Home » Organic Chemistry » Isoprene Rule

Isoprene Rule

In 1887, German Chemist Otto Wallach introduced isoprene rule and mentioned that thermal decomposition of almost all the terpenoids produce isoprene as one of the products, and this led to suggestion that the skeleton structures of all naturally occurring terpenoids can be built up of isoprene units.

It was recognized that an isoprene unit could be connected in different ways to produce the variety of carbon atom arrangements found in terpenoids.

Isoprene (2-methyl)

An isoprene unit is the carbon skeleton of isoprene (ignoring the double bonds).


For example: Myrcene contains two isoprene units.


Formally, in biosynthesis of terpenoids, two or more isoprene molecules are linked to one another. Linking between two isoprene molecules could occur in three ways, given that the head and tail of the molecules are primarily involved in the linkage.

1. Head to Head or 1-1 Link

The head of one isoprene molecule could link with the head of another isoprene molecule.

Head to Head or 1-1 Link

2. Head to Tail or 1-4 Link

In this arrangement, head of one isoprene unit could link with the tail of another isoprene molecule.

Head to Tail or 1-4 Link

3. Tail to Tail or 4-4 Link

Tail to Tail or 4-4 Link

Special Isoprene Rule

Ingold in 1925 pointed out that isoprene units in natural terpenoids were joined ‘head to tail’. The head is being the branched end of isoprene unit. It should be noted however, that this rule, which has proved very useful, can only be used in guiding principle and not as a fixed rule, several exceptions are reported, for e.g., carotenoids are joined tail to tail (4-4 link) at their centre.

Cyclic terpenes also contain links that are neither 1-1, 1-4 or 4-4, which are called cross links.

Special isoprene rule states that in most naturally occurring terpenes, there are no 1-1 or 4-4 links.





Related Posts
Nucleophilic Substitution Reactions

Nucleophilic Substitution Reactions Nucleophilic substitution reaction is a type of organic reaction in which nucleophile (an electron pair donor) reacts Read more

Chemical Bonding: Definition, Types and Examples

Chemical Bonding Chemical Bonding is the phenomenon in which two atoms combine together and results into formation of molecule or Read more

Lewis Structures

Lewis Structures Lewis structures of an atom denote the valence electrons of an atom by placing dots around the symbol Read more


Enantiomers Enantiomers are the optical isomers which are non-superimposable mirror images to each other. They are also known as chiral Read more

Leave a Comment

Your email address will not be published. Required fields are marked *

Translate »
Scroll to Top