Preparation and phase behavior of acetyl monoglycerides

A homologous series of 1‐monoacetyl‐3‐monoglycerides in which the fatty acyl group varies from C 14 to C 22 in even C intervals has been prepared by partial acetylation of the appropriate 1‐monoglycerides. Crystallization and silica gel chromatography were used to isolate the diglycerides in high purity. The isomeric monoacetyl monostearins were prepared either by application of a similar procedure to 2‐monostearin or by acetylation of 1‐tetrahydropyranyl‐3‐monostearin with subsequent removal of the blocking group to obtain 2‐acetyl‐1‐monostearin. In the homologous series of 1‐acetyl‐3‐monoglycerides from monomyristin through behenin the stable form at room temperature, called form I, is a tilted double chain length form, existing up to the melting point for myristin through stearin but transforming to a stable α form before melting for arachidin and behenin. The α form, low melting and entirely metastable for myristin through stearin, transforms reversibly to sub‐α on cooling. Both α and sub‐α are perpendicular forms and probably of single chain length structure. An interesting feature is the two‐step transformation of sub‐α to α on warming; the nature of this stepwise transformation, observed for all but myristin, is discussed. A nearly perpendicular double chain length β‐like form occurs for palmitin and stearin and perhaps myristin. This form, which has not been well characterized thermally, though possibly entirely metastable, is conceivably a stable form at low temperatures.