Use of fourier transform infrared spectroscopy to follow the heterocumulene aided thermal dehydration of phthalic and naphthalic acids

Article abstract of DOI:10.1366/000370206779321346

Fourier transform infrared (FT-IR) spectroscopy has been successfully employed to follow the formation of phthalic anhydride and 1,8-naphthallc anhydride on heating their corresponding acids. The effects of three heterocumulenes, cyanamide, dicyandlamide, and sodium cyanate, on the temperature of formation of the anhydrides were also investigated using this method. It was found that the carbodlimides cyanamide and dicyandiamide dramatically lowered the temperature at which thermal dehydration of the acid led to anhydride formation. It was noted that cyanamide had a stronger catalytic effect than dicyandiamide, presumably due to the electron-withdrawing effect of the amidine group. Sodium cyanate was also found to promote the thermal dehydration of the acids to form the corresponding anhydrides. Under more severe conditions, phthalic acid anhydride formed is seen to react further, leading to the formation of phthalimide. The discrepancy between the products obtained with cyanamide and sodium cyanate leads to the conclusion that, unlike earlier claims, imide formation is not due to the reaction of the anhydride with the urea formed but with sodium cyanate itself. However, only the phthalic anhydride flve-membered ring system is sufficiently reactive towards the CNO^- nucleophile to form the imide; the six-membered 1,8-naphthalic anhydride system does not react in this way.

Full text of DOI:10.1366/000370206779321346

Use of Fourier Transform Infrared Spectroscopy to Follow the
Heterocumulene Aided Thermal Dehydration of Phthalic and
Naphthalic Acids
MURIEL L. A. RIGOUT and DAVID M. LEWIS*
Department of Colour and Polymer Chemistry, The University of Leeds, Leeds LS2 9JT
Fourier transform infrared (FT-IR) spectroscopy has been successfully
employed to follow the formation of phthalic anhydride and 1,8-
cyclic and acyclic anhydrides, appropriate reagents are often
added. Many have been described in the literature;2–12 most
add to the carboxylic acid group, forming an activated
naphthalic anhydride on heating their corresponding acids. The effects
of three heterocumulenes, cyanamide, dicyandiamide, and sodium
derivative of the starting acid. The activated acid derivative
cyanate, on the temperature of formation of the anhydrides were also
is then susceptible to attack by the weakly nucleophilic oxygen
investigated using this method. It was found that the carbodiimides
of a free acid molecule, resulting in the formation of the
anhydride. A particular group of potential dehydrating agents
cyanamide and dicyandiamide dramatically lowered the temperature at
which thermal dehydration of the acid led to anhydride formation. It was
are the heterocumulenes. In this paper three heterocumulene
dehydrating agents, cyanamide, dicyandiamide, and sodium
cyanate, have been studied. To compare the effectiveness of the
above agents, anhydride formation upon thermal dehydration
of the di-acids phthalic and 1,8-naphthalic acids was studied in
the absence and in the presence of these agents. Fourier
transform infrared (FT-IR) spectroscopy has been found by
previous workers13–24 to be useful for monitoring anhydride
formation and was thus used in this case.
noted that cyanamide had a stronger catalytic effect than dicyandiamide,
presumably due to the electron-withdrawing effect of the amidine group.
Sodium cyanate was also found to promote the thermal dehydration of the
acids to form the corresponding anhydrides. Under more severe
conditions, phthalic acid anhydride formed is seen to react further,
leading to the formation of phthalimide. The discrepancy between the
products obtained with cyanamide and sodium cyanate leads to the
conclusion that, unlike earlier claims, imide formation is not due to the
