Alkaloids are a chemically heterogenous group of basic nitrogen
containing substances found predominantly in higher plants and also
occur in lower plants, animals, microorganisms and marine organisms.
Alkaloids usually contain one or two nitrogen atoms although some like
ergotamine may contain up to five nitrogen atoms.
The term alkaloid was coined in 1819 by the pharmacist Meissner and
meant simply alkali-like molecules.' True Alkaloids ' were defined as
compounds meeting the additional four qualifications namely:
I) Nitrogen is a part of heterocyclic ring
Ii) The occurrence of compound is restricted to plant kingdom
Iii) The compound has complex molecular structure
Iv) The compound manifests significant physiological activity
Their manifold pharmacological activities have always excited man's
interest, and since early times selected plant products have been used as
poisons, euphoriants, psychedelics, stimulants or as medicines. Many of
our modern drugs now contain the same compounds or synthetic
analogues, and the pharmacological and toxicological properties of these
compounds are thus of immense interest and importance.
1.Classification and Structural types
Some of the methods for classification of alkaloids as follows:
i) Biosynthetic Origin
This method depends on types of precursors or building block
compounds used by living organisms to synthesize complex structures.
The disadvantage with this method is that the relationship of alkaloids to
each other and to their precursors is not always apparent.
This classification is based on pharmacological activity.
This is done according to the source of compound without reference
to the chemical types.
The chemical classification of alkaloids is universally adapted and
depends on the type of heterocyclic ring structure present.
It is usual to classify alkaloids according to the amino acids (or their
derivatives) from which they arise. Thus the most important classes are
derived from the amino acids ornithine and lysine; or from the aromatic
amino acids phenylalanine and tyrosine; or from tryptophan and a moiety
of mavelonoid origin; and a number of compounds are also derived from
anthranilic acid or from nicotinic acid.
HETEROCYCLIC RING SYSTEMS
N N N N
H H H
pyrrolidine pyrrole piperidine pyridine
N N N
quinoline isoquinoline indole dihydroindole
quinolizidine pyrrolizidine tropane
C C N
benzylisoquinoline purine -phenylethylamine
2. Qualitative tests and general properties
Alkaloids are generally bitter in taste and are optically active.This
property is useful for resolution of racemic acids.
The alkaloids are mostly crystalline,colourless,non-volatile
solids,which are insoluble in water , but soluble in polar organic solvents
(methanol,chloroform,ether etc.).Some alkaloids, however,are
water-soluble liquids (for example coniine,hygrine,nicotine).Few
alkaloids are coloured(for exemple berberine is yellow).Nearly all basic
alkaloids form salts with inorganic acids,which are soluble in water and
insoluble in organic solvents.The knowledge of solubility of alkaloids and
their salts is utilized in their isolation.
Alkaloids yield precipitates or characteristic colours when reacted
with solutions of molybdic acid,picric acid,chloroplatinic acid,potassium
mercuric iodide etc.Therefore the presence of an alkaloids in a plant is
detected by using various reagents such as Mayer's reagent (potassium
mercuric iodide),Wagner's reagent (iodine dissolved in potassium
iodide),Dragendorff's reagent (potassium bismuth iodide),Hager's reagent
(saturated solution of picric acid in water),chloroplatinic acid (H2PtCl6),
Frohde's reagent (molybdic acid) etc.
These precipitates may be amorphous or crystalline and are of various
and Dragendorff's ) etc.Caffeine and some other alkaloids do not give
3. Isolation and purification
The knowledge of the solubility of alkaloids and their salts is also of
considerable pharmaceutical importance.Not only are alkaloid
substances often administered in solution but also the differences in
solubility between alkaloids and their salts provide methods for the
isolation of alkaloids from the plant and their separation from the
non-alkaloid substances.For many alkaloids are basic and occur as salts
of 2-hydroxybutane-1,4--dioic acid(malic acid),or of
1,3,4,5-tetrahydroxycyclohexane(quinic acid).They can thus be extracted
into acid solution using aqueous HCl,tartaric,or citric acids.Essentially
neutral alkaloids like colchicine or piperine,which are in fact
amides ,remain in the organic phase ,while most other alkaloids are
isolated after basification and extraction into ethyl acetate.
The almost invariably subsequent purification of the crude alkaloid
mixtures is effected by chromatography using silica or alumina,and then
recrystallization of the partially purified compounds from solvent systems
like aqueous ethanol ,methanol/chloroform,or methanol/acetone.
With few exceptions salts of alkaloids are water-soluble.The
following procedure is used for alkaloids isolation.
