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Infrared spectroscopic investigation of tannins.

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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. ARSHADA. BEGand Z. A. SIDDIQUI
(Eingegangen am 14. August 1968) *
SUMMARY:
Infrared spectroscopy is suggested as a diagnostic method for the characterisation 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 a t 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 gekennzeichnet. Beide Gruppen zeigen charakteristische Absorptionen, die zur Erkennung 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. I n the simple sugar like glucose, for instance, it is
found that it has a well defined spectrum in the 2 - 15p range. The macromolecule 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 t o the amino and carboxyl groups.
*
Revidierte Fassung, eingegangen am 20. Februar 1969.
67
M. ARSHADA. BEGand 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 t o distinguish between the two classes by means of infrared spectroscopy. 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 obtained 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 36003200, 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 t o 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 deformation. The medium intensity band a t 870 cm-1 is due t o 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 correlated. Tannic acid is known to be penta-m-digalloylglucose. Hydrolysable
tannins are the polyesters of gallic acid or m-digallic acid with various polyhydric 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. I n glucoses the ring vibrations occur a t 917 f 13 cm-1 for a-anomers and 920 f 5 cm-1 for the j3anomers. 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 t o 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 t o 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
1595 s
1520 m
1460 s
1405 s
3350-3150 sbr 1725 s 1625 s
1700 s
1710s 1625s
1725s 1625vs
3640 vs
3500-3175s
3450 s
Chebulinic
acid
Ellagic
acid
Myrabollam
nuts
1530m
1450 s
1587s
1515 m
1450 s
1400 s
1540 s
1440 s
1740 s 1625 s
1720 s
3500-3050 sbr
Tmic
acid
1540m
1470 msh
1450 m
1710s 1620s
3390 s
3280 s
3030 s
Gallic
acid
I
I
I
I
I
Table 1. Infrared absorption maxima of hydrolysable tannins.
I
-
1220 w
1200 m
1110 s
1330 s
1225 m
1350-1320sbr
1235-1175sbr
1205 s
1115 s
-
1220 s
1205 s
1170 m
1100 w
stretching
1340 sbr
1265 m
1370-1300 sbr
1250-1150 sbr
1370 m
1335 s
1307 s
1245 s
I
I
1030 s
960 w
1055 s
925 m
1065 s
935 m
1080 s
1030 s
970 mbr
940 mbr
915 mbr
1047 msh
1025 s
960 w
OH
deformation
W
0
M
p-
4
c
1670 s
1610s
1685 s
-
-
-
3390 s
2941 m
3360 s
3280 ssh
3425 vs
Henna
(leaves)
Green Tea
(leaves)
Sakur
1710s 1625s
3510 s
3355 s
Haritaki
(nuts)
1550m
1530 wsh
1515 wsh
1450 s
1405 s
-
1565 msh
1440m
1550 msh
1450 m
1420-1350 mbr
1380-1290sbr
-
1351 w
1314 w
-
1300-1250 mbr
1390-1280 ss
1370-1300 mbr
1245 sbr
1360-1300 mbr
1250 s
1250-1175sbr
1200 sbr
111Om
1258 s
1117 mbr
1250 s
1200 msh
1135 m
1110 ssh
1250-1150 sbr
1100 sbr
1176 sbr
1110-1030mbr
1070 m
1040-1025 mbr
1065 mbr
1030 mbr
1070 s
1030 s
1035 s
1030 m
v = very, s = strong, m = medium, w = weak, sh = shoulder, br = broad, ss = strong sharp, ms = medium sharp, sbr =
strong broad, mbr = medium broad.
1725 s 1670 s
3400-3175 sbr
Bahera
(nuts)
(galls)
1640 sbr
1450 m
1735 s 1620 s
3570-3175 sbr
Chest nut
(wood)
1456m
1440 w
1710s 1615s
3450 s
1695 sbr
1640 s
Pomegranate
(Peel)
-
3500 s
3450 s
(Bark)
Walnut
M. ARSIIADA. BEGand Z. A. SIDDIQUI
Table 2.
Infrared absorption maxima of condensed tannins.
