249
CHAPTER V
TJL7IUTI0LET SPflCTKOSCOPX
1
Ultraviolet spectroscopy, the oldest physical method,
employed in the analysis of chemical substances, was
developed at the beginning of the 33th century and
has become one of the important analytical tools for
the structural analysis of synthetic and natural organic
compounds.
Besides this, it has provided valuable
information about the allied structural parameters,
2
such as tautomsrisa, association of organic molecules,
3
4
dissociation of acids and bases, and reaction rates.
A
survey
of early developments in the ultraviolet
6
spectroscopy has been given by Braude*
This chapter
gives a brief account of the basic principles
underlying ultraviolet spectroscopy and its applications,
with special reference to organic compounds, and also
presents an account of its utilisation for the
quantitative evaluation of terpenoids and their
binary mixtures^
250
? *i
.ggflgfji LU m iskt* a£ m uyA pJ&t. M sste& m M
Spectrophotometry deals with the measurement of
radiant energy transmitted by a system at a specific
wavelength*
411 the molecules of a system, possess
the property of absorbing electromagnetic radiations;
in the case of organic compounds, this property is
generally localised in some particular groups of
atoms, and therefore by measuring the amounts of
radiation absorbed by a molecule it is possible to
know some of its structural parameters*
4s a result
of the absorption of electromagnetic radiations by
the molecule, the electrons around the nuclei undergo
transition between the ground state and the excited
state*
These transitions give rise to electronic
spectra; the transition of electrons from the ground
state of the molecule to its excited state produces
nabsorption spectrum*, while the transition of
electrons from the e&clted state of the molecule to
its ground state gives rise to "emission spectrum'*.
In the study of organic xoleeules absorption
spectroscopy is preferred to emission spectroscopy
because there are very little chances of decomposition
and molecular transformation In this method of
analysis*
Emission spectroscopy can, on the other
hand, be used with those molecules which are
to thermal and electrical excitations.
3table
mx
The absorption of light la ultraviolet region
generally follows I*ambert-Beer law which is
mathematically expressed ass
lo
x » log —
where
*
c
& c b|
( 1)
4 is absorbance, IQ is the Intensity of
incident li#it» I is the Intensity of transmitted
iigfrt, c is the concentration of the solution*
b represents the thickness of the solution layer,
and £ represents the molar extinction coefficient.
In such cases where the molecular wei^it of a compound
is unknown, the intensity of absorption is expressed
1 cm*
as the
value, which represents the absorbance
1% solution of the substance in a 1*0 cm* cell,
of a
this value Is related to the molar extinction
coefficient by the expressions
1 cm*
2D £ * Ejg
x mol* wt*,
\hen
(2)
of a pare substance at the same wavelength
and in the saute solvent, in which it is determined in
the test substance, is known, the percentage of absorbing
substance in the test solution can be calculated from
the eolations
100 x e L 6®* (observed)
* % of absorbing
________ lZ__________________
substance
1 cm*
(pure substance)
(3)
252
V. 2
4 Resume of tha Developments
The theory and practice of ultraviolet spectroscopy
6
i» fully established*
the absorption of light in
ultraviolet region brings about the transition of
electrons from bonding orbitals to the anti-bonding
orbitals.
In organic molecules the electrons from
CT-orbital, TT^-orbitalt and n-(non-bonding) orbital
*
are promoted to cr -antibonding orbitals and
*
H"f -antibonding orbitals, since n-»o?bitals do not
take part in Dond-formation , there are no anti«bonding
orbitals associated with them.
The following types
of electronic transitions are involved In the
ultraviolet absorptions
Cf — * cf*t 0 —> & * * 0 — > rrf*. and
Since the
O
n f — » rTT .
' cr* transitions r e t ir e
energy,
the saturated hydrocarbons do not absorb in ordinary
ultraviolet region.
These and some other saturated
alcohols and ethers,which fail to absorb between
200 ap and 1000 ap, are therefore used as solvents
for spectral determinations.
Those compounds which
contain non-bonding electrons on oxygen, nitrogen,
sulphur, or halogen atoms involve n — » <r* transitions
and absorb in ordinary ultraviolet region.
