Die Angewandte Makromolekulure Ghemie 7 (1969) 67-78 (Nr. 63)

From the Central Laboratories, Pakistan Council of Scientific & Industrial Research :

Off University Road, Karachi-32.

Infrared Spectroscopic Investigation of Tannins

By M. ARSHAD A. BEG and Z. A. SIDDIQUI

(Eingegangen am 14. August 1968) *

SUMMARY:

Infrared spectroscopy is suggested as a diagnostic method for the characteri- sation and qualitative estimation of the two classes of tannins. Gallic acid, tannic acid and chebulinic acid have been taken as model compounds for the hydrolysable and catechin for t,he condensed tannins. The former class is marked by the presence of strong absorption maxima at 1710 - 35 cm-1. The two classes have characteristic pattern of absorption, from which it is possible to characterise the particular type of tannin.

ZUSAMMENFASSUNG :

Die Infrarotspektroskopie wird als Methode zur Charakterisierung und zur qualitativen Bestimmung der beiden Gruppen von Gerbstoffen vorgeschlagen. Gallussiiure, GerbsBure und Chebulinsgure wurden als Modellverbindungen fiir die hydrolysierbaren und Catechin fiir die kondensierten Gerbstoffe genommen. Die erstgenannte Gruppe ist durch starke Absorptionsmaxima bei 17 10 - 35 cm-1 ge- kennzeichnet. Beide Gruppen zeigen charakteristische Absorptionen, die zur Er- kennung des Gerbstofftyps dienen konnen.

The infrared spectra of several macromolecules show only the broad outlines of the molecule i. e. their main absorption bands are due to the most prominent groups in the compounds. In the simple sugar like glucose, for instance, it is found that it has a well defined spectrum in the 2 - 15p range. The macro- molecule cellulose on the other hand has diffuse bands corresponding to the hydroxyls while the region above l o p is almost transparents. Similarly the polypeptides do not have any prominent bands in this particular region, and have only broad bands corresponding to the amino and carboxyl groups.

* Revidierte Fassung, eingegangen am 20. Februar 1969.

67

M. ARSHAD A. BEG and Z. A. SIDDIQUI

Tannins are similar substances. They also absorb intensely in the hydroxyl and carbonyl region. It is intended to discuss the spectra of the tannins in terms of their simple units so that these spectra are of diagnostic value in the characterisation of these compounds.

Tannins have been known for a long time to be of two kinds : the hydrolysable and the condensed type. The basic unit of the hydrolysable tannins is gallic acid while that of the condensed type is catechin, and on this basis it should be easy to distinguish between the two classes by means of infrared spectro- scopy. We have isolated the tannins from several known sources and have studied their spectra. For the correlation we have recorded the spectra of the basic units gallic acid and chebulinic acid5 for the hydrolysable and catechol and catechin for the condensed type.

Experimental

Tannin extracts of the local vegetable materials were prepared by the standard procedures of leaching. A 5 yo solution of this extract or of the commercial tanning material was filtered hot to remove the suspended impurities. The hot filtrate was reacted with a 10 yo aqueous solution of lead acetate. The tannate so obtained was washed with hot water and then suspended in a ten-fold excess water and treated with a 5 Yo solution of oxalic acid. The mixture was refluxed for four hours, filtered and the filtrate evaporated to dryness in a rotary evaporator. The dried material was extracted with ether to remove any trace of oxalic acid. The tannins so ob- tained were kept well desiccated. Tannic acid, gallic acid and catechol used were the B.D.H. reagents. The spectra were recorded both in Nujol mulls and potassium bromide pellet on a Beckman IR 5 spectrophotometer.

Discussion

The spectra of the tannins are marked by intense absorptions in the 3600- 3200, 1700-1600, 1300-1200, and 800-7OOcm-1 regions. Except the first two regions where the absorption bands are broad and strong, others are either of medium intensity or weak. The spectra of the tannins fall broadly into two classes corresponding to the two types. The mixed type having the features of both hydrolysable and condensed have bands common to both.

