Terpenes are hydrocarbons found in essential oils derived from various plant sources. Their general formula is (C5H8)n. Terpenes are classified based on the number of isoprene units they contain and can be monoterpenes, sesquiterpenes, diterpenes, triterpenes, or polyterpenes. They occur naturally in fruits, flowers, leaves, bark, and roots, and are responsible for the scents of many plants and essential oils.

1. Terpenes
Definition
Terpenes are isomeric hydrocarbons obtained from the essential oils such as
eucalyptus oil, sunflower oil, sandalwood oil, palmrosa oil etc. Their general formula is
(C5
H8
)n
. Their oxygenated derivatives such as alcohols, aldehydes and ketones with general
formula (C5
)n
are known as terpenoids.
eg : Dipentene, myrcene (Terpenes), citral, menthone (Terpenoids).
Occurrence
They occur in the fruits, flowers, leaves, stem barks and roots of nearly all the plants
that have pleasant smell. This pleasant smell is due to the presence of certain steam volatile
oils known as essential oils. The following table gives the list of some terpenes or
terpenoids and their source.
Terpenes (or) Terpenoids Source (essential oil)
Citral Lemon
Geraniol Rose
Santalal Sandal wood
Zinziberene Ginger
Cineole Eucalyptus
Pinene Turpentine
Camphor Camphor
Linalool Jasmine
Terpineol Cardamon
Classification
Terpenes are classified on the basis of the number of C5
(isoprene) units present in
them.

2. Class No. of isoprene units Example(s)
a) Hemiterpenes one isoprene unit (C5
) -
b) Monoterpenes two isoprene units (C10
) Citral, limonene
c) Sesquiterpenes three isoprene units (C15
) Farnesol, zinziberene
d) Diterpenes four isoprene units (C20
) Phytol
e) Triterpenes six isoprene units (C30
) Squalene
f) Polyterpenes several isoprene units (C5
)n
Rubber
Terpenes are further subdivided into acyclic or cyclic terpenes. Acyclic terpenes
posses open chain structure (myrcene) and cyclic terpenes posses ring structure (limonene).
Cyclic terpenes are further classified into monocyclic (limonene) bicyclic (-pinene) or
tricyclic (abietic acid) according to the number of rings present in them.