Medium-chain aliphatic compounds bearing oxygen-containing functional groups—such as alcohols, ketones, or carboxylic acids—have attracted increasing attention due to their potential as bioactive agents in pest management. These compounds have demonstrated diverse biocidal properties, including insecticidal, antimicrobial, fungicidal, and nematicidal activities. In this study, the nematicidal potency of three structurally related C9 aliphatic ketones—2-nonanone, 3-nonanone, and 5-nonanone—was evaluated against Bursaphelenchus xylophilus, the pinewood nematode (PWN). These isomeric ketones differ in the position of the carbonyl group, providing a useful model for examining structure–activity relationships (SAR) among positional isomers.

The direct-contact bioassays performed at 1 mg/mL revealed that 2-nonanone exhibited the highest nematicidal activity, causing 92.3±1.2% mortality on the PWN, followed by 3-nonanone at 80.1±0.8%, while 5-nonanone showed significantly lower activity at 17.1±0.5%. The results suggest a strong dependency of bioactivity on the position of the carbonyl group along the carbon chain. The increasing efficacy from 5- to 2-nonanone suggests that proximity of the carbonyl group to the terminal end may enhance activity, for example by enhancing membrane interaction or disrupting nematode metabolic processes. These findings underscore the importance of molecule structure analysis in designing effective nematicidal agents and support further investigation into terminally positioned oxygenated medium-carbon chain aliphatic compounds as potential leads. This work highlights that subtle structural differences within homologous series can significantly influence bioactivity and provides a foundation for developing targeted, biodegradable nematicides derived from simple aliphatic frameworks.