Odd–Even Effects in Acylated 2-Aminopyrazines: A Systematic Examination of Structure–Property Relationships

Abstract

This study presents a systematic investigation of a possible odd–even effect in a homologous series of acylated 2-aminopyrazine derivatives, focusing on the relationship between alkyl chain length and macroscopic properties. A series of N-(pyrazin-2-yl)alkylamides (PZ2–PZ9) was synthesized and structurally characterized (PZ2-PZ8) using single-crystal X-ray diffraction. The results revealed that even-numbered alkyl chains promote more efficient crystal packing, with all chains aligned in the same direction, whereas odd-numbered compounds exhibit alternating orientations, leading to packing inefficiencies. The even-numbered derivatives consistently exhibited higher melting points and densities compared to the odd-numbered derivatives. The results also indicate that decreasing molecular polarity can translate into increased resistance to water uptake at the bulk level. In this series, only the shortest alkyl chain and the most polar compound (PZ2) exhibited moisture sensitivity. Overall, subtle variations in molecular structure are shown to significantly affect macroscopic properties in predictable ways, thus providing further insights for the rational bottom-up design of functional organic materials.