249 chapter v tjl7iuti0let spflctkoscopx 1 ultraviolet spectroscopy...
TRANSCRIPT
249
TJL7IUTI0LET SPflCTKOSCOPX
1Ultraviolet spectroscopy, the oldest physical method,
employed in the analysis of chemical substances, was
developed at the beginning of the 33 th 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 ultraviolet6
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^
CHAPTER V
?*i .ggflgfji LUm iskt* a£ m uyApJ&t. M sste& m M
250
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 3table
to thermal and electrical excitations.
mx
The absorption of light la ultraviolet region
generally follows I*ambert-Beer law which is
mathematically expressed ass
lo cx » log — * & c b| ( 1)
where 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
of a 1% solution of the substance in a 1*0 cm* cell,
this value Is related to the molar extinction
coefficient by the expressions
1 cm*2D £ * Ejg x mol* wt*, (2)
\ hen 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 (3)
1 cm*(pure substance)
The theory and practice of ultraviolet spectroscopy6
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 n f— » rTT .
Since the O ' cr* transitions retire 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
252
V. 2 4 Resume of tha Developments
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. The olefinic
double bonds show at 160— 180 a^n
the absorption between 180— 190 aap is also caused
by * 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-hexatrienea 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 12
such as alkyl, aisino, and phenolic groups, have a
marked influence on its absorption spectrum; alkyl13
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* the14
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 a2jB
large number of natural products, such as carotenoids, 19 20 21
alkaloids, anthocyanins, natural p la n t s ,22 23 24
flivonoids, steroids, antibiotics, and coumarins*
It has been successfully employed in the identification25 26
of heterocyclic compounds Including furans, purines,
and pyrimidines* Ultraviolet speetroscopy has found
an important application in the cgialltative and
quantitative analysis of essential oil components.
file volatile constituent of the family Compositae—
eosmen*--gives four absorption bands at 272,
273, 296, and 309*7 cap.* 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 a31
carbonyl group and ethylenic linkage*
266
27
Ultraviolet speetroscopy has been useful in the
identification of some isomeric terpenoids, such as32
cugenol and iso-eugenol. Eugenol shows a low intensity
band at 279 m while lso~«igeaol shows a low intensityn ^3
band at 256 mu* 0C— and p — vetlvoaes, and safrole and 34 I
iso-safrole 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 the37structures of terpenoid alcohols, and terpenoid
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 the41
authenticity of some essential oils and the estimation42
of some of their components•
¥*3 Work Done
41 44The 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*
256
38
m
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 deter mi 03 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*
V*4 Experimental
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% at (4)‘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 4 and B
respectively, 4 ^ and B are the standard
S1 cm. E1 cm*
extinction coefficients of 4 and B respectively, and
4B k is the molar extinction coefficient of the B
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
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 m ju ( £ | 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* The results
are presented in table 7*1. 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
V.5 Bonita and Discussion
X Cm M )
FIG.V.l. U . V . ABSORPTION OF CITRAL.
o o O oo <7> CO N ID
1VH1IO dO 39VlN30d3d
FIG
. V
.2.
MOLAR
EXTIN
CTIO
N
COEFFIC
IEN
T
VS
. PER
CEN
TA
GE
OF
CIT
RA
L.
260
U.V. Spectroscopic Data of Citral and Myrcene Mixtures
TABLE 7.1
Cone, of citral .4
Cone* of myrcene
..... aas...x
Percentage „4 of citral
10 .. ...........8
14*973 «» 100 13,253
12*576 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
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* i graph
between tiie percentage of eugenol and £was plotted*
A (KVA )
FIG.V.3. U.v. ABSORPTION OF EUGENOL.
S2&3A&£a si y s r n ia ix n w m si 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 49 .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
261
UJoL l .L_UJoO
H*o
HXUJ
cr<
O O O O O Q O O O O O e n C O N - i D W ^ ^ t r O e M —
o2UJo3UJ
u_oUJ
o<(-zUJocrUJCL
CO>
UJ<J
UJ
Oo
2Of-ozHXUJ
cr<Oz
>6U_
10N39n3 JO 39VlN30d3d
•90
•80
•70UJo2 -60 <CD
O -50(j) cQ< -40
•30
•20
•10
100
2 2 0 2 4 0 2 6 0
X C 'm /O
280
FIG.V.5. U .V . ABSORPTION OF CARVONE
TiBi® ?.3
II* ?• Spectroscopic Data of Carvone and OK -terpinene Mixts.