Some
compounds do not show any absorption above
210 spi,
but there is usually some absorption in the shorter
wavelengths| the intensity of absorption goes on
ass
increasing continuously towards shorter wavelengths.
Such compounds are said to show end-ibsorption.
This is in part due to n — =>cS * transition near 200 mp
and such molecules usually contain a lone pair of
electrons.
The transition of electrons from nT- trf*
orbitals is associated with unsaturated centres in the
molecule,
since these transitions require low energy,
molecules absorb at longer wavelengths.
double bonds show
the absorption between
by
The olefinic
at 160— 180 a^n
180— 190 aap is also caused
* transitions , while n —> rf* transitions
exhibit the absorption at 275— 295
The absorption spectra of identical functional groups
in different molecules are always dictated by their
structural environment! the absorption spectra are
greatly Influenced by solvent— solute interactions,
association of molecules, dipole moments, and
conjugation*
The isolated non-conjugated chroaophoric
groups exhibit absorption at almost the s*me
wavelengths in various molecules, but the pres m e of
two or more chrooophoric groups, particularly when
they are in conjugation with each other, shifts the
absorption band towards longer wavelengths.
7
1,3-butadiene absorbs at 217 m , while 1 ,3 ,5-hexatriene
a
8
shows A malf at 266 wfn9 Benzene gives two absorption
bands i one at 193 wp. and the other at 230— 270 Jftt*
264
The introduction of substituent* on benzene (melons,
10
11
such as alkyl, aisino, and phenolic groups, have a
marked influence on its absorption spectrum; alkyl
13
groups and fused benzene rings shift the absorption
maxima of benzene towards longer wavelengths*
tlie carbonyl group of aldehydes and ketones by
virtue of n —>cr* transitions show an absorption at
130— 160 ap.
The unconjugated carbonyl groups
exhibit a weak band near 280 *§»! this band occurs due
to the presence of a lone pair of electrons on
carbonyl oxygen atom*
on the other band* the
14
semicarbazones, oximes, and 2s4 dinitrophenyl*
15
hydrazones of carbonyls give a stronger absorption
band which is used for their structural investigation*
The aliphatic aside and diazogroups show two bands
eachs the former gives a characteristic
band at
1©
236 nm and the latter exhibits a strong band at
17
220 apa.
The azomethine and cyanide ehromophores do
not show any selective strong band between 200-1000 qtu
Ultraviolet spectroscopy has facilitated the
identification and structural determination of a
2jB
large number of natural products, such as carotenoids,
19
20
21
alkaloids, anthocyanins, natural p l a n t s ,
22
23
24
flivonoids, steroids, antibiotics, and coumarins*
It has been successfully employed in the identification
25
26
of heterocyclic compounds Including furans, purines,
12
266
27
and pyrimidines*
Ultraviolet speetroscopy has found
an important application in the cgialltative and
quantitative analysis of essential o il components.
file volatile constituent of the family Compositae—
eosmen*--gives four absorption bands at
273, 296, and 309*7 cap.*
272,
plattner and Heilbronner
have reported the spectroscopic data of a&uleaes and
five aethylazulenes and observed that introduction of
methyl groups in these compounds shifts the
absorption band towards longer wavelengths* &llaai and
30
West have determined the U.V. spectra of semicar’oazones
and the semicarbazones of irone, eucarvone, and
related ketones*
They have also found that the
abnormal absorption spectrum of umbellulone was obtained
due to the presence of an unusual chromophoric group
consisting of cyclopropane ring in conjugation with a
31
carbonyl group and ethylenic linkage*
Ultraviolet speetroscopy has been useful in the
identification of some isomeric terpenoids, such as
32
cugenol and iso-eugenol.
