FRIEDEL-CRAFTS REACTIONS BASIC INFORMATION

Several chemicals are manufactured by application of the Friedel-Crafts condensation reaction. Efficient operation of any such process depends on:

1. The preparation and handling of reactants
2. The design and construction of the apparatus
3. The control of the reaction so as to lead practically exclusively to the formation of the specific products desired
4. The storage of the catalyst (aluminum chloride)

Several of the starting reactants, such as acid anhydrides, acid chlorides, and alkyl halides, are susceptible to hydrolysis. The absorption of moisture by these chemicals results in the production of compounds that are less active, require more aluminum chloride for condensation, and generally lead to lower yields of desired product.

Furthermore, the ingress of moisture into storage containers for these active components usually results in corrosion problems.

Anhydrous aluminum chloride needs to be stored in iron drums under conditions that ensure the absence of moisture. When, however, moisture contacts the aluminum chloride, hydrogen chloride is formed, the quantity of hydrogen chloride thus formed depends on the amount of water and the degree of agitation of the halide.

If sufficient moisture is present, particularly in the free space in the container or reaction vessel or at the point of contact with the outside atmosphere, then hydrochloric acid is formed and leads to corrosion of the storage container.

In certain reactions, such as the isomerization of butane and the alkylation of isoparaffins, problems of handling hydrogen chloride and acidic sludge are encountered. The corrosive action of the aluminum chloride–hydrocarbon complex, particularly at 70 to 100oC, has long been recognized and various reactor liners have been found satisfactory.

The rate of reaction is a function of the efficiency of the contact between the reactants, i.e., stirring mechanism and mixing of the reactants. In fact, mixing efficiency has a vital influence on the yield and purity of the product. Insufficient or inefficient mixing may lead to uncondensed reactants or to excessive reaction on heated surfaces.