reaction of the anhydride with the urea formed but with sodium cyanate
itself. However, only the phthalic anhydride five-membered ring system is
sufficiently reactive towards the CNOꢀ nucleophile to form the imide; the
six-membered 1,8-naphthalic anhydride system does not react in this way.
EXPERIMENTAL
Index Headings: Fourier transform infrared spectroscopy; FT-IR spec-
troscopy; Dehydration of carboxylic acids; Carboxylic acid; Carboxylic
acid anhydride; Thermal dehydration; Carbodiimide aided dehydration.
Materials. Phthalic acid (1,2-benzenedicarboxylic acid,
98%), cyanamide (99%), sodium cyanate (96%), and HCl
(37%) were purchased from Aldrich Chemicals. Phthalic
anhydride (1,2-benzenedicarboxylic anhydride, 99%), 1,8-
naphthalic anhydride (1,8-naphthalenedicarboxylic anhydride),
and dicyandiamide (99%) were obtained from Avocado. NaOH
(.97%) was provided by Fisher Scientific. 1,8-Naphthalic acid
(1,8-naphthalenedicarboxylic acid) was obtained from the
hydrolysis of the corresponding anhydride. 1,8-Naphthalic
anhydride was suspended in 0.03 M NaOH solution. The
temperature was then maintained at 50 8C for 5 hours, after
which time 1,8-naphthalic anhydride had hydrolyzed to the
water-soluble sodium di-carboxylate salt. The solution was
subsequently filtered in order to remove any un-hydrolyzed
anhydride. 1,8-Naphthalic acid was precipitated by adding
dilute HCl, filtered under vacuum, then washed in dilute HCl.
The purity of the sample obtained was monitored by FT-IR
spectroscopy.
Thermal Dehydration. One gram (1.00 g) of carboxylic
acid was ground in a pestle and mortar with an appropriate
amount of dehydrating agent (1:1 molar ratio). Typically, 0.025
g of the mix was placed in aluminum foil and sealed. The
sample was subsequently heated in a heat press (transfer press
model BMSI.20, A. Adkins and Sons Ltd, Leicester, UK), the
accuracy of the temperature being controlled via the use of a
thermocouple (Electronic Thermometer type 1602, Comark
Electronics Ltd, Sussex, UK).
INTRODUCTION
Carboxylic acid anhydrides are highly valuable as acylating
agents, since they can undergo a wide range of nucleophilic
addition reactions under relatively mild conditions. Their
formation reactions are thus of high interest, and although
many routes for their synthesis are known, new catalytic
pathways can prove invaluable for specific applications. It is
widely known that carboxylic acids can be thermally
dehydrated to yield their corresponding anhydrides. This is
particularly true for di-acids that form five- and six-membered
ring anhydrides, which are thermodynamically favored due to
the low ring strain. In addition, the elimination of water
molecules provides a strong driving force for the reaction.
Thus, cyclic anhydrides are relatively easily formed by high-
temperature dehydration of the corresponding di-acid, while
carboxylic acids not capable of forming cyclic anhydrides
seldom undergo purely thermal dehydration.1 During the
course of a multi-step synthesis process, it was required that
a cyclic anhydride should be formed from the corresponding
di-acid. Since the temperature to achieve the above had to be
relatively low (below 190 8C), due to the thermosensitive
nature of other compounds present, a number of investigations
were carried out. In order to facilitate the formation of both
Fourier Transform Infrared Analysis. Fourier transform
infrared analysis was performed on a PerkinElmer Spectrum
One Spectrometer (PerkinElmer Instruments, UK) using the
PerkinElmer Diamond Golden Gate sampling attachment,
which obtains attenuated total reflectance (ATR) spectra.
Received 10 December 2005; accepted 2 October 2006.
* Author to whom correspondence should be sent. E-mail: ccddml@leeds.
ac.uk.
0003-7028/06/6012-1405$2.00/0
Ó 2006 Society for Applied Spectroscopy
Volume 60, Number 12, 2006
APPLIED SPECTROSCOPY
1405