Eugenol shows a low intensity
band at 279 m while lso~«igeaol shows a low intensity
n
^3
band at 256 mu* 0C— and p — vetlvoaes, and safrole and
34
iso-safrole
I
have also be mi identified by comparing
their U.V. spectra*
the U.V. spectroscopy has revealed
the presence of OC--and p — unsaturated ketonic group
35
36
in irone and lso*thujone, and has confirmed the
37
structures of terpenoid alcohols,
and terpenoid
256
38
hydrocarbons, such as (0<-phillandrene, myrcene, and
39
ocimene*
The U*V. spectra of twenty-three hydrocarbons
<
\ aY 220 to 320 sp.) have been reported by 0* cannor
and Ooldbatt*
The unconjugated dienes, such as limonone
and T -terpinene show a continuous spectrum without any
characteristic band*
Ultraviolet spectroscopy has been
successfully employed is the determination of the
41
authenticity of some essential oils and the estimation
42
of some of their components•
¥*3
Work Done
41
44
The method# of surve, et.al and Fearns, et.al
have proved of immense utility in the evaluation of
binary mixtures.
These methods have been used for the
quantitative evaluation of the constituents of some
essential oils.
In the present study Surve, et.al’ s
method of mixing a compound with another compound,
which shows no absorption at the
of the test
substance, has been utilised for the estimation of
citral, pulegono, sugenol, and carvone in binary
terpenoid mixtures,
Citral has been estimated in the
oil of lemongrass and carvone has been estimated in the
oil of caraway*
The values obtained were found to be
in conformity with the chemical values.
Fearn’ s method
of estimating the constituents of a binary mixture has
been applied to estimate citral, carvone, and eugenol
in artificial binary mixtures*
m
V*4
Experimental
The present investigation was carried out with the
help of Beckman spectrophotometer • The solutions of
•4
various terpenoids (conc* 10 M) studied during the course
of investigation were prepared in n*hesune«
The
absorption maxim of each terpenoid was determined and
selected as the standard wavelength for further studies
on the terpenoid*
The compound under study was mixed with another
terpenoid, which showed negligible absorbance at the
of the compound to be estimated*
The absorbance
of the binary mixture was determi03 d and its molar
extinction coefficient was calculated*
4 calibration
curve was plotted between the concentration and molar
extinction coefficient of the terpenoid*
These plots
were used to estimate the compound in some samples of
essential oils*
The values obtalad were found to be in
conformity with the chemical values, within xo error
percentage of 0*2 to 0*36*
4
set of simultaneous equations (Eq. 4 and Bq* 6)
previously used by Fearns have also been applied to
estimate the amount of terpenoids in binary mixtures
of known composition*
258
100 X A B 3 *
it A
« * Of 4 X 4 JJ
at ^
a . __
X
E•1 cm*
1 cm*
% of B X B„l%
(4)
at
‘1 esu
J of U
4 y
at
K1 cm*
\ahere
4 and B are the two components of the binary
mixture,
and A 2 ire the absorption maxima of
respectively,
4 ^
and
B
S 1 cm.
extinction coefficients of
4B
k
B
4 and B
are the standard
E1 cm*
4 and B respectively, and
is the molar extinction co efficien t of the
1 cm.
binary mixture*
These equations have been applied to
the following mixturest
(a)
eitral and ayrcene,
(b)
carvone and eugenol, and
(c )
eu^enol and <!X-terpinene*
259
V.5
Bonita and Discussion
Mtlsaatlon of Citral in Presence of Myrcene
m e U.V. absorption spectra of citral is presented
in Fig* V*l*
It shows maximum absorbance at 238 vp>
< 8 | 13,500) while the absorption maxima of myrcene
is observed at 224 mju ( £ | 1,456).
411 the
measurements of absorption of the binary mixtures of
citral and myrcene w«re determined at 238 myu and molar
extinction coefficients were calculated*
are presented in table 7*1.
The results
The calibration curve
between the molar extinction coefficient and percentage
of citral (fig . 7 .2 ) was utilised for the determination
of citral content in lemongrass oilf the percentage
of citral in this oil was found to be
( 8 1 9,150)
and was in accord with the chemical value*—59.0%
(Fig* V.2| AjJ)*
Estimation of Eugenol
Presence of Of -terpinene
The absorption spectrum of eugenol is presented
in Fig* V .3 .
It shows two absorption maximal one at
231 m/OL and the other at 282 m^u.
