Phase Transition, Conformational Disorder, and Chain Packing in Crystalline Long-Chain Symmetrical Alkyl Ethers and Symmetrical Alkenes

The effect of incorporation of a central functionality (ether linkage/olefin double bond) in an n-alkane chain on its conformation related properties has been studied using DSC, IR, and wide-angle X-ray scattering (WAXS) techniques. These investigations have also included the influence of alkyl chain length on phase transitions, conformational disorders and chain packing of thus obtained symmetrical alkenes and symmetrical alkyl ethers. The molecules studied involved four symmetrical ethers from 13-oxapentacosane (C 2 5 ) to 19-oxaheptatriacontane (C 3 7 ) and three symmetrical alkenes from 15-triacontene (C 3 0 ) to 19-octatriacontene (C 3 8 ) having a constant chain length difference of four carbons among them. These compounds are analogues of n-alkanes that are known to exhibit a series of chain length-dependent premelt phase transitions. Symmetrical ethers exhibit a more pronounced effect on the depression of both melt and premelt transition temperatures than symmetrical alkenes. The enthalpy of the melt transition showed an increase in the case of symmetrical ethers but a decrease in the case of symmetrical alkenes as compared to the parent n-alkanes. However, the enthalpy of premelt transition has decreased for symmetrical ethers but is not altered significantly for symmetrical alkenes. IR absorption spectra of all symmetrical ethers and alkenes revealed the occurrence of almost all common conformational defects of n-alkanes at premelt and melt transition temperatures. The WAXS patterns indicate that the crystals of shorter chain (C 2 5 and C 2 9 ) symmetrical ethers may have a helical monoclinic crystal packing, whereas longer chain (C 3 3 and C 3 7 ) symmetrical ethers and all symmetrical alkenes have orthorhombic crystal packing at room temperature (ca. 23 °C). The effect of chain length on conformational properties is more pronounced in symmetrical alkyl ethers than in symmetrical alkenes and n-alkanes.