The isoprene rule states that terpenoids are composed of repeating five-carbon isoprene units that are joined together in a head-to-tail fashion through their first and fourth carbon positions. This is demonstrated by facts showing isoprene can be dimerized in the lab to form the ten-carbon terpene dipentene, and polymerized to form rubber, with the isoprene units repeating to create the carbon skeleton of terpenoids.

1. Isoprene rule
1) The C5
unit of Terpenes is isoprene (2-methyl, 1,3 - butadiene). The carbon skeleton of
terpenoids are divisible into carbon skeleton of isoprene units. Therefore, terpenoids are
known as isoprenoids.
2) The isoprene units are joined together through C1
and C4
positions. That is the isoprene
units are joined in a regular head to tail fashion. This is known as isoprene rule.
3) The isoprene rule is further proved by the following facts.
a) Isoprene may be dimerised in the laboratory to give the terpene dipentene.
2 C5
H8
C10
H16
Isoprene Dipentene
b) Isoprene may be polymerized to give rubber like product.
n C5
H8
(C5
H8
)n
Isoprene Rubber