Isoprene and Isoprene Rule

Isoprene: 

• Isoprene (2-methyl-1,3-butadiene)is a colourless volatile liquid. It is an organic compound which formula is CH₂=C(CH₃)CH=CH₂

•   Isoprene is one of the nature’s favourite building blocks. It either alone or in the combination with other unsaturated compound is used to make polymeric compound.

(A polymeric compound is giant molecule which is formed by the combination of many small or similar compound bonded together)

Butyl rubber is one of such example which is formed by isobutene and a small amount of isoprene.

•  It occurs not only in rubber, but in wide variety of compounds isolated from plants and animal source.  

Isoprene Rule:

Terpenes, which are found in the essential oils of many plants, have carbon skeletons made up of isoprene units. In this compound, the isoprene units are joined in the multiples way. This idea leads to the basis of the “isoprene rule".

Linking between two isoprene molecules could occur in three ways:

1         Head to head or 1-1 link

2         Head to tail or 1-4 link

3         Tail to tail or 4-4 link

1)   1-1 or head to head link: In this linking the head of one isoprene could link with the head of another isoprene molecule.  

2)   1-4 or head to tail link: The head of one isoprene molecule could link with the tail of another isoprene molecule.

3)   4-4 or tail to tail link: The tail of one isoprene molecule could link with the tail of another isoprene molecule.

Isoprene rule states that, in most naturally occurring terpenes like Myrcene, Limonene, Retinol etc there are 1-4 link.

Myrcene

Limonene

Retinol or Vitamin A

However, there are some other terpene which show 4-4 link also. They are called irregular terpene. Eg:

β-Carotene