Question

In organic chemistry lab we just completled a "Lidocaine Synthesis". I have some questions to clarify before we take the in class exam.

1. Based on apperence and melting point; how can we validate if the crystals are pure lidocaine?

2. When using the seperatory funnel in extraction, we washed the organic layer with water and would shake then drain the aqueous layer out. After a couple of washes with water, we then use HCL to extract and collect this aqueous layer seperatley. Following the extraction with HCL we then again washed the organic solution with water. Any of the washes with water were collected then combined at the end of using seperatory apparatus. Why would we wash the oganic layer with water again after using HCL? Does it yeild more product?

3. What compund were we dissolving in the aquoues extract?

Answer 1

1. A "pure" compound, if it's white, you can actually see the purity by seeing the color. If the original compound is white, then, you should see it whiter. Now, basing on melting point, you need to see if the compound has one melting point reported, or a range of melting point. For example, let's suppose that melting point for lidocaine is 231 °C, and you got a range of 226-229 °C of melting point. This means that although you have your compound, it's not 100% pure, cause you didn't get one melting point. The range in melting point (majority of cases of course) mean that the compound would need a recrystallization process to get a closer melting point and not a range. or at least to lower this range.

2. Actually you are washing with water again, to eliminate any trace of HCl remanent in the organic layer. As HCl is soluble in water, when you make the extraction with HCl, there are still some traces of HCl in the whole layer, and when you washed with water, you are "cleaning" the organic layer, and therefore, you can get more yield.

3. In this step, you are actually dissolving the base of the compound, by turning into the chloride salt of that base. LAter, with this you can do the conversion to lydocaine.

Hope this helps.