The powdered plant material is first extracted with petroleum ether
for the removal of fats.In a general procedure recommended by Manske
the material then is extracted with methanol or ethanol and the dark
solution is separated from cellulose and other insoluble materials and
evaporated to dryness.Water is added to the residue ,the mixture is
acidified to pH 2 and extracted with ethyl acetate or ether for the removal
of non-basic molecules. The aqueous solution is basified to pH 10 and
extracted with ethyl acetate or ether and organic layer evaporated to
dryness to obtain a mixture of water insoluble organic bases.These
alkaloids are further purified by fractional crystallization of
salts.Alternatively the purification is achieved by using column
chromatography or counter-current distribution method.
Water-soluble alkaloids are obtained from aqueous solution by the
precipitation with ammonium Peinecke's (H[Cr(NH3)2(SCN)4]) solution
followed by ion exchange chromatography of the precipitated salt.The
freeze-drying of the eluent gives the water-soluble alkaloids.
Isolation and purification of an alkaloid from a plant material may not
always be a simple procedure as described above because some alkaloid
bearing plants contain a complex mixture of several
alkaloids.Therefore,the isolation of a pure alkaloid from these plants may
become extremely laborious procedure sometimes due to the repeated use
of chromatography or crystallizations.
Also,in rare instances stream distillation can be employed,for example
with low molecular weight or volatile liquid alkaloids such as
nicotine ,sparteine and coniine are most conveniently isolated by
distillation.An aqueous extract of plant is made alkaline with caustic soda
or sodium carbonate and alkaloid is distilled off with steam.The distillate
is extracted with solvent to isolate the desired alkaloid.
4.General methods of structure determination
In structure determination of alkaloids,a variety of general chemical
methods and more recently physical methods are employed.Many of the
following chemical methods have been particularly useful in alkaloids
In general,elemental composition is obtained from combustion analysis
and after determination of molecular weight,molecular formula is
calculated.The measurement of optical rotation indicates the presence of
The alkaloids mostly contains one or more oxygen atoms,which may
be present as hydroxyl,methoxy,methylenedioxy,carbonyl ,carbonyl
ester,lactone,amide,lactam,epoxide groups or ether linkage.The nature of
oxygen atom is ascertained using the following usual chemical reactions.
4.A. Chemical Methods
i) Hydroxyl group
The acetylation (acetic anhydride, pyridine) or benzoylation (benzoyl
chloride,,pyridine) indicates the presence of hydroxyl group together with
amino group (-NH or -NH2) in the molecule.If molecule contains
hydroxyl group or -NH group then the number of these groups can be
estimated by acetylation or Zerewitinoff's method.In the former method,
the acetate derivative is hydrolyzed with known volume of 1N NaOH and
the amount of base required for hydrolysis (estimated by titration)
indicates the number of hydroxyl and any amino groups.In the latter
method,hydroxyl and amino groups are estimated by the treatment with
methyl magnesium iodide.The volume of methane obtained is measured,
which indicates the number of -OH (and any N-H) groups.(One mole of
alkaloid containing one -OH or -NH group = 22.4 liters of CH4 at S.T.P).
R-OH + CH3-CO-Cl → R-OCO-CH3
R-NH-R1 + CH3-CO-Cl → R-N(COCH3)-R1
ii) Zerewitinoff's method
R-OH + MeMgI → R-OMgI + CH4
R-NH-R' + MeMgI → R-N(MgI)-R' + CH4
If hydroxyl group is present it may be alcoholic or Phenolic.Phenolic
compounds are soluble in sodium hydroxide and are reprecipitated by
carbon dioxide.They give colouration with ferric chloride.If the alkaloid
does not respond to thede tests for phenol, the hydroxyl group may be
If alcoholic group is present, then the nature of this group (usually
secondary,tertiary) is verified by oxidation or by dehydration to
ii) Carbonyl group
The presence of carbonyl group is ascertained by usual reactions with
hydroxylamine, semicarbazide or 2,4-dinitrophenyldrazine when the
corresponding oxime,semicarbazone or 2,4-dinitrophenyldrazine is
formed.The carbonyl group may be present as an aldehyde or a ketone.
This distinction can be made from Tollen's reagent.Aldehydes give silver
mirror whereas ketones are unreactive.
iii) Carboxyl group
The solubility of an alkaloid in bicarbonate or ammonia and
reprecipitation with carbon dioxide indicates the presence of carboxyl
group.The number of carboxyl groups may be determined volumetrically
by titration against a standard solution of barium hydroxide using
phenolphthalein as an indicator.
The formation of ester on treatment with alcohol in the presence of
dehydrating agent also indicates the presence of carboxyl group.
iv) Methoxy group
The detection and estimation of methoxy group can be achieved using
Zeisel's method,which is similar to the Herzig-Meyer method for
N-methyl group determination.In this method aknow weight of alkaloid is
boiled (120℃) with hydroiodic acid,which cleaves methoxy group with
the formation of methyl iodide,this is then estimated by absorption in
ethanolic silver nitrate and the precipitated silver iodide is filtered,dried
and weighted.From the weight of silver iodide ,the number of methoxy
group can be calculated.
