In situ reaction of various long chain hydrocarbon precursors with cellulosic substrate inside plasma zone were investigated. Each of the selected precursor compounds had C12 alkyl chain as the hydrophobic moiety to impart hydrophobicity to the substrate and a functional group to aid reaction with the active sites of the substrate. The selected compounds were dodecyl acrylate (DA, with acrylate group), lauryl alcohol (LA, with hydroxyl group), dodecanoic acid (DODAC, with carboxylic acid) and dodecane (DD, without any functional group). Treatment was carried out in-situ with these compounds using helium gas as a carrier. After the treatment, durable hydrophobicity was obtained in all cases with water absorbency time of substrate increasing to > 1 h in the case of DA and LA, up to 20 min in the case of DD and only up to 9 min in the case of DODAC. The functional groups were found to have profound effect on the extent of functionality obtained and the plasma parameters required to obtain effective reaction with the substrate. Both plasma zone and the functionalized surfaces were characterized to understand the chemistry involved with the different precursors.