262
Cone* of carvone
-4j BS, X 10
Cone* of percentage -terpinene of carvone
—4gas* x 10
8
14*9203 - 100 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* ofpulegsne
x 10*4
Cone, of linalool
-4jus. x 10
percentage of pulegone £
16*189 • 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 lo w aixtures of pul.gon. *>4 llnalool <
263 m^i) were manured at 253 aja because the molar
extinction coefficient of pylegone was the hipest
O O O O O '<3 0 0 0 0O O ' C O N O m ^ M N —
3NOAdVO JO 39VlN30«3d FIG
V.6
. M
OLAR
EX
TIN
CTIO
N
COEFFIC
IEN
T
VS-
PER
CEN
TA
GE
OF
CA
RV
ON
E.
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.
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 .
U.V. ipectroscopit Data of Carvone and Linalool Mixta.
Estimation of Carvone in Presence of Linalool
T.iBLi tf.o
done, of ’... Cone, o#......' "percentagecarvone w4 linalool of carvone ggms. x ID* gjas. x icT
11.8560
14.6200
9.3714
8.374
2.113 ll.o79
4.627
3.879
5.137
70.812
66.952
62.069
100
14.432
18,001
14,353
13,470
12,866
6 , 6 4 3
FIG. V-7.
220 2 4 0 2 6 0 2 80 300
A C^-A)
U-V. ABSORPTION OF PULEGONE.
3 2 0
o o o o o o o o o oO <J> CO N ^ IO I 1 r o c v l —
3NOD31fld JO 39VlN3Dd3d
FIG
-V
.8.
MOLAR
EXTIN
CTIO
N
COEFFIC
IEN
T
VS
.
PE
RC
EN
TA
GE
OF
P
UL
EG
ON
E.
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
was ©Alma**©<5 V. 8* V*ll, and ¥.14).
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
# » • * $fts. x id
9.673 5. 099 0.42 0.296.993 8.620 0.30 0.274.157 u .o sa 0.20 0.34
865
Table v .7
&>fflpound Concentration „ density .........
1
Citral
Myrcene
13.776
11.325
0*63
0*04
0.07
0.43
Table \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
Table r.9
Optical Density of Carvoae and Eu^enol Mixtures
Cqims. of '"' 1 Pone*" off ^ " S bSSu I f lm f l jC IZ Z Icarvone . Eugeaol * \ Os
,«*4 A 1 • 236 ^ 2 « 231gm». x 10 gms* x 30
13*0271 3*2840 0.66 0.19
G«T4*ii v.**OviL v « *•) 0.30
3.5432 11*3276 0.25 0.47
336
T & m V, 30
Confound Concentration■•4
#B»* X 30
Optical Density at
....23§ ...............* 231
Carrcne 15.0203 o.so 0*09
Eugenol 16.340 0*07 0.59
TABLE V .ll
percentages of Carvone and Eugenol in Bin^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
33.8386wmUw *********** 24.6081 76.3736 76.3919
T^MUb ¥.12
Optical Density of Eugenol and O^-terpinane Mixta.
ConoY'of'... Cone, of ... " " r T OpticIi Densltyeugenol -terpiaane .
0 3 * • at l y " 4 j n s . » 1 & T 4 1 a 2 3 1 ______________ a 88 2 8 6
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?
207
T<U&£ V. 13
CCMOpOttOd Concentrition .. , ..Qpiioii Density _ ..................... . . . .
gas. x lD-<i 1, * ^ 2 * <^5
Eugenol 15*4673 0*33 0*05
-terpiaeno 13*5763 0*03 0*43
T4BLS V#14
p«reantag«t of Bugenol aue &>t«rpineBe in Binary Mixts
* ereanta*® of mitral P erceatage of CX -terpinen#
Added..... .... found . idded ........._....
