Determination of Enantio-Purity of Products - Assignment Example

Determination of Enantio-purity of Products Introduction Optical purity is a parameter that can be applied to determine the chemical nature of a compound. Optical purity is based on the fact that different compounds have different chemical structures and would therefore rotate plane polarized light differently. Optical purity is very important because it can be used to determine whether a particular compound contains impurities. Determination of the optical purity of a compound can be achieved by determining the specific optical rotation of that compound. The result can then be compared to the optical rotation of an enantiomer which has been chemically predetermined. Specific optical rtation [a] is calculated as = observed rotation in degrees/pathelngth(dm)× concentration (gML). This can also be represented as a/l ×c Experimentally optical purity is identified as ee and can be calculated and further defined as: %ee = [α] D sample/ [α] D Pure enantiomer × 100 Therefore the %ee represents the extent to which a particular compound is in relation to the pure enantiomer. Another method of calculating optical purity is where the formulae is where: %ee = (R)- (S)/ (R)+ (S) × 100 For acids the optical purity is always positive while for bases it is always negative. Hence for a mixture having 90% (R)-enantiomer and 10% (S)-enantiomer has an optical purity of 80%. Materials and Reagents -Round bottomed flasks. -Steam bath. -Condenser. - Sodium hydroxide. -Ethanol. -Acetyl phenylalanine methyl ester. -Pasteur pipette. -Universal indicator -Dichloromethane. Experimental Procedure 0.3 molar sodium hydroxide was collected in a beaker and subtilisin Carlsberg protease enzyme was also obtained. The crude N-Acetylphenylalanine was placed in a 100ml conical flask and then water was added and 1 ML of universal indicator. The pH was then adjusted to 7.5 by adding 0.3M NaOH stepwise from a Pasteur indicator. 3M hydrochloric acid was added to adjust the pH back because it exceeded 8.0. The enzyme was then added slowly and using NaOH to ensure that the pH was maintained at approximately 7.5. Once the pH was stable the solution was transferred to a 250ml separating funnel and the thereafter extracted using dichloromethane. The Aqueous phase was then acidified using 1M hydrochloric acid and then extracted using dichloromethane. Answers A) Sample specific optical rotation= +12 Pure isomer specific optical rotation= -48 %ee = 12/48= 25% B) Proline reacts differently to other amino acids because proline has a cyclic structure and proline also lacks a primary amino group as compared to other amino acids. Experimental Procedure 2g of Phenylalanine methyl ester was obtained from the service room. 100ML of phenylalanine methyl ester was added to 10% sodium carbonate solution. 50ML of dichloromethane was added and the mixture shaken briefly. The aqueous layer was extracted by dichloromethane and collected in 250ML conical flask. The extract was the dried using sulphate and then the hydrated sodium sulphate was filtered using a fluted filter paper. Tri-ethylamine was added followed by acetyl chloride. The reaction was then swirled for additional ten minutes. The reaction mixture was then analyzed using ninhydrin. The organic mixture was then washed by sodium carbonate followed by 3M hydrochloric acid. The flask was then weighed and the quantity of N-Acetyl methyl ester determined. The bottom of the acid was then run off in a layer beaker and then ethyl acetate layer into a clean flask. The acid layer was further extracted using ethyl acetate. Ethyl acetate layer was further placed into 100ML separating funnel and the ethyl layer washed with 3M hydrochloric acid. The hydrochloric acid was basified using to deprotonate the codaine. 3M sodium hydroxide was added t the hydrochloric acid layer. The solution was then checked if it was basic using litmus paper. The basic solution was then transferred into 100ML separating funnel. The ether layer was then washed with 3M sodium hydroxide followed by brine. The ether was placed in a flask and dried over anhydrous magnesium sulphate. The mixture was then filtered in a flask and then boiling chips were added until the mixture was dried in a fume chamber. The sample was then transferred in clean test tube. Determining the Number of Compounds in unknown Tablet Three tablets of unknown compounds A, B and C were obtained from the service room. One tablet was crushed using labelled mortar and pestle and the powder transferred in a 25ML measuring cylinder. 5ML of 10%sodium hydroxide was then added stepwise, the sample will froth depending on the constituents of the tablet. 5ML of ethyl acetate was then added to the sodium hydroxide tablet mixture. On aluminum baked silica plate a line was ruled 1cm into the plate. The tip of the t.l.c plotter was then immersed into the ethyl acetate layer. I cm of the plotter was then allowed to flow into the plotter. The spotter was then held vertically over ruled line on the silica and then the spotter was lowered to until the tip just touched the plate. The tip was then removed quickly ensuring that the spot was as little as possible. Similar spots were made on the standard solution. The same place was spotted continuously until the solution in the spotter was all used. Thin Layer Chromatography Thin layer chromatography is a technique used to separate components of compounds. The technique behind thin layer chromatography is placing a spot of the material to be analysed on one end of the strip. The strip is made of a thin aluminum sheet that is coated with a thin layer of silica gel. The t.l.c technique was used to determine the identity of compounds in the unknown tablets. After the spot is placed on the strip the strip is dipped in a solvent. As the solvent rises the components of the mixture are then partitioned between the solid phase and the liquid phase. Compounds having higher affinity for the adsorbent will then migrate slowly as compared to other compounds. RF value is a term used to compare how different compounds migrate. RF is given by = distance a substance travels/distance travelled by the solvent. Read More