FIG. 1. Anhydride formation upon thermal dehydration of phthalic acid.
Spectra were recorded at 4 cmꢀ1 resolution using 100 scans per
spectrum, at a scan speed of 0.50 cm sꢀ1. Data was collected
and processed with Spectrum 5.0.1 software (PerkinElmer
Instruments, UK). For some of the peaks of weakest intensity,
and for those appearing as shoulders on other peaks, the
second-derivative plot of the spectrum was used to accurately
determine the wavelength of absorption.
FIG. 2. Infrared spectra of (A) phthalic acid and (B) anhydride. The carbonyl
symmetrical and anti-symmetrical stretches are also labeled.
literature.26–29 As for phthalic acid, anhydride formation can be
assessed by monitoring the anhydride symmetrical and anti-
symmetrical carbonyl stretches at 1769 and 1734 cmꢀ1 and the
acid carbonyl stretch at 1680 cmꢀ1, free from interference from
Differential Scanning Calorimetry. Differential scanning
calorimetry (DSC) was performed using a DSC 2010 (TA
Instruments Ltd) under a nitrogen atmosphere, from room
temperature. Typically, 5 to 10 mg of sample was heated at a
rate of 10 8C minꢀ1 with a purge gas flow of 200 cm3 minꢀ1
.
TABLE I. Vibrational assignments of phthalic acid and anhydride.a
Data was recorded using Thermal Advantage software (TA
Instruments Ltd).
Phthalic acid Phthalic anhydride
Assignment
3064
2961
2862 s, br
2645
2519
1978 vw
1940 vw
1867 vw
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
w
w
3094
3053
ꢁꢁꢁ
w
w
m(PhH)
m(PhH)
m(OH)
RESULTS AND DISCUSSION
Anhydride Formation in the Absence of a Dehydrating
Agent. The thermal dehydration of phthalic acid to form
phthalic anhydride in the absence of any dehydrating agent, as
illustrated in Fig. 1, was first considered.
w
w
ꢁꢁꢁ
overtone and combination m(C–OH) and
d(COH)
ꢁꢁꢁ
1985
1931
1902
1849
1807
1790
vw
vw
vw
s
vw
m
overtone and combination x(PhH)
(summation bands)
The FT-IR spectra of phthalic acid and phthalic anhydride
are presented in Fig. 2. The vibrational assignments of the
bands observed are summarized in Table I. The anti-
symmetrical carbonyl stretch band for the acid, at 1662
cmꢀ1, is of strong intensity and, as seen in Fig. 2, is in a region
where the anhydride hardly absorbs. The monitoring of the
decrease in intensity of this particular band can thus give clear
information regarding the amount of phthalic acid present,
while being relatively free of interference from the absorption
bands of the anhydride that might eventually form. Similarly,
the absorbance values of the symmetrical and anti-symmetrical
carbonyl stretches of phthalic anhydride, at 1849 and 1755
cmꢀ1, respectively, can be successfully used in order to assess
the amount of phthalic anhydride present, regardless of the
presence of phthalic acid.
Some of the spectra recorded, from the heating of phthalic
acid alone for 30 seconds at various temperatures, are shown in
Fig. 3. Anhydride formation, as seen by the appearance of
peaks at 1849 and 1755 cmꢀ1, is detected at and above 190 8C.
The percentage of phthalic anhydride present at each
temperature, listed in Table II, was calculated from the
absorbance of the anti-symmetrical carbonyl stretch bands of
phthalic acid and anhydride. For this purpose, the spectra of an
equimolar sample of acid and anhydride were first recorded,
and all subsequent anhydride percentages were calculated from
the ratio of absorbance of the carbonyl anti-symmetrical stretch
bands thus obtained (thereby accounting for the difference in
intensity of the bands of acid and anhydride).
ms(C¼O)
overtone m(C–O)
Fermi resonance splitting of mas(C¼O)a
overtone and combination x(PhH)
1782 vw
ꢁꢁꢁ
ꢁꢁꢁ
1755
ꢁꢁꢁ
vs
mas(C¼O)
mas(C¼O)
1662
ꢁꢁꢁ
s
1650
1599
1572
1518
1471
1423
ꢁꢁꢁ
vw
m
w
w
m
w
1585
m
d(CCC) (quadrant stretch)
d(CCC) (semicircle stretch)
1538 vw
1496
1455
ꢁꢁꢁ
w
w
1399
ꢁꢁꢁ
m
d(C–O–H)
1384
1360
1336
1288
ꢁꢁꢁ
w
m
m
w
ꢁꢁꢁ
Fermi resonance splitting of d(CCC)b
ꢁꢁꢁ
ꢁꢁꢁ
1262
ꢁꢁꢁ
m
m(C–OH)
m(C–O)
d(CCC)
d(PhH)
d(PhH)
d(PhH)
x(PhH)
x(PhH)
m(C–O)
1257
1213
1170
1110
1072
1007
ꢁꢁꢁ
vs
w
m
s
w
w
1211
1154
1140
1070
1005
974
ꢁꢁꢁ
m
w
w
m
w
w
904
s
894 m, br
ꢁꢁꢁ
xoop(OH. . .O)
ꢁꢁꢁ
888
839
vw
m
m
x(PhH)
x(PhH)
x(C¼O)
830
795
w
w
800
a All frequencies are given in cmꢀ1. Relative intensities: s, strong; m, medium;
w, weak; vw, very weak; vs, very strong; and br, broad. Vibrational modes: m,
stretching; d, deformation (all kinds of); and x, wagging; subscripts: s,
symmetrical; as, anti-symmetrical; and oop, out-of-plane.
The FT-IR spectra of 1,8-naphthalic acid and anhydride are
shown in Fig. 4. Vibrational assignment of the bands observed,
summarized in Table III, have been extensively reported in the
b
Assignment from Ref. 25.
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Volume 60, Number 12, 2006