78.3840 7 £.8403 21.6160 21*5597
40*7973 40 *2198 *k> • 2027 5&.5S02
12*9343 20*0596 U,*075? 79*9809
Hotes and References
1. Ultraviolet speetroscopy is based on the principleof 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*, et.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*, et.al, The Cheat. Action of U.V. Ravi (Reinbold pub* C o *|l*fH (1941 )|
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 5 7
6* West, w*, et*al, Chem* jyrniis,* of Spectros. in Weisserberg, a* (Id7) Xigm . Qrl. Chem. (Interscience} H.X*) m m g i l H
Jaffe, H.H. and orchin, M*, Theory 4, ipplic. of Spectros* (John Wileyj H.2C.) (1962).
7* Saakula, £* anaew. Chem* (1934) 657.
8* Hovlon, £* Qrg. Chem* (1949) J£t 1*
9* Henri, V*. Phys. Radium (1922) J , 181.
10* Horton and Stubbs, g, Chem* aoc. (1940), 1349*
11* Kllngatedt, Comnt. land. (2922) 812j (1923) 176.248#
12* iiobertson and Matsen, gm M * Gheau aoc. (1350) 72.5250.
13* lUyneord and Roe, proc. Roy, soc. (1935) a 15 2. 299*
14* Evans and Olllaa, a.E. » ,£• Chen* Soc. (3943), 666.
15. Roberts & Oreen, £* J&. Chem. soc. (1946) £§, 214.
16* sheinfcer, Doklady akad. flauk. (1931) 2Z, 1043.
17* hjasperger, Cnea. &qu. (1928) &*, 123.
18* Fieser, Xi*F«, et.al, j£. S2SJ6* (1948) 800*
i»* L t o u g &*&* al iftfeAa Atotollflft(Lilly ResearchLab*, Indianapolis) Ind.) (I960).
20* iiODinson *ud Todd, £. Chett. Soc. (2932), 2299*
21* Sbx, Mature (1946) j£g, 18.
22. oil lam, 4.B. and Heiibron, I.M ., aiocham. 1* (1936)1253.
23. Hochstein, et.al, 4* ^a* Cheau Sqc* (1953) 2&» 6455.
24* Hershenson, jyt* & 3LlSjLlLkft i&&* S.E*£$£* ( «***. press) (1956).
26* Barton, £>. H. R* and lOad, £. Chea. &fi£. (1966), 2086.
26. Bentley, et.al, j£. vJaem* soc* (1951), 2301*
27. Marshall, J.R. and Walker, J . , -ibid- (1961), 1004*
28. Sorensen and Sorensen, acta* Cfoea. Bcaad* (1954) J , 284.
29* plattner, p *a# and Heilbronner, E«, &sl£. Chen,(1947) 910) (1948) J i , 804.
30. OLllam, a.E. and West, T .F ., £. CJaem. £&&. (1942), 483.86.
31* (H 11am, i*£* and West, T .F . , (1946), 95*98.
38* Haves, Y.E. , Helv. GhLa* Acta (1951) 369-70.
32. fespe and Boltz, Analyst. Chen, (1962) jg|, 664.
269
270
34. crymble, et.al, £• Chem. Soc. (19IX)
35. GiUaa, 4.1. and West, T .P ., Hatuure (1941) 148. 114*
38* C&llam, a.E. and West, T .F ., £• Chea. |j££. (1941),811-14.
37. Baden, Eelv. Chlcu Aeta (1951) j& , 1632-34.
38. Diarotii and Trautiaann, Beg. (1936) 669.
39. Walker and Hopkins, £. Chea. Sac. (1952) 22, 4209.
40. O'Connor and cpldblatt, malart.t. Cfrea. (1954) j£,1726.
41. Tattautser, et.al, Ind. &ag. C.kea. (1944; AS# 62k-24.
42* Montes, rnal. is sen, -Alla. Argentina ( Jjy54> Jg, 30-37.
43. survv, et.al* £,. & Bqc4« (1958) §>, 724.
44. Fearns, et.al, -iblfr. (I960) £1, 355-56.