Starch Propionate and Butyrate

Starch propionates, with various degrees of propionation up to the tripropionate, have been prepared by refluxing com starch with propionic acid and propionic anhydride. The lower propionates are water-soluble, but the tripropionate is soluble only in organic solvents. Starch tripropionate and starch tributyrate are easily prepared by treating swollen starch with pyridine and the acid anhydride. These triesters are more soluble in organic solvents than the starch triacetates. 

Both starch tripropionate and tributyrate form clear plastics which, although brittle, are somewhat softer than those of typical whole-starch triacetates. This fact is in keeping with similar results noted for cellulose esters and sustains the view that the mutual attraction of the starch chains is continually rendered less effective as the aliphatic chain increases in length. 

Starch esters of either mono-, di-, or trichloroacetic acid can be prepared by heating starch with the appropriate acid or by treatment with the acid anhydride and sulfuric acid. Starch triacetates can be chlorinated by phosphorus pentachloride at 135-140 to produce a starch trichloroacetic ester.  

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