Gallic acid, tannic acid and chebulinic acid5 are the three very useful model compounds for the study of the hydrolysable tannins. For gallic acid the strong and broad band at 3500-3300 cm-1 is assigned to hydroxyl stretching, the 3100 cm-1 medium intensity band to C-H aromatic stretching and the strong and sharp bands a t 1710 cm-1 to the C= 0 stretching of the carbonyl

68

IR-Spectroscopy of Tannins

group. The bands a t 1545, 1475, 1445, 1370, 1340, and 1305cm-l are the skeletal vibrations of the ring. The medium to high intensity bands a t 1620 and 1245 cm-1 are consistent in most of the polyhydroxy compounds and hence are assigned to 0-H bending and C-OH stretching, respectively. The peaks a t 1205 and 1100 cm-1 fall in the region of C-H in-plane bending and are assigned to these vibrations. The sharp and strong band a t 1030 cm-1 is common among terpenoidal alcohols and hence is assigned to C- OH defor- mation. The medium intensity band a t 870 cm-1 is due to the out-of-plane bending of the isolated hydrogen in the benzene ring and the 732 and 700 cm-1 are due to the ring deformation modes.

On the basis of this assignment the spectrum of tannic acid may be correla- ted. Tannic acid is known to be penta-m-digalloylglucose. Hydrolysable tannins are the polyesters of gallic acid or m-digallic acid with various poly- hydric alcoholsl. Tannic acid is, therefore, a close model of the tannins. The spectrum of glucose pentaacetate corresponds quite closely with tannic acid. Various other esters of glucose have similar features. It is therefore suggested that the spectra of the ester type tannins would be marked by the occurrence of pyranose and furanose skeletal modes. The variations usually occur in the type of carbohydrate esterified, and the latter have weak but characteristic absorptions in the 900-800 cm-1 region. For example the band a t 875 cm-1 in tetrahydropyran has been attributed to a ring vibration2. This may also include contributions from C-0-C antisymmetric stretching. The 813 cm-1 band is assigned to the ring breathing frequency. In glucoses the ring vibra- tions occur a t 917 f 13 cm-1 for a-anomers and 920 f 5 cm-1 for the j3- anomers. The 844 f 8 cm-1 band in the a-anomer and the 891 & 7 cm-1 band in the j3-anomer are assigned to C-H deformation mode and the 775 f 10 cm-1 band in the a- or /I-anomer are assigned to the ring breathing frequency. The acetates of gIucose absorb a t 843 & 4 cm-1 and a t 753 & 17 cm-1 for the a-anomer and a t 890 f 8 cm-1 and 753 f 17 cm-1 for the ,%anomer3,Q. All these absorptions are noted in the spectra of tannins. For tannic acid weak absorptions are recorded a t 860 and 756cm-1 and on the present basis it should be a p-anomer.

Besides these weak to medium intensity absorptions which indicate the type of carbohydrate skeleton present, tannic acid records strong and broad bands a t 3550-3200 cm-1 which is the hydroxyl stretch. The split band a t 1740 and 1725 cm-1 is the C=O stretch due to the ester carbonyl group; the splitting showing an inequivalence in the orientation of the carbonyls. The strong band a t 1620 em-1 is observed in polyphenols and is most likely due to the 0-H bending vibrations. The 1545 and 145Ocm-1 bands are also quite strong. According to the above arguments both of them should be due to the skeletal

69

Tab

le 1

. In

frar

ed a

bsor

ptio

n m

axim

a of

hyd

roly

sabl

e ta

nnin

s.

