The role of activated carbon in organic synthesis

Activated carbon is one of the commonly used substances in organic synthesis experiments. It is mainly used to decolorize and remove impurities. According to the basic experiment book of chemistry, the standard use method of activated carbon is thermal filtration and decolorization before recrystallization. In fact, activated carbon has other uses. Skillfully using the properties of activated carbon to distinguish its advantages and disadvantages will bring a lot of convenience to the experiment of organic synthesis.

The functions of activated carbon in organic synthesis mainly include decolorization, adsorption and filter aid. Usually, in one operation of activated carbon, it mainly shows one aspect, and the functions of other aspects are secondary.

1. Decolorization

The most common function of activated carbon is decolorization. According to and polarity analysis, activated carbon can be regarded as non-polar material, which can be used to adsorb non-polar and small polar pigments, and is suitable for use in large polar solvents. Most of the pigments contained in the material belong to non-polar or small polar pigments, so activated carbon is the most commonly used decolorizer, and the most commonly used solvents are water and alcohol. Generally, when decolorization is required, the polarity of pigment does not need to be considered. Decolorization is directly carried out with activated carbon. The decolorization effect is judged by observing the change of solution before and after decolorization.

The general operation process is as follows:

Add the decolorizing substance into a certain amount of solvent, heat it to fully dissolve, add a certain amount of activated carbon, stir for a period of time, heat filter and concentrate the filtrate. The substance to be decolorized is solid or liquid containing visible pigment, mostly solid; The amount of solvent is generally 3-10 times, which is too small and difficult to operate, resulting in large loss during thermal filtration; Too much cost is too high and unnecessary; Solvents are generally large polar solvents, such as methanol, ethanol and water. If crystallization is required after decolorization, the most suitable solvent needs to be selected; The amount of activated carbon is generally 5-10% of the solute (i.e. the substance to be decolorized), which can be increased or decreased according to the situation; The mixing time generally ranges from 30 minutes to 2 hours, which can be increased or decreased according to the situation; The filtrate shall be treated as appropriate. If the appearance color changes little before and after decolorization, activated carbon can be added for repeated decolorization; If recrystallization is required, direct cooling crystallization or cooling crystallization after appropriate concentration; If the substance to be decolorized is liquid, it is generally concentrated to dry.

2. Impurity removal

The impurities here mainly refer to insoluble substances, such as inorganic salts, dust and physical impurities. In fact, the decolorization of activated carbon also belongs to impurity removal, but the decolorization removes the impurities absorbed by organic visible light. The impurity removal process is very simple, which is similar to the decolorization process. After full dissolution, add activated carbon, stir, filter and concentrate directly. In fact, simple impurity removal can be achieved by direct total solution filtration without adding activated carbon. The addition of activated carbon is mainly to make use of its role of filter aid, which is beneficial to filtration.

3. Adsorption

Adsorption is mainly aimed at tar and viscous impurities. If these substances are directly filtered without activated carbon, the filter medium will be blocked. After adsorption with activated carbon, the general effect is obvious. When adsorption is mainly used, activated carbon can be replaced by silica gel or diatomite, with little difference. Generally, in the process of using activated carbon, activated carbon shows three functions of decolorization, impurity removal and adsorption at the same time. Colored impurities enter into activated carbon molecules, tar and viscous impurities exist between activated carbon particles. During filtration, activated carbon helps to filter insoluble impurities. The three functions cannot be separated.

The function of activated carbon in organic synthesis is actually very simple, but other problems often occur in the use of activated carbon. The most common problems are:

The decolorization effect of activated carbon is poor, and the color of filtrate is still heavy after decolorization for many times; The decolorization loss of activated carbon is large, and the loss of decolorization is more than 10%; Activated carbon penetrates the filter paper and cloth, and the product contains a small amount of activated carbon; When adding activated carbon for filtration, the filtration is very slow, and the filter paper and filter cloth are always blocked; The reaction kettle after using activated carbon is very difficult to wash, and it can't be cleaned in any way;

The poor decolorization effect of activated carbon is usually related to the polarity of pigment, and the same decolorizer can not be applied to all pigments; The large loss of decolorization is generally due to the excessive adsorption of the product on activated carbon; The main reason why activated carbon penetrates the filter paper and cloth is the wrong selection of activated carbon model; The filter cloth of clogging filter paper is not only related to the model of activated carbon, but also related to the particle size of insoluble matter; As for the difficulty of washing the reactor, it is related to the nature of activated carbon itself.