General methods for the determination of structure of terpenoids

1. General methods for the determination of structure of terpenoids
The structural determination of a terpenoid involves the following steps.
i) Determination of molecular formula :
The molecular formula is determined by elemental analysis and molecular weight
determination.
ii) Physical constants :
The physical constants like melting point, boiling point, solubility and specific
rotation (for optically active compounds) are determined. Absolute configuration of
terpenoids can be studied using optical rotatory dispersion measurements.
iii) Detection and estimation of olefinic bonds :
The presence of olefinic bond is detected by the decolourisation of bromine water.
The number of olefinic bonds can be determined by quantitative hydrogenation and
ozonolysis.
iv) Detection of conjugation :
Presence of conjugated system of double bonds is detected by Diels-Alder reaction,
optical exaltation and by UV spectroscopy. max was observed for various terpenoids and
the same was verified by Woodward-Fieser rules (calculated max
).
For example,
v) Detection of nature of carbon skeleton :
The carbon skeleton of the terpenoid can be determined by dehydrogenation using
zinc, sulphur or selenium etc.

2. vi) Functional nature of oxygen :
The functional nature of oxygen available in a terpenoid can be hydroxyl, aldehydic,
ketonic or carboxyl.
a) Hydroxyl group : The detection and estimation of hydroxyl group can be made either by
acetylation or by using 3,5-dinitrobenzoyl chloride.
b) Aldehydic or ketonic group : Aldehydic or ketonic groups can be detected by reagents
like sodium bisulphite, hydroxylamine, phenyl hydrazine etc.
c) Carboxyl group : The carboxyl group can be detected using a saturated solution of
NaHCO3
. It is estimated volumetrically using Ba(OH)2
solution.
vii) Degradative oxidation :
a) Oxidation with chromic acid converts the c-methyl ( C–CH3
) group into acetic acid.
C - CH3
⃗CrO 3
H
2
SO
4 CH3
COOH
The other reagents used for oxidative degradation are ozone, acid, alkaline or neutral
KMnO4
, HNO3
etc.
viii) Other spectral studies :
IR spectroscopy and NMR spectroscopy can also be used in the structural elucidation
of terpenoids.
ix) Synthesis :
Finally the structure of the terpenoid is confirmed by unambiguous synthesis.