SEPARATING AND PURIFYING THE PRODUCT

Now that the reaction is over, you may think that the hard work is finished. Not so. Chemists often refer to the next stage of the process as the work-up of a reaction. 

A typical work-up procedure may be as simple as the addition of another reagent (in organic reactions this is often water or dilute acid) in order to finish or quench a reaction but it may be a lengthy series of procedures, taking far longer than the actual reaction itself. Nevertheless, the work-up stage of a reaction may be critical to its success. 

Sometimes the work-up of a reaction is simple. Take the case of the last of the three experiments that we considered, the preparation of the complex [ Fe(CO)2(il -C5H5) 2] Here we saw a sophisticated experiment where we had to take precautions to exclude air and moisture in order to ensure success. This reaction has a very easy work-up procedure. After the reaction mixture has cooled to room temperature, deep-red crystals of the desired product form. The crystals are simply filtered off from the solution and dried in the air. This is an example of a complicated reaction where the desired product is the only solid obtained and so its isolation is very simple. 

Now let us consider the reaction of a carbonyl group with a Grignard reagent (CH3MgBr) to produce an alcohol. You will often see this reaction written as 

This is not quite a true representation of the experiment. The methyl group adds to the carbonyl group, but the product is not the alcohol, but a species known as a metal alkoxide (an ionic complex between the negatively charged oxygen atom and the positively charged metal) 

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High-pressure liquid chromatography (HPLC) is used to separate and purify the products of laboratory reactions. 

The intermediate organolithium compound 358 shown in equation 89 is sufficiently stable at low temperature to be determined by NMR spectroscopy. Its presence can also be deduced after quenching with an electrophile, separating and purifying the product 359 and characterizing its structure by physical methods, including IR, H and NMR spectroscopies and MS. ... 

The semi-continuous type of reactor with the large capacity was comprised of a pyrolysis chamber, a catalytic cracking chamber and a separation and purifying section. The feed plastic material was melted and decomposed in the pyrolysis chamber held at the ambient pressure and at the temperature 723-783 K, and fed to the catalytic cracking chamber. A reflux condenser was used to separate and purify the products formed in the chamber and individual factors were obtained using fractional distillation apparatus [26]. Different types of reactors are being utilized depending on the type of feed and the expected products from the pyrolysis. 

Chemical reactors are the heart of processing plants. Unlike all the other unit operations, it is in the reactor where the molecules that make up the raw materials undergo profound changes, transforming into valuable commercial products. Chemical changes occur only in the reactor. Other operations involve the preparation of raw materials to feed the reactor or to separate and purify the products. 

Once you have completed a reaction, separated and purified the product(s), there are still questions to be answered ... 

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