I I

I I

I O

H

defo

rmat

ion

I st

retc

hing

I

I

Gal

lic

3390

s

1710

s 16

20s

1540

m

1370

m

1220

s 10

47 m

sh

acid

32

80 s

14

70 m

sh

1335

s 12

05 s

1025

s 30

30 s

14

50 m

13

07 s

1170

m

960

w

1245

s 11

00 w

Tm

ic

3500

-305

0 sb

r 17

40 s

1625

s 15

40 s

1370

-130

0 sb

r -

1080

s ac

id

1720

s 14

40 s

1250

-115

0 sb

r -

1030

s 97

0 m

br

940

mbr

91

5 m

br

Che

bulin

ic

3350

-315

0 sb

r 17

25 s

1625

s 15

95 s

acid

17

00 s

1520

m

1460

s 14

05 s

Ella

gic

3640

vs

1710

s 16

25s

1587

s ac

id

3500

-317

5s

1515

m

1450

s

1400

s

Myr

abol

lam

34

50 s

17

25s

1625

vs

1530

m

nuts

14

50 s

1340

sbr

1205

s 12

65 m

11

15 s

1330

s 12

20 w

12

25 m

12

00 m

11

10 s

1350

-132

0sbr

-

1235

-117

5sbr

1065

s 93

5 m

1055

s 92

5 m

1030

s 96

0 w

p- W

M 0

Wal

nut

3500

s

-

1695

sbr

1440

w

(Bark)

3450

s 16

40 s

Pom

egra

nate

34

50 s

17

10s

1615

s 14

56m

(P

eel)

Che

st n

ut

3570

-317

5 sb

r 17

35 s

1620

s 14

50 m

(w

ood)

Hen

na

3390

s -

1640

sbr

1550

msh

(l

eave

s)

2941

m

1450

m

1420

-135

0 m

br

Gre

en Tea

3360

s

-

1670

s 15

65 m

sh

(lea

ves)

32

80 s

sh

1610

s 14

40m

Saku

r 34

25 v

s -

1685

s -

(gal

ls)

Bah

era

3400

-317

5 sb

r 17

25 s

1670

s 14

05 s

(nut

s)

Har

itak

i 35

10 s

1710

s 16

25s

1550

m

(nut

s)

3355

s 15

30 w

sh

1515

wsh

14

50 s

1360

-130

0 m

br

1250

s

1370

-130

0 m

br

1245

sbr

1390

-128

0 ss

1300

-125

0 mbr

-

1351

w

1314

w

-

1380

-129

0sbr

1110

-103

0mbr

1176

sbr

1250

-115

0 sb

r 11

00 s

br

1110

ssh

1250

s 12

00 m

sh

1135

m

1258

s 11

17 m

br

1200

sbr

111O

m

1250

-117

5sbr

1030

m

1035

s

1070

s 10

30 s

1030

mbr

1065

mbr

1070

m

1040

-102

5 m

br

v =

ver

y, s

= s

tron

g, m

= m

ediu

m,

w =

wea

k, s

h =

shou

lder

, br

= b

road

, ss

= s

tron

g sh

arp,

ms =

med

ium

sha

rp,

sbr =

st

rong

bro

ad, m

br =

med

ium

bro

ad.

4

c

M. ARSIIAD A. BEG and Z. A. SIDDIQUI

Catechin

Table 2. Infrared absorption maxima of condensed tannins.

