Isoprene Rule, Special Isoprene Rule and Violations Of Isoprene Rule

What is isoprene rule?

“It States that the skeleton structure of all terpenoids are built up of isoprene Units or 2-methyl 1,3-butadiene”.

     The isoprene rule derived form the following facts.

(a) The empirical Formula if almost all terpenoids is C5H8.

(b)  The thermal decomposition of all terpenoids gives isoprene as one of the products   

Eg:-Rubber on destructive distillation yields isoprene as the products.

The isoprene rule has been confirmed by the following facts.     

 (¡) Isoprene,When Heated to 280℃ yield a (Dipentene)
 (¡¡)  Isoprene may be polymerized to yield a rubber like product.

Special Isoprene Rule                       

This rule proposed by Ingold in 1925.According to this rule “the isoprene units in terpenoids are joined by head to tail linkage or 1,4-linkage”. (The branched end of isoprene units was considered as head and other end as the tail).

Violations  Of  Isoprene Rule
Carbon content of certain terpenoids are not a multiple of five.
Cryptone, a naturally occuring ketonic terpenoid contains nine carbon atoms,it can not be divide in to two isoprene unite.

In certain terpenoids isoprene rule is Violated .                         

 Lavandulol is composed of two isoprene units are linked through C3 and C4.