Purification of organic compounds

The experimental operations in organic chemistry which occur with greatest frequency are those which are concerned, directly or indirectly, with the isolation and purification of organic compounds. It is necessary therefore to describe in detail the chief methods of purification. Before doing so, however, the criteria of purity (and their observation) must first be discussed, so that when the purification has been attempted, its success can at once be checked and confirmed. 

Some information on hazards related to chemicals and solvents that are used in the book is listed together with the preparation procedures and in the appendix. However, the list is not exhaustive and the reader is referred to other sources (e.g., [55,56]).The description of a number of techniques that are used in organic laboratory during for analysis, separation, and purification of organic compounds is out of the scope of this book. Therefore, the reader is refeered to some exellent textbooks on practical organic chemistry (e.g. [57,58]. 

All reactants, unless otherwise stated, were purchased from Sigma Aldrich (97- -%). Trap-by-trap distillation were used for further purifications of organic compounds [53,58,59,61]. Ultra Pure air (containing <1 ppm hydrocarbons) was obtained from Matheson Inc. Carrier gases for GC (hydrogen, helium and air)... 

When the liquids form a constant-boiling mixture, they can not be separated in pure condition by fractional distillation. The boiling-point of a mixture of ethyl alcohol and water, which contains 96 per cent by weight of the former, is lower than that of pure alcohol. As a consequence, when a mixture of the two substances is subjected to repeated fractional distillation, the constant-boiling mixture is obtained. In order to prepare pure alcohol it is necessary to remove the water from the mixture by chemical means. Very few cases of this kind are met with in the purification of organic compounds. 

The types of examples that use ion exchange include general chemistry (salt conversions and preparation of deionized water), purification of organic compounds, environmental applications (trace enrichment and interference removal), food and beverage samples (sample clean-up), pharmaceutical and biological samples (trace enrichment), metal ions (trace enrichment), and humic substances (hydrogen saturation), to name but a few. These examples are explained in the following sections. 

In the last decade, supported liquid membranes based on ionic liquids (SILMs) have been successfully applied in separation and purification of organic compounds, involved in the synthesis of pharmaceutical and fine chemicals, (alcohol, esters, organic acids and amino acids) and mixed gases [16-25],... 

Sublimation plays an important role in the purification of organic compounds on a micro scale since it involves little loss of material, is convenient and fast, and provides very efficient purification. An essential condition for its application is, however, that the substance has sufficient vapor pressure at the temperature to be used. 

For the MCAT, we should be familiar with the basic features of separation and purification of organic compounds. In this cheater, we will review some of the techniques that are commonly used by organic chemists. 

Melt crystallization is an important separation, purification, and concentration technique used in the chemical, pharmaceutical, and food industries. Crystallization from melt is a very powerful separation process for the purification of organic compounds up to very high purities of 99.99%. Hence, the objectives of melt crystallization (purity, separation, or concentration) are quite often different compared to crystallization from solution (purity and defined crystal size distribution). 

Crystallization is an excellent method of purification of organic compounds. Insoluble impurities may be removed by hot gravity filtration. Mixed solvents may be used when a single solvent is not suitable. 

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