R-OMe + HI R-OH +MeI AgI
(Estimate ppt gravietrically)
v) Methylenedioxy group (-O-CH2-O-)
If an alkaloid,on heating with hydrochloric or sulfuric acid yields
formaldehyde,the presence of methylenedioxy group is
indicated.Formaldehyde thus obtained can be estimated gravimetrically
after conversion to dimedone derivative.
+ HCl HCHO
vi) Amide, lactam, ester,lactone groups
These groups can be detected and estimated through acid or alkaline
-CONH2 + NaOH → -COONa + NH3 ↑
-COOR + NaOH → -COONa + ROH
vii) Epoxide and ether linkage
Epoxide or ether linkages are cleaved by the addition of hydrogen
bromide or hydroiodic acid.
viii) Nature of nitrogen
In the majority of alkaloids nitrogen atom(s) is present in heterocyclic
ring therefore it can be secondary or tertiary.
The acetylation or benzoylation can distinguish tertiary amine from
secondary amine,the former being inert whereas the latter gives acetate or
benzoate derivative.This distinction can also be done by treatment with
HNO2 or methyl iodide or oxidation with 30% hydrogen peroxide.
The presence of N-methyl group is often detected by distillation of
amine with sodalime or estimated by the treatment with hydroiodic acid
at 150-300℃ and conversion of methyl iodide produced to silver iodide
as mentioned for estimation of methoxy groups.
>N-H + HNO2 → >N-NO+H2O
>NH + CH3I → >N-Me + HI
>N + CH3I → >N+ -Me I-
>N + H2O2 → >N+-O-+ H2O
>N -Me + CaO → CH3NH2
>N -Me + HI → N-H + MeI AgI
ix) Tertiary methyl group
The presence of C-Me group is quantitatively estimated by Kuhn-Roth
oxidation (K2Cr2O7/H2SO4) to acetic acid,which is distilled off and
titrated against standard base.
-C-Me + K2Cr2O7/H2SO4 → CH3COOH
4.B. Degradation of alkaloids
The following methods are used to find out structural fragments of
i. Hofmann exhaustive methylation.
ii. Emde's degradation.
iii. von Braun's method.
v. Alkali fusion.
Now we discuss the mainly used methods of Hofmann degradation
and Emde's degradation.
i) Hofmann degradation
The alkaloids is converted into the quaternary ammonium salt by the
treatment with excess of alkyl halide and the resulting salt is then
converted into the more basic hydroxide by reacting with silver
oxide.subsequently the quaternary hydroxide salt is heated at 200℃ to
give an olefin with the elimination of a tertiary amine and water
R-CH2-CH2-N+R3 + OH- → R-CH=CH2 + H2O +NR3
The elimination generally proceeds by an E2 elimination in which the
β-hydrogen and quaternary nitrogen groups are present in the
Hofmann's degradation fails in the following cases:
i. with tetrahydroquinoline
ii. with aromatic heterocyclic rings
iii. With compounds having no β-hydrogen atom
Thus the reaction fails in the case of derivatives of
1,2,3,4,-tetrahydrodimethyl quinolium hydroxide,which on pyrolysis
merely loses methanol without fission of ring.However,Emde degradation
results in cleavage of ring.
ii) Emde's degradation
The alkaloid is converted to quaternary ammonium salt by refluxing
with alkyl halide and the resulting salt is subjected to reductive cleavage
by the treatment with sodium amalgam in alkanol or sodium in liquid
ammonia or by catalytic hydrogenation.Tetrahydroquinoline and
tetrahydroisoquinoline quarternary salts undergo Emde's degradation.
4.C. Physical Methods
Recently physical methods are used,in conjunction with chemical
reactione to elucidate structure of alkaloids.Such is now the level of
sophistication of the associated instruments that it is possible to determine
a structure in a matter of days given a few milligrams(or less) of a pure
Infrared spectrum gives information about many functional
groups.Ultraviolet spectra are used to indicate the nature of unsaturation
or aromatic rings. NMR spectroscopy is more versatile for detecting
many function groups, the nature of protons,carbons, heterocyclic rings
etc.Mass spectral fragmentation gives the information about molecular
weight and degradation of the skeleton.
Single crystal X-ray analysis has offered means for determining or
confirming stereochemistry as well as distinguishing between alternate
structures that appear to fit well for a particular alkaloid.Further support
for the stereochemistry can be obtained by using optical rotatory
dispersion or circular dichroism studies.