Limonene belongs to very large and miscellaneous group of natural compounds called terpenes. The molecules of terpenes are formed by linking of two or more isoprene (2-methyl-1,3-butadiene) units. The linking can be done as head-to-head, head-to-tail, or tail-to-tail link (picture 1). The second one being the most common. Cyclic terpenes usually contain another types of linking called crosslinks. The isoprene rule (C5 rule) says – in most naturally occurring terpenes, there are no 1-1 or 4-4 links. Terpenes that disobey this rule are irregular.
Based on number of isoprene units linked together, terpenes are classified to subgroups such as monoterpene (2), sesquiterpene (3), diterpene (4), …, polyterpene (n). Terpenes can be acyclic, monocyclic or polycyclic.
Limonene is a cyclic terpene. At normal temperature and pressure, limonene is a colorless liquid that is stable. It is oxidized to carveol or carvone (picture 2) when exposed to the air and moisture. It occurs naturally as (R)-enantiomer that is responsible for the strong odor of citrus fruits.. Racemic mixture of (R)- and (S)- isomer is called dipentene and is obtained by heating up to 300 °C.
30.65 g of the orange peel from the outer part of orange was weighted and placed into 250 mL round bottom flask. 150 mL of fresh water were added and the distillation apparatus was set up. The mixture was boiled vigorously for more than one hour and 100 mL of distillate were collected. The distillate was extracted two times with 25 mL portions of n-hexane. Aqueous layer was discarded. Extracts were combined together, dried over anhydrous sodium sulfate and filtered. The solvent was removed and the product was weighted. The yield was 165 mg. According to the mass of the peel it contains 0,54 % of limonene.