The absorption of
binary mixtures of eugenol and (X-terpinene (
mjtt ) were measured at
231 mji because the
extinction coefficient of eugenol at this wavelength
was higher ( 8 $ 7,240) than at 282 mju*
Four mixtures
X Cm M)
FIG.V.l.
U . V . ABSORPTION
OF
CITRAL.
o
o
1 V H 1IO dO
FIG. V .2 . MOLAR
o O o
<7> CON ID
39V lN 30d3d
EXTINCTION
COEFFICIENT
VS.
PERCENTAGE
OF
CITRAL.
260
TABLE 7.1
U .V. Spectroscopic Data of Citral and Myrcene Mixtures
Cone, of
citral
.4
Cone* of
Percentage
myrcene
„4 of citral
..... aas...x 1 0 .. ...........
8
100
13,253
2*790
31*811
11,056
9.673
6*699
62*926
9,030
6*993
8*630
44*073
7,615
4.1b?
11.086
27*271
5,859
14*973
«»
12*576
TiELE V .2
!!•?• spectroscopic Data of Eugeool and (X-terpinene Mixta.
conc* of
eugenol _4
*p* x 10
C one * of
p ere enta ge
•terpinene of eugenol
gas* x 10*4
•
16*3140
£
100
7,221
13*3190
2*276
32*2414
6,550
10*963
3*954
61*1876
5,770
4*213
12*476
25*2577
4*473
of eugenol and QC -terpinene, containing different
amounts of each terpenoid (table V*2), were prepared
and molar extinction coefficients calculated*
between tiie percentage of eugenol and
i graph
£was plotted*
A (KVA )
FIG.V.3.
U.v. A BSO R PTIO N
O F EUGENOL.
261
S2&3A&£a s i y s r n i a i x n w m s i Q ^ t r a a w a i
The U.V* absorption spectrum of carvone (Fig* V .5)
shows absorption maxima at 235 m
( ^ j 19,000) %felle
CX -terpinene shows absorption maxima at 265 m u.
The molar extinction coefficients of six binary mixtures
of carvone and (X-terpinene, containing varied amounts
of each component, are jglven in table V .3 .
The
calibration curve between the percentage and molar
extinction coefficient of carvone Is presented in
Fig* V*6*
This curve was utilised for the estimation
of carvone in the commercial sample of the oil of
caraway and the oil of caraway obtained from the seeds
of the plants from the state of Jammu and Kashmir*
The commercial sample showed the molar extinction
coefficient UplSo corresponding to 4 9 .3£ (Fig* V*6}A^)
of carvonei its chemical value was found to be 48%.
The oil from the state of Jammu and Kashmir showed the
molar extinction coefficient 11,32® corresponding to
54% of carvone (Fi<j* V*6jlg) while its chcuical value
was found to be 51*6%*
Estimation of pule gone in presence of Mnaloai
The U*Y* spectrum of pulegone is given in Fig* V .7.
Its absorption was measured from its solution in
spectroscopic ethanol*
It shows two absorption peaks*
one at 253 mju. and the other at 316 myu.
The absorptions
o
2
UJ
o
3
UJ
ou_
UJ
UJ
o
L l.
L_
oO
o
<
(z
UJ
o
cr
UJ
CL
UJ
H*
o
CO
>
UJ
<J
H
X
UJ
cr
<
UJ
O
o
2
O
fo
z
H
X
UJ
cr
<
O
z
>
O O O O O Q O O O O
O e n C O N - i D W ^ ^ t r O e M
10N39n3 JO 39VlN30d3d
6
U_
—
100
•90
•8 0
•70
UJ
o
2
<
-60
CD
O -50
(j)
cQ
< -40
•30
•20
•10
220
240
260
X C 'm /O
FIG.V.5. U .V . ABSORPTION
280
OF CARVONE
262
TiBi® ? .3
II*?• Spectroscopic Data of Carvone and
Cone* of
carvone
-4
j^BS, X 10
14*9203
OK-terpinene
Cone* of
percentage
-terpinene of carvone
—4
gas* x 10
-
100
Mixts.
