ is a milder reducing agent than LiAlH₄ and can be used in protic solvents, such as ethanol. Conversely LiAlH₄ must always be used in aprotic solvents, such as tetrahydrofuran, and under extremely rigorous anhydrous (moisture free) conditions. LiAlH₄ also requires a separate acidic work up step where reduction with NaBH₄ does not. For more on these reagents see Eğe sections 14.4 and 21.3 B.

Schlesinger, H.I.; Brown, H.C. et al. J. Am. Chem. Soc.1952, 75, 186.

Reaction

Reaction Scheme:

Note the stoichiometry of the reaction. Each molecule of NaBH₄ can reduce up to 4 carbonyl groups since it delivers 4 hydrides per NaBH₄ molecule. With that in mind try drawing the balanced equation for this reaction. Hint: one H on the alcohol comes from a hydride and the other comes from the acidic proton on CH₃CH₂OH (ethanol).

Lab Objective

In part 1 you will reduce 9-fluorenone using the procedure described in the next tab. In part 2 you will reduce an unknown ketone also using the method described. Your objective is to determine if the ketone unknown can be reduced by NaBH₄ to form an alcohol, to compare the two reactions (part 1 & 2) and to determine the identity of the unknown ketone.

Procedure

Reagents

• 9-fluorenone (part 1)
• Unknown ketone (part 2)
• Ethanol
• NaBH₄

In a dram vial, dissolve 0.1 g of 9-fluorenone (part 1) or unknown aromatic ketone (part 2) in 2 mL of 95% ethanol, and cool the solution in ice (most ketones will produce a fine suspension). Add to this solution or suspension 20 mg of sodium borohydride (a large excess). If you have a suspension the suspended ketone solid will dissolve. The reaction mixture should warm up and it may be necessary to add more ethanol if it has evaporated (especially when starting with 9-fluorenone). After 15 minutes, add 1 mL of water and heat the solution to the boiling point. Dilute the hot solution with hot water (1–2 mL) to the point of saturation indicated by cloudiness.* Collect the crystalline precipitates generated upon cooling the mixture to room temperature using vacuum filtration. Recrystallize the reduction product.

*Note three of the unknown aromatic ketones should produce liquid products after reduction. If your unknown product is a liquid the mixture will not become cloudy upon addition of 2 mL of water. In this case you will perform a microscale extraction to isolate your product. Add 4–5 mL of diethyl ether to the mixture and mix to allow the product to migrate into the organic layer. Transfer the ether layer into another dram vial and wash with an equal volume of brine (saturated solution of NaCl). Pipet out the ether layer and dry it over anhydrous magnesium sulfate. Remove the magnesium sulfate by gravity filtration and evaporate the organic solvent by applying a stream of nitrogen gas.

Formation of the DNP-derivative to test for the presence of an aldehyde or ketone

Ketones and aldehydes will react readily with DNP (2,4-dinitrophenylhydrazine) to form a distinctive precipitate, which will range in color from yellow to orange depending on the degree of conjugation. The formation of the DNP-derivative is often used as a diagnostic test for the presence of an aldehyde or ketone functional group. The derivatives are especially useful because they are easily recrystallized and characterized, whereas ketones and aldehydes are notoriously difficult to purify and characterize. In the reduction of a ketone with NaBH₄, the DNP reagent can be used to quickly and easily test for the completion of the reduction reaction by indicating the presence (or absence of) ketone starting material.

Procedure

In a vial dissolve a small amount of the reaction mixture in 95% ethanol (about 50 mg per 2 ml ethanol). Add the DNP reagent drop wise. If the ketone starting material is still present a precipitate (yellow in color) will instantly form. This precipitate is the DNP-derivative and indicates that the reduction reaction is not complete.

Techniques

Reagents

Characterization

Determine the purity of the products in part 1 and 2, as well as the success of each reaction using TLC. Collect the Infrared spectrum of each product and the unknown ketone starting material. Characterize the starting materials and products of part 1 and 2 by m.p. Use the Beilstein test for halogens to rule out ketone product 1 (the only unknown with halogen) from table 2-1 (next tab). Use Reaxsys or SDBS to search for literature IR spectra for the unknown ketone and its corresponding alcohol product for comparison to the spectra you obtain.

Table 2-1