Study of Catalystic Cyclisation of (+)-Citronellal with Zn/y-Alumina as Catalyst = Mempelajari Reaksi Siklisasi Katalistik (+)-Sitronelal dengan Katalis Zn/y-alumina.

The reaction of catalytic cyclisation of (+)-citronellal has been done using y-alumina and Zn/y-alumina as catalysts. Zn/y-alumina catalyst was prepared by impregnation of ZnBr2 into solid of y-alumina in methanol as a solvent. The calcination process is performed on Muchalal reactor at 400 °C. The reactions were carried out under nitrogen gas atmosphere by mixing 5 mL sample and 1.0 g catalyst at 90-95 °C for 3 hours. The product of reaction was analyzed using GC, FT-IR and GC-MS. The reaction of cyclization of (+)-citronella) with y-alumina catalyst did not give targeted product as well as reaction with Zn/y-alumina catalyst. Whereas, the reactions with Zn/y-alumina catalyst gave four compounds of isopulegol stereoisomer. Conversion of (-)-Isopulegol, (+)-Neoisopulegol, (+)-lsoisopulegol and (+)-Neoisoisopulegol were 64,29%, 18,80%, 9,20% and 3,32% respectively. For that reason, the transformation of (+)-sitronelal using Zn/yalumina was considered to be the best catalyst. A reaction mechanism is proposed where the (+)-citronellal molecule binds to a catalyst Lewis acid site via the aldehyde oxygen and the Tr-electrons of the double bond. Subsequent protonation of the aldehyde via a neighbouring Bronsted acid site initiates the cyclisation to isopulegol. Keywords: catalytic cyclisation, impregnation, (+)-isopulegol