Catechin
3484 vs
Quebracho
Mimosa
Goran
Mangrove
3570-3390
vsbr
-
3450-3400
vs
-
3030 s
2940 s
2900 s
1730 w
3400 vs
3500 vs
3330 vs
-
3450-3125
sbr
-
-
-
1725 w
1755-1710
1618 s
1520 wbr
1473 s
1439 ssh
1393 ssh
1613 vs
1515 s
1450 s
1380 msh
1625 vs
1515 w
1450 s
-
-
1351 wbr
1300 sbr
1615 s
1525 w
1440 w
1390-1360
wbr
1300-1220
sbr
-
2915 ssh
-
2941 m
Remarks
OH stretching
CH stretching
W
1307 ssh
-
1615 vs
1515 s
1440 vs
-
ring
vibration
1360 sbr
-
ester linkage
1284 s
1280 s
1290 vs
1261 ssh
1250-1 170
sbr
1240 vsbr
-
-
-
-
1138-1135
sbr
1110 s
1050 mbr
1120-1040
sbr
1030 vs
-
-
-
-
-
1198-1190
sbr
1138 ss
1114 m
1069 m
1053-1 040
sbr
1029 m
1007 ssh
-
1150 s
1180 vs
1110 s
1045-1025
sbr
-
1110 sbr
1075 vs
980 m
1282-1265
vsbr
1250-1170
vsbr
ring
vibration
O H deformation
-
1010 vs
980 m
-
-
-
875 m
-
-
-
850 s
-
-
OH bending
980 s
870 mbr
O H out-ofplane bending
820-760
mbr
-
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
3400-3125 vs
3390 vs
3205 ssh
2924 ssh
1754 m
1724 wsh
1705 msh
1661 msh
1639 msh
1634 ssh
1613 s
1558 msh
-
-
2950 s
1710 sbr
2800 s
1615 vs
1600 vs
1515 s
1515 m
-
1527 w
1435 sbr
1315 sbr
1450 msh
1440 s
1361 sbr
1210 vsbr
1280-1190 sbr
-
1100 sbr
1105 sbr
1030 sbr
870 mbr
765 mbr
720 mbr
1050 sbr
820 mbr
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 characteristic 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. ARSHADA. BEGand Z. A. SIDDIQUI
C6’
40003000 2000 1500
1ooO900 800
700
8
K
.-B
E
gc
Wavelength (p)
Infrared
spectra
of
tannins,
hydrolysable
type (recorded in potassium broFig. 1.
mide pellets), from top to bottom : “Sakur” (Tanarix indica) ; “Haritaki”
(Terminalia sp.) ; Chebulinic acid; “Chestnut” (Castanea sp.) ; “Bahera”
(Terminalia bellerica).
cm-’
1000900 800
30002000 1500
I
74
I
I
I
I
,
3
4
5
6
7
,
I
,
,
I
700
I
I
I
,
,
8 9 1011121314
Wavelength (p)
Fig. 2 . Infrared spectra
of tannins, hydrolysable
type (recorded in potassium bromide pellets),
from top to bottom: “Myrabollam”
(Tenninalia
chebula) ; “Pomegranate
peel” ; “Wdnut”.
IR-Spectroscopy of Tannins
cm-’
400 3000 2000 1500
I
1000900 800
I
,
1
700
I
t
._
E
5
:
Waveiength (p)
Fig. 3. Infrared spectra of tannins, hydrolysable type (recorded in potassium bromide pellets), from top t o 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 chebulinic 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. BEGand 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
sc
.B
f
C
$
1000900 800
50003000 2000 1500
700
Fig. 4. Infrared spectrum of tannin, condensed type (recorded in potassium bromide pellet) : Catechin.
-1
cm
40C 313000 2000 1500.
I
3
Fig. 5.
76
4
5
#
6
I
7
1000900 800
I
I
I
0
9 10 11
Wavelength (p)
8
,
I
700
1
12 13 14 15
Infrared spectra of tannins, condensed type (recorded in potassium bromide pellets), from top to bottom: “Goran” (Rhizophorasp.); “Mimosa”
(Acadia mollisima); “Quebracho” (Quercus lerentii); ‘‘Mangrove”.
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 corresponding 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. ARSHADA. BEGand Z. A. SIDDIQUI
I n 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 characteristic 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
2
3
4
5
A. RUSSEL,W. G. TEBBENSand W. F. AREY,J. h e r . chem. SOC.65 (1943) 1472.
S. C. BTJRKETand R. M. BADGER,
J. h e r . chem. SOC.72 (1950) 4397.
S. A. BARKER,E. J. BOURNE,M. STACEYand W. H. WHIFFEN,J. chem. SOC.
[London] 1954, 171.
S. A. Barker, E. J. BOURNE,R. STEPHENS
and W. H. WHIFFEN,J. chem. SOC.
[London] 1954, 3468.
The Chemistry & Technology of Leather, Vol. I1 (American Chemical Society
Monograph Series),p. 108.
78
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