Reduction of camphor using sodium borohydride

REDUCTION OF A KETONE

OBJECTIVES: To perform a reduction of a ketone functional group suing NaBH4. To identify the isomer obtained by NMR.

Background

Reduction of carbonyl groups.

NaBH4 and LiAlH4 are common reagents utilized for reduction of ketones and aldehydes. They are called “metal hydrides” and they eract delivering an hydride (H-). In reactions with ketones and aldehyde they can be used interchangeably (unless there are other functional groups that could potentially react too). NaBH4 is considered a “milder” reducing agent, while LiAlH4 is considered “stronger”. For instance, esters, carboxylic acids can be reduced only using LiAlH4. Creation of a new stereogenic center upon reduction of an achiral ketone is expected to result in a racemic mixture of enantiomers that are indistinguishable by NMR.

Reduction of camphor

However, camphor is chiral. One side of the bicyclic ring is much more sterically hindered than the other side, making it possible for an achiral reducing agent such as NaBH4 to be selective for one

isomer. You will determine the isomers using NMR. Both isoborneol and borneol are obtained from reduction. The dominant isomer can be predicted based on the mechanism of the reaction, and the ratio of these isomers can be determined experimentally by NMR analysis. 3 As in the case of camphor, the NMR spectra are quite complicated. However, the proton adjacent to the alcohol in both compounds appears in a clear region of the 1H NMR spectrum that can be integrated to find the isomer ratio.

Distinguishing Borneol and Isoborneol Using NMR

You are given a 1H-NMR spectrum of borneol as a reference. Obtain 1H NMR spectra of the product mixture from Procedure B. In the 1H NMR, look closely at the protons on the alcohol carbon. (Hint: these typically appear at 3.5-4.0 ppm). The ratio of these groups of peaks corresponds to the percentages of isoborneol and borneol. In Delta, use integration to determine the relative amounts of the two products. You should use the zoom tool to expand the region, then integrate the CHOH signals; use the "cut integral" tool to separate the integrals so that you can calculate a ratio. The easiest way to do this is to select the smaller one (so it is yellow), and type "1".

Experimental

Start with 500 mg of camphor. Dissolve in methanol in an Erlenmeyer flask to make a 1.15 M solution. Cool the solution on an ice bath, then add 1 molar equivalent of NaBH4 in three portions over 10 min,

see below. After all of the sodium borohydride is added, boil the mixture on a sand bath (hotplate) for 2 minutes. Pour the product mixture into ice and rinse the flask with ~1.25 mL of methanol into the ice. Collect the white precipitate on a Hirsch funnel. Dissolve the solid in about 5-8 mL of ether in small erlenmeyer, dry with CaCl2, transfer

to a tared round-bottom flask, then remove the ether using a rotary evaporator. Calculate a percent yield of the crude dry product. Take

an 1H NMR spectrum of the product to determine purity and identify products

References

1. Based on a microscale experiment on the oxidation of 9- fluorenol to 9-fluorenone. Jones, C. S.; Albizati, K. J. Chem. Educ. 1994, 71, A271-A272.

2. Based on a microscale experiment on the reduction of camphor to isoborneol Pavia, Lampman, Kris Advanced Organic Laboratory, 3rd ed. Saunders: New York, 1985.

2

POST LAB

1) Write the introduction for the reduction experiment in not more than 5 sentences

2) Write the abstract using not more than 4 sentences

3) Write a short discussion (include the following points: 1) mechanism of the reaction, 2) which isomer is formed in higher yield based on the NMR analysis, 3) Discuss the NMR and how it helps in evaluating the final product

3) Write the experimental section (NMR analysis

using the following format (Ex: 1H-NMR (CDCl3)

δ1.2 (t, 2H, J = 6.1Hz), 1.8 (s, 9H), 2.0 (s,

3H), 2.3 (t, 2H, J= 6Hz).

4) Write the conclusions section.