8
18,045
13*0371
3*0712
@0*9340
16,430
11*0352
4*5370
60.90
13,936
8*7434
6*4371
57*5960
12,420
3*5765
10*113?
26*1245
7,800
1*6367
12*5630
10*8988
5,864
table v . 4
U.V. Spectroscopic Data of Fulegone and Limlool Mixts*
oonc* of
pulegsne
x 10*4
16*189
Cone, of
linalool
percentage
of pulegone
£
-4
jus. x 10
•
100
8,115
1^*348
2*446
83*466
7,300
8*610
4*973
63*388
6,386
4*173
9*486
30*661
4,727
of b l o w aixtures of pul.gon. *>4 llnalool <
263 m^i) were manured at 253 aja because the molar
extinction coefficient of pylegone was the h ip est
C ARVON E.
OF
VS- PERCENTAGE
COEFFICIENT
EXTINCTION
O O O O
O ' C O N
O
' <3 0
m ^
0
M
0
N
3NOAdVO JO 3 9 V lN 3 0 «3 d
0
—
FIG V.6. MOLAR
O
O
263
( 6 } 8,150) at this wavelength.
The molar extinction
coefficients of four binary mixtures are given in
table V .4 and the calibration curve between the
percentage and molar extinction coefficient of pulegone
is presented in Fig. V.8.
Estimation of Carvone in Presence of Linalool
Six mixtures of carvone and linalool containing
varied amounts of these terpenoids were prepared
(table V .5 ) and their absorbance was determined at
235 Qjii.
The molar extinction coefficients were
calculated! percentage of carvone was compared with
the values obtained from the calibration curve (Fig. ?.@)
plotted for the bleary mixture of carvonc and
(X -terpineae.
Tue molar entice tion coefficients
calculated from the absorbance of carvone and linalool
mixtures are tabulated in table ? .5 .
T.iBLi tf.o
U.V. ipectroscopit Data of Carvone and Linalool Mixta.
done, of ’... Cone, o#......' "percentage
carvone w4 linalool
of carvone
gms. x ID*
gjas. x icT
14.6200
g
100
18,001
11.8560
3.879
70.812
14,353
9.3714
4.627
66.952
13,470
8.374
5.137
62.069
12,866
2.113
ll.o79
14.432
6 ,6 4 3
220
240
FIG. V-7. U-V. ABSORPTION
260
280
A C^-A)
300
OF P U LE G O N E .
320
VS.
CO
N
^
IO
I
rocvl
3NOD31fld J O 3 9 V l N 3 D d 3 d
FIG-V.8. MOLAR EXTINCTION
COEFFICIENT
PERCENTAGE
OF P U L E G O N E .
o <J>
o o o o o o 1o o o
O
—
as4
al £giaaLl», laffiaa
The absorbance of the three binary mixtures, used
to find out the applicability of Fearn’ s procedure,
was determined at two wavelengths, corresponding to the
Absorption maxima of each component of the mixture.
She values ire given in tables v .6 , 7 .9 , and V .12.
?he absorbance of individual components of the
mixtures was Also determined ( tables V .7 , ¥ .10, and
V.13) at these wavelengths.
The values were
substituted in Peam* s slrmlt&neous e^ations and the
percentage of each coapocoot in the binary mixture
V. 8* V *ll, and ¥.14).
was ©Alma**©<5
The values were in the range of the actual amounts
present.
T4BLf; f.@
Optic il density of Citral and Myrcene Mixts.
Cone .'"of... bone.' "of "
citral
Hyrcene
#»• *
9.673
6.993
4.157
$fts. x id
5. 099
8.620
0.42
u .o s a
0.20
0.30
0.29
0.27
0.34
865
Table v .7
&>fflpound
density
Concentration „
.........