Quebracho Mimosa Goran Mangrove Remarks

3484 vs

-

2915 ssh -

-

1618 s 1520 wbr 1473 s 1439 ssh 1393 ssh

- 1307 ssh

1284 s

1261 ssh

1198-1190 sbr

1138 ss

1114 m 1069 m

1053-1 040 sbr

1029 m 1007 ssh

-

-

-

3400 vs

3330 vs

2941 m -

-

1613 vs 1515 s 1450 s

1380 msh

1351 wbr

-

-

1280 s

1250-1 170 sbr -

1150 s

1110 s 1045-1025

sbr -

- 980 m

-

-

-

3500 vs

3450-3125 sbr - -

1725 w

1625 vs 1515 w 1450 s

- -

- 1300 sbr

1290 vs

1240 vsbr

-

1180 vs

1110 sbr 1075 vs

-

- 1010 vs 980 m

875 m

850 s

3570-3390 vsbr -

3030 s 2940 s

1755-1710 W

1615 s 1525 w 1440 w

1390-1360 -

wbr -

1300-1220 sbr -

-

-

1138-1135

1110 s 1050 mbr

sbr

-

- - -

-

-

3450-3400 vs -

2900 s -

1730 w

1615 vs 1515 s 1440 vs

- -

1360 sbr -

1282-1265 vsbr

1250-1170 vsbr -

1120-1040 sbr -

1030 vs

-

- 980 s 870 mbr

820-760 mbr -

OH stret- ching

CH stret- ching

OH bending

ring vibration

ester linkage

ring vibration

O H deforma- tion

OH out-of- plane bending

v = very, s = strong, m = medium, w = weak, sh = shoulder, br = broad, ss = strong sharp, ms = medium sharp, sbr = strong broad, mbr = medium broad.

72

IR-Spectroscopy of Tannins

Babool Sundri Acacia sp.

3450-3125 vs -

2950 s 1710 sbr

1615 vs

1515 s

1435 sbr

1315 sbr -

1210 vsbr

1100 sbr

1030 sbr 870 mbr 765 mbr 720 mbr

3400-3125 vs -

2800 s -

1600 vs

1515 m

1450 msh 1440 s 1361 sbr

1280-1190 sbr

1105 sbr

1050 sbr 820 mbr

3390 vs 3205 ssh 2924 ssh 1754 m 1724 wsh 1705 msh 1661 msh 1639 msh 1634 ssh 1613 s 1558 msh 1527 w 1524 msh 1449 w

1379 sbr 1351 sbr 1307 ssh 1282 w 1266 ssh 1258 ssh 1220 sbr 1170 ssh 1156 sbr 1117 sbr

1099-1042 sbr

v = very, s = strong, m = medium, w = weak, sh = shoulder, br = broad, ss = strong sharp, ms = medium sharp, sbr = strong broad, mbr = medium broad.

modes of the ring. The occurrence ofa broad maximum a t 1330 cm-1 is charac- teristic of ester groups. The band a t 1200 cm-1 is due to a C-0-C linkage, possibly a cyclic ether. When this vibration is compared with the absorption in the 850 and 750 cm-1 region mentioned above, the assignment seems justified. Another consistent band which has also been noted in the case of gallic acid occurs a t 1030 cm-1 and may have the same origin viz. -0-H deformation with contributions from the - C- 0 vibration.

73

M. ARSHAD A. BEG and Z. A. SIDDIQUI

C 6 ’ 40003000 2000 1500 1ooO900 800 700

8 .- B E g

K

c

Wavelength (p) Fig. 1. Infrared spectra of tannins, hydrolysable type (recorded in potassium bro-

mide pellets), from top to bottom : “Sakur” (Tanarix indica) ; “Haritaki” (Terminalia sp.) ; Chebulinic acid; “Chestnut” (Castanea sp.) ; “Bahera” (Terminalia bellerica).

cm-’ 30002000 1500 1000900 800 700

I , I I

I I I I , , I , I I , ,

3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 Wavelength (p)

Fig. 2. Infrared spectra of tannins, hydrolysable type (recorded in potas- sium bromide pellets), from top to bottom: “My- rabollam” (Tenninalia chebula) ; “Pomegranate peel” ; “Wdnut”.

74

IR-Spectroscopy of Tannins

400

t ._ E 5 :

- cm-’ 3000 2000 1500 1000900 800 700

I I , 1 I

Waveiength (p)

Fig. 3. Infrared spectra of tannins, hydrolysable type (recorded in potassium bro- mide pellets), from top to bottom: Ellagic acid; “Henna leaves”; “Green Tea”; Gallic acid; Tannic acid.

It might be difficult to conclude the structure of the tannins from the infrared spectra, but from the pattern of absorption of the carbohydrates and gallic acid an approximate structure can be given, on the lines similar to the other macromolecules like cellulose, luteose and laminarins.

