Packing of aliphatic molecules

Abstract

AbstractIt is generally not appreciated that almost all physical properties of solids depend primarily on the shape of the molecule, which controls regularity and density of packing and thus such properties as melting point, specific gravity, hardness, etc. The melting point is of special interest; it characterizes the solid → melt transition.This contribution follows a novel method of assessing shape and packing arrangement of molecules, based on four considerations: (1) common‐sense probability, (2) packing characteristics, relative to compactness and balance of structure, (3) comparative studies of the melting‐point/chain‐length curves of a number of related series of homologous compounds of known and unknown configuration, and finally (4) actual packing experiments with different types of molecular models. The data so gained should also agree with known x‐ray measurements.Of importance and comparatively widely studied is the configuration of the fatty acid glycerides. For triglycerides, two (principal) arrangements have been proposed so far, namely, the “tuning fork” and the “chair” structures; these are assumed to be universally accepted. An unnamed third arrangement, not unlike a capital letter E might be regarded the basic shape. The symmetrical one‐acid diglycerides are thought to form a U‐type structure.It is shown that none of the arrangements named above is really the most probable. The symmetrical or more generally all 1,3‐diglycerides take the shape of straight‐axis molecules which form twin molecules held together by hydrogen bonds. The triglycerides also form straight‐axis molecules; the chain of the acid‐radical in the 2‐position runs parallel to the axis. The composite chain of all the glycerides can be regarded as truly planar, since it shows one negligible deviation only. The twin diglyceride molecules constitute ideal packing units with respect to both closeness and regularity. The packing structure of the triglycerides, particularly of the one‐acid type, is not much less ideal. The latter can form two kinds of twin molecule, i.e., one molecule and one‐and‐one‐half molecules long, as well as multiple (in particular eightfold molecules) two molecules long. The importance of hydrogen bonding is discussed in connection with all types of glycerides. Reference is also made to the packing of ringmolecules of planar symmetry and of aliphatics of spherical symmetry. It is pointed out that the concept of the unit cell offers little advantage in discussion of packing properties of molecular crystals, in particular in the case of long‐chain compounds; rather, it complicates the issue.