Fusion and solid-to-solid transitions of a homologous series of alkane-α,ω-dinitriles

Abstract

Fusion and solid-to-solid transitions of a homologous series of 11 alkane-α,ω-dinitriles NC-(CH₂)_n-CN, where n = (1 to 10 and 12), were investigated by differential scanning calorimetry (d.s.c.). The temperatures of fusion increased as a function of the number of carbon atoms in the alkyl chains and showed a marked odd–even effect with even terms displaying higher values. Molar and massic enthalpies and entropies of fusion also showed odd–even alternation. Solid-to-solid transitions were detected for all compounds, except dodecanedinitrile. Comparison was made with the literature values for temperatures, enthalpies, and entropies of fusion of linear alkanes and alkane-α,ω-diamines.

Introduction

The knowledge of the changes in thermodynamic quantities during transitions is of great importance in the study of the structure and stability of organic compounds as well as in many thermodynamic calculations. Alkane-α,ω-dinitriles (hereinafter called alkyldinitriles) are an industrially important class of compounds whose thermodynamic properties have received little attention. The thermodynamics of their solid-to-solid and solid-to-liquid phase transitions have only been investigated for the first three members of the series, malononitrile [1], succinonitrile [2], [3] and glutaronitrile [4]. This paper provides an overall picture of the solid phase behaviour of 11 linear alkyldinitriles NC-(CH₂)_n-CN, where n = (1 to 10 and 12). It also examines the dependence of the thermodynamic parameters of fusion on the number of carbon atoms in their alkyl chains. The odd and even terms of homologous series of variously terminally substituted linear alkyl compounds are known to behave differently with respect to both their fusion and sublimation thermodynamic properties and other physical and chemical quantities, such as density, molar volume, solubility in various solvents [5], [6], [7], [8], [9], [10], [11]. Correlations with crystal structures and packing patterns have often been invoked for this carbon-number-based alternation [12], [13], [14], [15], [16], [17].

We have therefore measured by differential scanning calorimetry (d.s.c.) the temperatures and enthalpies of solid-to-solid and fusion transitions of 11 linear alkyldinitriles as a part of our recent investigations of homologous series of model compounds [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Comparison was made with the literature values for temperatures, and enthalpies and entropies of fusion of both isoelectronic linear alkanes H₃C-(CH₂)_n-CH₃ [5], [11] and alkane-α,ω-diamines H₂N-(CH₂)_n-NH₂ [16].