0.1 Extraction of carvone

Extraction of carvone

Objective

The purpose of this laboratory exercise is to take a natural product, in this case spearmint leaves and caraway seeds, and extract carvone from them.

Background

One of the most fascinating areas in organic chemistry is the study of stereochemistry. The most complicated part of most syntheses is to ensure that your product has the appropriate stereochemistry. An enormous amount of work is expended by chemists to ensure that they have better stereochemical control. Interestingly, living organisms have no problem producing stereochemically pure products. While it is difficult to make an enantiomerically pure compound in lab, plants and animals do it constantly with no error. Living organisms are also much more sensitive to stereochemical differences than many laboratory tests. Using most techniques, two enantiomers behave exactly the same. They have the same melting point, boiling point, and density, and will react identically with achiral chemicals. The only way to distinguish between different enantiomers in the laboratory is to measure their optical activity or treat them with other enantiomerically pure chemicals. Living organisms, on the other hand, are excellent at distinguishing between different enantiomers. The nose, for example, is one of the most sensitive tools for distinguishing some enantiomers. Two different enantiomers can smell completely different even though they behave the same as one another in many other tests.

In this lab, you will study two naturally produced enantiomers, (+)- and (-)-carvone. Both of these chemicals are edible in small quantities and are used as flavoring oils in industry. They have the exact same molecular structure except they are mirror images of one another.

You will extract carvone from both caraway seeds and spearmint leaves by soaking the plant matter in the proper solvent, and then you will use thin layer chromatography (TLC) and IR to compare the compounds you’ve extracted. The extraction is based on the fact that carvone is very soluble in methylene chloride and methanol. Therefore, soaking seeds or leaves in either of these solvents will cause compounds to leach out of the plant matter and into the solvent. The reason we do not use the same solvent for both extractions is because methanol dissolves carvone more readily but also dissolves many other substances. There are many materials in caraway seeds that are soluble in methanol that using this solvent results in a highly contaminated sample. In spearmint leaves, fewer contaminants are extracted into the methanol, so it is an ideal solvent for carvone extraction. Methylene chloride does not dissolve carvone well, but it is an effective solvent for the extraction of carvone from caraway seeds, where it is abundant.