the production of thymoquinone from thymol and carvacrol

CHAPTER 5
OXIDATION OF MONOTERPENES
5.1. Definition of Monoterpene
Essential oils are made up of many chemical constituents. No two oils are alike
in their structure or their effects. Some of the main constituents found in essential oils
are alcohols, esters, ketone, ether, phenol, terpenes..
Each of these can be broken down into numerous smaller units. For example
terpene classification includes monoterpenes, sesquiterpenes, sesquiterpene lactones,
Di-terpenes, etc. Terpene is a compound whose carbon skeleton can be divided into two
or more units identical with the carbon skeleton of isoprene as seen in Figure 5.1
(Monteiro et al. 2004, Solomons 1988). The basic building block of many essential oils
is a five-carbon molecule called an isoprene. Most essential oils are built from isoprene.
When two isoprene units link together, they create a monoterpene; when three join, they
create a sesquiterpene; and so forth. Triterpenoids are some of the largest molecules
found in essential oils. They consist of 30 carbon atoms or six isoprene units linked
together.
C H 3
C H 2 = C - C H = C H 2
2-Methyl-1,3-butadiene
(Isoprene)
head
Figure 5.1. Isoprene structure
1 2 3 4
C - C - C - C
(Source: Monteiro et al. 2004, Solomons 1988)
Oxygenated monoterpenes are major components in the essential oils. Carvacrol,
thymol and p-cymene are p-menthane type of aromatic monoterpenes, which can be
found in the essential oils of aromatic plants (Martin et al. 1999).
C
Isoprene unit
tail
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