The correlation of the spectrum of chebulinic acid is in conformity with the assignments made above. A striking difference between the spectra of chebu- linic acid and tannic acid is the simplicity and sharpness of the bands of the former. The broad and strong bands in the latter suggest interaction among the various groups and should be an indication of a polymeric structure. The spectrum may be taken as an evidence of the formation of longer chains as in polysaccharides. The pyranose skeleton for the two compounds is, however, borne out from the similarity of their spectra.

M. ARSHAD A. BEG and Z. A. SIDDIQUI

Proceeding on these lines it may be possible to classify the hydrolysable tannins into one class. Fig. 1, 2 and 3 show a close resemblance of the spectra of “Terminalia chebula”, “Haritaki”, “Walnut”, “Pomegranate”, “Bahera”, “Chestnut”, “Sakur”, “Henna”, and “Sumac”. The spectra of all these compounds have strong and rather broad maxima a t 1725 and 1620 cm-1, a

s c .- B f $ C

50003000 2000 1500 1000900 800 700

Fig. 4. Infrared spectrum of tannin, condensed type (recorded in potassium bro- mide pellet) : Catechin.

40C -1 cm

313000 2000 1500. 1000900 800 700

I # I I I I 0 , I 1

3 4 5 6 7 8 9 10 11 12 13 14 15 Wavelength (p)

Fig. 5. Infrared spectra of tannins, condensed type (recorded in potassium bro- mide pellets), from top to bottom: “Goran” (Rhizophora sp.); “Mimosa” (Acadia mollisima) ; “Quebracho” (Quercus lerentii) ; ‘‘Mangrove”.

76

IR-Spectroscopy of Tannins

weak band a t 1545 cm-1, a medium intensity band a t 1450 cm-1, and strong and broad bands a t 1330, 1200, and 1030 cm-1. The peaks in the lower region are quite weak and give information on being resolved properly, which is possible only in pure compounds.

The second class of tannins known as the condensed type have catechin as the basic unit. Their spectra recorded in Fig. 4 and 5 have dominant bands cor- responding to this nucleus. This type has no carbonyl or ester group and hence the absence of a strong band a t 1710-35 cm-1 can easily distinguish them from the hydrolysable ones. The spectra of this type have three bands viz. a t 1620, 1520, and 1450-60 cm-1 which are all characterist.ic of the 0-H bending and the aromatic ring as described earlier. The pattern of their absorption is the same as in catechin, i. e. the intensity decreases in the order: 1620,1450,1520. The other bands occur a t 1300-50, 1200 and 1030 cm-1 and correspond to the phenols and polyhydroxy compounds. These bands are broad and like the hydrolysables reveal the polymeric nature.

The mixed type has bands common to the above two classes of tannins. This is borne out from their spectra, recorded in Fig. 6.

Wavelength (p)

Fig. 6. Infrared spectra of tannins, mixed type (recorded in potassium bromide pellets), from top to bottom : “Keekar” (Acacia sp.); “Sundri” (Heriticra minor) ; “Babool” (Acacia arabica).

The above discussion has revealed the efficacy of this study in not only classifying the tannins but also in distinguishing them from the non-tannins.

M. ARSHAD A. BEG and Z. A. SIDDIQUI

In order to make it suitable for quantitative estimations suitable cells have to be designed. We are at present working on the feasibility of exploiting a quartz cell for this study. Quartz is transparent from 2-7 ,u region where the characte- ristic pattern of absorptions corresponding to the tannins occur and the quantitative estimation of tannins by this method will be the subject of a future communication.

1 A. RUSSEL, W. G. TEBBENS and W. F. AREY, J. h e r . chem. SOC. 65 (1943) 1472. 2 S. C. BTJRKET and R. M. BADGER, J. h e r . chem. SOC. 72 (1950) 4397. 3 S. A. BARKER, E. J. BOURNE, M. STACEY and W. H. WHIFFEN, J. chem. SOC.

4 S. A. Barker, E. J. BOURNE, R. STEPHENS and W. H. WHIFFEN, J. chem. SOC.

5 The Chemistry & Technology of Leather, Vol. I1 (American Chemical Society

[London] 1954, 171.

[London] 1954, 3468.

Monograph Series), p. 108.

78