A oil compound recently purified by bulb to bulb distillation. When synthesizing a library of compounds, we characterize their properties for later workers to match and compare. This involves H-NMR, C-NMR, IR, Rf, accurate mass spec and maybe elemental analysis or a crystal structure. If it is a solid at room temperature we are expected to measure its melting point. However if it is an oil a melting point is not required. While it is possible to freeze an oil and measure the point that it melts as it returns to room temperature, this is not expected because melting point is mostly an indication of purity that can be performed simply. Measuring low melting point compounds is also hard for other workers to repeat. Furthermore, a simpler method to determine the purity of an oil is to measure their refractive index.
Refractive index is a dimensionless number which is a measure of the speed that light travels through a medium compared with the speed of light in a vacuum. For example, the refractive index of water is 1.33 meaning that light travels 1.33 times slower in water than it does in a vacuum. Refractive index is always more than 1 and for most compounds it is between 1.3 and 1.7. It is typically measured to four decimal places as long as the substance is clear (allows light to pass through). When measured at a standard temperature (20 °C) it is, like melting point, constant for each compound. Refractive index, like melting point, is also easily altered by a small amount of an impurity .
Refractive index is a dimensionless number which is a measure of the speed that light travels through a medium compared with the speed of light in a vacuum. For example, the refractive index of water is 1.33 meaning that light travels 1.33 times slower in water than it does in a vacuum. Refractive index is always more than 1 and for most compounds it is between 1.3 and 1.7. It is typically measured to four decimal places as long as the substance is clear (allows light to pass through). When measured at a standard temperature (20 °C) it is, like melting point, constant for each compound. Refractive index, like melting point, is also easily altered by a small amount of an impurity .
Isoeugenol, Methyl eugenol and Methyl isoeugenol. It is common to measure the refractive index of an oil in the fragrance chemistry industry. This is largely due to a higher proportion of end use compounds being oils, but it can also be used to assess the desirability of essential oil extracts which are not pure compounds but are combinations of fragrant oils with ratios that vary depending on how the plant has been grown. The refractive index for an ingredient can be compared with previously measured batches of ingredients. Very similar compounds have refractive indexes that are generally close but are readily distinguished by this method. For example the refractive indexes for Isoeugenol, Methyl eugenol and Methyl isoeugenol at 20 °C are 1.5720-1.5770, 1.5320-1.5360 and 1.5660-1.5690 respectively. The maker of a fragrant product (eg perfumer) may use this knowledge to test the purity of a purchased batch of an ingredient in a simple way and compare it to literature values as well as their own earlier samples. This is important for keeping the end product consistent for the consumer, whether it be a perfume, cosmetic or cleaning product. Refractive index allows purity testing without access to an NMR and fulfills the same function as melting point for solid compounds.
Despite refractive index being a well established method for purity of oils, most journals don't require it for novel compounds even though they require melting points. While this seems strange to me I am relieved; my University lacks the equipment to measure refractive index and it would be tedious extra work to have it measured elsewhere.
