Butylated hydroxy benzylidene ring: an important moiety for antioxidant synergism of semicarbazones

The compounds having two or more antioxidant functions in their structure exhibit antioxidant synergism that generally increase the antioxidant activity of these compounds. In this study, two series of semicarbazone ( 7a – j and 7a′ – j′ ) bearing butylated ortho and para hydroxy benzylidene ring were prepared for the investigation of antioxidant synergism. This study found that intramolecular hydrogen bond can form in semicarbazones due to the inappropriate position of hydroxyl group on benzylidene ring, which adversely affects the antioxidant synergism. As consequence, butylated para hydroxyl benzylidene phenyl semicarbazone ( 7a ) (IC 50 12.27 µM) showed ~ 4.3 times and ~ 2.7 times better antioxidant activity than compounds 7a′ (IC 50 53.30 µM) and BHT (IC 50 32.63 µM), respectively, in DPPH assay. In addition, based on the solubility in trimethylolpropane trioleate (TMPTO) as synthetic base oil and obtained IC 50 results, oxidation stability of synthesized compounds was also evaluated by two kinds of differential scanning calorimeter (DSC) test, namely temperature ramping DSC and programmed temperature DSC. Thermogravimetric analysis is also performed for the thermal stability assessment. TMPTO incorporated with 0.25 mass% of 7c and 7e were found better oxidative (around 2 times) and thermal resistance than BHT. This DSC results showed another important aspect of semicarbazones that proper modification of semicarbazones can be used in the synthetic lubricant oil as a potential antioxidant. Thus, the results of this study are promising which can be taken under consideration to design and prepare more efficient multipotent semicarbazones. Graphic abstract