Replacement of hydrogen by halogen in aliphatic dicarboxylic acids and nitriles

In principle, what has been said for halogenation of monocarboxylic acids applies also to that of dicarboxylic acids there are preferential -bromination, preferential -chlorination under ionic reaction conditions, decrease in the proportion of a chloro derivatives in radical chlorination, and in most cases direct or indirect reaction by way of the acid halides. There is, however, the additional complication that two diastereoisomers — the meso- and the racemic form — can usually arise, e.g., with, a/-dihalo dicarboxylic acids and their esters. 

5-Dichloroadipic acid 569 If chlorine is passed at 80°, with exclusion of light, into adipoyl dichloride containing FeCl3 (5 g per mole of dichloride) or I2 (2 g per mole of dichloride) until absorption reaches 2 atoms of Cl per mole of dichloride, subsequent esterification of 

Chlorination of nitriles RCH2CN gives mainly, a -dichloro nitriles, under ionic conditions and even under conditions (UV-irradiation) that favor radical reactions. Acetonitrile is readily chlorinated to trichloroacetonitrile since the chloro nitrilium salt that is first formed is chlorinated much faster than acetonitrile itself. 

It is therefore advantageous to pass in HC1 (ca. 1 part per 10 parts of nitrile) below 36° and then to chlorinate at not too high a temperature (35-420).571 Monochloroacetonitrile is obtained from monochloroacetamide and P4O10 in trimethylbenzene.572 

Glutarodinitrile,573 and also adipodinitrile, heptanedinitrile, and octanedinitrile,574 are converted into the a,a,aV-tetrachloro dinitriles when chlorine is passed into them at 70 to 75°, and hydrolysis of these products by boiling concentrated HC1 gives the corresponding tetrachloro dicarboxylic acids. 

---