In this unit, we shall consider some of the methods commonly employed for the purification, qualitative analysis and quantitative analysis of an organic compound. The various methods used for this purpose are: Distillation under reduced pressure 7.
Besides reading the lab manual you will need to do a little bit more. To help you understand the chemical basis of this exercise, you should review Sections 3. Pay particular attention to the use of pKa values. You should also review the appropriate pages in the Mohrig and pay keen attention during your lab talk to acquaint yourself with extraction, washingdrying agents, and recrystallization.
Extraction is a particularly useful means of separating organic compounds if one compound in the mixture can be chemically converted to an ionic form. The ionic form is soluble in an aqueous layer and can be extracted into it.
Other, non-ionic organic compounds in the mixture will remain dissolved in the organic solvent layer. Separation of the two layers results in the separation of the two compounds. The extent to which an acid-base reaction proceeds to completion depends upon the relative acidity of the reactants and products.
Reactions occur so that stronger acids and bases are converted into weaker conjugate base and conjugate acids, respectively.
The pKa value of the acids gives a measure of the acidity of each compound. Stronger acids have smaller pKa values and their conjugate bases are weaker.
The position of an acid-base equilibrium can then be predicted from knowledge of the pKa values of the acids involved. Take a look at the following acid-base reactions in Figure 1, paying particular attention to the position of the equilibrium and its relationship to the pKa values given.
The reactions of a carboxylic acid and a phenol with bicarbonate ion. Note that the carboxylic acid has a lower pKa than the conjugate acid of bicarbonate ion carbonic acid. The reaction, therefore, proceeds to products.
The reaction of a phenol, however, favors the reactants since the pKa of phenol 10 is larger than that of the carbonic acid 6. Acid-base reactions favor the side with the weaker acid that is, they favor the side with the larger pKa. So, extracting a mixture of these two compounds with bicarbonate results in the ionization and extraction of a carboxylic acid in the presence of phenol thus separating the two compounds from one another.
Now, look at the reaction in Figure 2 where we use a stronger base to do the reaction: The reactions of a carboxylic acid and a phenol with hydroxide ion. Note that in both cases, the reactions favor the formation of products.
Therefore, extracting with hydroxide ion would result in the ionization and extraction of both compounds at he same time.
A close look at these two figures indicates that separating a mixture of a carboxylic acid and a phenol would best be done using bicarbonate ion since only the carboxylic acid is converted into its conjugate base by bicarbonate.
The conjugate base of the carboxylic acid, being an ionic species, is soluble in the aqueous layer while the phenol left unionized would remain dissolved in the organic layer. However, if we were to extract with hydroxide ion, both the carboxylic acid and the phenol would be converted into their conjugate bases see figure2.
The conjugate bases, again are both ionic species and therefore soluble in the aqueous layer. This means that both compounds would be extracted at the same time, resulting in no separation. A neutral compound will not react with either bicarbonate ion or hydroxide ion since a neutral compound does not have protons acidic enough to be removed by these bases.
Therefore, such a compound will remain dissolved in the organic layer, no matter which base is added. For example, a mixture of neutral compound and a carboxylic acid can be separated using bicarbonate ion since only carboxylic acid will be ionized by the bicarbonate ion.
Once extracted, the carboxylic acid and phenol can both be recovered by adding HCl to the aqueous solutions. The carboxylate ion and phenoxide will both be protonated by HCl, resulting in the formation of the original carboxylic acid and phenol, neither of which is soluble in water so they precipitate from solution.
The solid can then be isolated by filtration. Figure 3 shows this chemistry for you.
The reactions of a carboxylate ion and a phenoxide ion with HCl. Since HCl is stronger acid than either of the conjugate acids, the products are favored in both cases.
The products, a carboxylic acid and a phenol, are insoluble in aqueous solutions and precipitate from solution.Other, non-ionic organic compounds in the mixture will remain dissolved in the organic solvent layer. Separation of the two layers results in the separation of the two compounds.
The extent to which an acid-base reaction proceeds to completion depends upon the relative acidity of the reactants and products. (v) Separation and Drying of Crystals. The crystals are separated from the mother liquor by filtration with the help on a Buchner funnel and a suction pump (Fig.
). The crystals obtained on the filter paper. are washed two or three times with small quantities of the pure solvent.
The separation conditions for ppm each of selenocystine (SeCys), selenomethionine (SeMet), and selenoethionine (SeEth) were adapted from Puskel et al..
The concentrations of methanol and TFA in the eluent were optimized by Puskel et al.  and further by Pearson and McCroskey . 9 SEPARATION AND PURIFICATION. IDENTIFICATION OF ORGANIC COMPOUNDS BY SPECTROSCOPIC TECHNIQUES T he separation of mixtures of compounds to give the pure components is of.
The residue organics are eluted from the resins via organic solvents, followed by solvent removal and subsequent bioassay for mutagenicity. Then the residue organics are fractionated via a coupled bioassay/analytical fractionation method which progressively focuses to the bioactive constituents of the complex mixture of residue organics.
Home → Separation and Purification of.I Organic Compounds Organic compounds when isolated from natural sources or prepared by organic reactions are seldom pure; they are usually contaminated with small amounts of other compounds which are produced along with the desired product.