No abstract

In the homologous series of n-alkanes two alternating effects are known, viz. the familiar alternation of melting points for the short-chain members and the recently reported alternation of transition points for members with chains of more than 20 carbon atoms.A further study of the phase behaviour of these hydrocarbons now reveals that both effects are due to some peculiarity in one of two structures in which n-alkanes may crystallize, viz. the Sp -modificaticn. As a consequence, the thermodynamic properties of this solid modification show an alternation; this effect, in its turn, is responsible for the specific behaviour of the melting and transition point temperatures.It is to be expected that the other crystal structure, viz. Sn, and the liquid do not show similar peculiarities.

The properties of the crystalline Sg-modifications of n-alkanes show alternation; this alternation effect is the basis of all alternation effects occurring in the general behaviour of n-alkanes, dso when other states of matter (liquid state or S,-modification) are involved. The properties of the latter states do not show alternation themselves.Quantitative experimental evidence by means of thermodynamic data has now been obtained. In terms of enthalpy the magnitude of the alternation effect is about 600 cal/mole, in terms of entropy and free enthalpy the magnitudes are about 1.5 cal/mole " C and 80 cal/mole, respectively. Generally the S,1-phases of n-alkanes with an even number of carbon atoms show relatively lower values for the absolute thermodynamic quantities as compared with the n-alkanes with an odd number of carbon atoms.