1
Citral
13.776
0*63
0.07
Myrcene
11.325
0*04
0.43
T able \
t.8
Percentages of Citral and Myrcene la Binary Mixtures
Percentage of ..citral......
p ereentaise o£ Myrcene.........
added
62.9260
33*0361
37.0740
36*9639
44.7893
44.0880
55.3105
55*9120
27.2? IS
27*0048
72.7285
72.9951
T able r .9
Optical Density of Carvoae and Eu^enol Mixtures
Cqims. of '"' 1 Pone*" off
carvone
. Eugeaol
*
,«*4
gm». x 10
gms* x 30
^ " S bS S u I f l m f l j C I Z Z I
\
Os
A 1 • 236 ^ 2 « 231
13*0271
3*2840
0 .6 6
0.19
G«T4*ii
v.**OviL
v « *•)
0.30
3.5432
11*3276
0.25
0.47
336
T&m
V, 30
Confound
Optical Density at
Concentration
■•4
#B»* X 30
.... 23§
............... * 231
Carrcne
15.0203
o.so
0*09
Eugenol
16.340
0*07
0.59
TABLE V .l l
percentages of Carvone and Eugenol in B in ^ y Mixts.
Percentage of carvone
percentage of JugraoX
Added..
Found
Added.... ... ..
Found
79.8991
80.7601
20*2009
19*2399
57.1143
4)7.5692
42.886?
42*4308
33.8386
wmUw***********
24.6081
76.3736
76.3919
T^MUb ¥.12
Optical Density of Eugenol and
ConoY'of'...
eugenol
03* •
at l y " 4
O^-terpinane Mixta.
Cone, of ... " " r T OpticIi Denslty
-terpiaane
.
jn s .
»
1& T4
1
a
2 3 1 ______________ a
8.9321
3.4632
0*27
0.14
6.3210
3*9413
0.22
0.20
4.3913
6*3724
0*16
0.29
2*1397
8.5994
0*09
0.3?
88 2 8 6
207
T<U&£ V. 13
CCMOpOttOd
Concentrition
.. ,
gas. x lD-<i
Eugenol
15*4673
-terpiaeno 13*5763
..Qpiioii Density _
1, *
^ 2 * <^5
..................... . . . .
0*33
0*05
0*03
0*43
T4BLS V#14
p«reantag«t of Bugenol aue &>t«rpineBe in Binary Mixts
* ereanta*® of m itral
P erceatage of
CX -terpinen#
Added.........
found
78.3840
7 £.8403
21.6160
21*5597
40*7973
40 *2198
*k>• 2027
5&.5S02
12*9343
20*0596
U,*075?
79*9809
.
idded ........._....
Hotes and References
1.
Ultraviolet speetroscopy is based on the principle
of the absorption of ii&it in the 200 to 800 aju
region of the s-pectrum. The historicai background
of the absorption spectroscopy ha& ueen given by
lUyser, H .t ftjjadbugh
(Leipzig)
(1908) £ AndH.
2.
among others*
Wilson, W*, e t .il, I .
0rg. Caaa. (1363) ££, 3 8 1 ;
Buraway, i. and Thompson, a*R* , -ibid- (1953) 77.1443.
3.
Braude, £*4*, £« Cheat, ^oc, (1948), 1971}
laborn, C *, Nature (1953), 3148, used U.V. for the
determination of acidity functions of concentrated
and non-a»jaeous acid solutions*
4*
U.V* spectroscopy has been used to determine the
unstable structures and reaction rates of some
organic compounds* For reference sees
Ellis, C *, e t.a l, The Cheat. Action of U.V. Ravi
(Reinbold pub* C o * | l * f H (1 9 4 1 )|
Roberts, J.i). and Watnabe, £ . ^yg* Ghem* koc. (1950)
5*
Braude, 1*4* in Braude and Hachod (Id *). Detn.
i&s*
ats. bv Phvs. Methods (4cad press* N .f .) (19©2) 1 ,
l&g 57
6*
West, w*, et*al, Chem* jyrniis,* of Spectros. in
Weisserberg, a* (Id7) X ig m .
Qrl. Chem.
(Interscience} H .X *) m m g i l H
Jaffe, H.H. and orchin, M*, Theory 4 , ipplic. of
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Baden, Eelv. Chlcu Aeta (1951) j & , 1632-34.
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Bqc4 « (1958) ^§>, 724.