Hexamethylenediamine from acrylonitrile

Another derivative of butadiene, hexamethylenediamine (HMDA), is used in the synthesis of nylon. We have already met this compound earlier in this chapter since it is made from acrylonitrile through adiponitrile. 

In 1965, the Monsanto Company started production of adiponitrile from acrylonitrile via an electrochemical route [Equation (26.113)]. Adiponitrile was then converted to either adipic acid or hexamethylenediamine by chemical means these two compounds are reactants in the production of nylon 66. 

Hexamethylenediamine is now made by three different routes the original from adipic acid, the electrodimerization of acrylonitrile, and the addition of hydrogen cyanide to butadiene. Thus, the starting material can be cyclohexane, propylene, or butadiene. Currently, the cyclohexane-based route from adipic acid is the most costly and this process is being phased out. The butadiene route is patented by DuPont and requires hydrogen cyanide facilities. Recent new hexamethylenediamine plants, outside DuPont, are based on acrylonitrile from propylene, a readily available commodity. 

Hexamethylenediamine (HMDA), a monomer for the synthesis of polyamide-6,6, is produced by catalytic hydrogenation of adiponitrile. Three processes, each based on a different reactant, produce the latter coimnercially. The original Du Pont process, still used in a few plants, starts with adipic acid made from cyclohexane adipic acid then reacts with ammonia to yield the dinitrile. This process has been replaced in many plants by the catalytic hydrocyanation of butadiene. A third route to adiponitrile is the electrolytic dimerization of acrylonitrile, the latter produced by the ammoxidation of propene. 

Hexamethylenediamine is discussed in Chapter 10, Sections 1 and 8. It is produced from adiponitrile by hydrogenation. Adiponitrile comes from electrodimerization of acrylonitrile (32%) or from anti-Markovnikov addition of 2 moles of hydrogen cyanide to butadiene (68%). 

Nylon 66 is a polymer manufactured by the condensation of adipic acid with hexamethylcnediamine, and in excess of I million ton year are manufactured in the USA atone. Adiponitrile is a convenient intermediate for the production of both hexamethylenediamine and adipic acid, the former by hydrogenation and the Latter by hydrolysis. In addition, acrylonitrile is available from the gas phase, catalytic oxy-amination of propylene. Hence, to complete the manufacture of nylon from propylene and ammonia, a procedure for the conversion of acrylonitrile to adiponitrile is required (Fig. 6.1). 

The modified process is summarized in Fig. 6.5 and employs an emulsion of acrylonitrile and 15% disodium hydrogen phosphate in water containing a low concentration of quaternary ammonium salt (0.4%). Thus, the concentration of the starting material in the aqueous phase is 7% (i.e. saturated) and the adiponitrile will extract back into the acrylonitrile within the cell. The quaternary ammonium ion used is hexamethylene-i /s(ethyldibutylammonium) it is prepared on the site by quaternization of hexamethylenediamine, an intermediate between adiponitrile and the nylon. It is therefore cheap but it is also convenient because traces are readily washed from the organic phase with water. Hence, the product isolation is straightforward and the aqueous and organic phases can be treated separately. The organic phase is merely washed with water and distilled. 

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