249 CHAPTER V TJL7IUTI0LET SPflCTKOSCOPX 1 Ultraviolet spectroscopy …shodhganga.inflibnet.ac.in/bitstream/10603/23985/10/10_chapter 5.p… · Ultraviolet spectroscopy, the oldest

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.