TABLE II. Percentage of anhydride formed through thermal dehydra-
tion of the corresponding acid.
Temperature
No Cyanamide as Dicyandiamide Sodium cyanate
dehydrating dehydrating as dehydrating as dehydrating
agent
agent
agent
agent
Phthalic acid/anhydride
90 8C
100 8C
110 8C
120 8C
130 8C
140 8C
150 8C
160 8C
170 8C
180 8C
190 8C
200 8C
ꢁꢁꢁ
ꢁꢁꢁ
0%
0% (8%a)
18%
36%
56%
85%b
ꢁꢁꢁ
0%
0%
0%
0%
98%
94%b
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
0%
22%
22%
24%
20%
0%
0%
0%
ꢁꢁꢁ
ꢁꢁꢁ
FIG. 3. Selected infrared spectra of phthalic acid heated for 30 s at (A) 180 8C,
(B) 190 8C, and (C) 200 8C. All conditions as described in the Experimental
section. The carbonyl symmetrical and anti-symmetrical stretches are also
labeled.
ꢁꢁꢁ
ꢁꢁꢁ
0%b
0%b
0%b
0%b
11%b
38%b
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
other bond vibrations. Significant amounts of 1,8-naphthalic
anhydride are thus first observed to form upon dehydration of
the acid after 30 seconds at 180 8C. Again, the percentages of
anhydride present, listed in Table II, were calculated from the
relative intensities of the acid and anhydride carbonyl stretch
bands.
ꢁꢁꢁ
ꢁꢁꢁ
1,8-Naphthalic acid/anhydride
ꢁꢁꢁ
90 8C
100 8C
110 8C
120 8C
130 8C
140 8C
150 8C
160 8C
170 8C
180 8C
ꢁꢁꢁ
ꢁꢁꢁ
0%
0%
0%
0%
0%
0%
0%
9%
16%
31%
35%
37%
44%
53%
89%
ꢁꢁꢁ
25%
42%
86%
100%
100%
ꢁꢁꢁ
ꢁꢁꢁ
17%
33%
100%
100%
ꢁꢁꢁ
Differential scanning calorimetry was performed on 1,8-
naphthalic acid and phthalic acid and their corresponding
anhydrides. The thermograms recorded are shown in Fig. 5.
Also shown in Fig. 5 is the thermogram of the phthalic acid
heated to 200 8C and then reheated. The thermogram of 1,8-
naphthalic acid shows two endothermic processes, the first one
at 185 8C and the second at 275 8C. The enthalpy change that
takes place at 275 8C can be attributed to the melting of the
anhydride, as confirmed by the thermogram of 1,8-naphthalic
anhydride produced independently. It is thus presumed that the
endothermic process at 185 8C corresponds to the thermal
dehydration of 1,8-naphthalic acid to form the corresponding
anhydride. These results seem to be in good accordance with
the temperatures at which anhydride formation was previously
observed.
In the case of phthalic acid, interpretation of the thermogram
is not as straight forward. While the anhydride is seen to melt at
135 8C and boil at 247 8C, the thermogram of phthalic acid
shows an endothermic process taking place at 195 8C only.
However, heating the acid to 200 8C and repeating the DSC
process shows two important points: (1) weighing the sample
revealed that considerable mass loss occurred during the first
DSC process; (2) no enthalpy changes are observed. It may
0%
0%
13%
17%
27%
81%
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
a
In the presence of hydrochloric acid.
b
Percentages must be taken as approximations due to the volatile nature of
phthalic anhydride at temperatures above 130 8C, which is likely to result in
some anhydride being lost previous to analysis.
therefore be deduced that the endothermic process observed in
the thermogram of phthalic acid corresponds to the thermal
dehydration of the acid, leading to anhydride formation. The
temperature is indeed in good accord with the temperature at
which anhydride formation was first observed in the heating of
phthalic acid in the heat press, as described above. Since this
temperature is above the anhydride melting point, it is
presumed that the small amount of anhydride thus formed is
subsequently lost through vaporization, leading to the results
observed in the thermogram produced in the second heating
step.
Cyanamide as a Dehydrating Agent. Carbodiimides have
been found previously to have a catalytic effect on the thermal
dehydration of carboxylic acids.1,2,30–32 Cyanamide in partic-
ular has been successfully employed in the past.13,33 Its relative
stability compared to other carbodiimides32 makes cyanamide
the dehydrating agent of choice. However, confusion still
remains as to its reactivity, and the mechanism of its
dehydrating effect with carboxylic acid to form anhydrides
has not been fully established.34
Cyanamide is known to exist, as illustrated in Fig. 6, in
equilibrium with its carbodiimide tautomer. The equilibrium
constant for this particular tautomeric equilibrium has been
calculated and found to be 1019 at 300 K in the gas phase,32
which accounts for the relative stability of cyanamide
compared to other carbodiimides. However, it is thought that
a very small amount of the carbodiimide tautomer is sufficient
to catalyze reactions.
FIG. 4. Infrared spectra of (A) 1,8-naphthalic acid and (B) 1,8-naphthalic
anhydride. The carbonyl symmetrical and anti-symmetrical stretches are also
labeled.
Carbodiimides have two centers of reactivity: the electro-
philic central carbon atom and the nucleophilic terminal
APPLIED SPECTROSCOPY
1407

TABLE III. Vibrational assignments of 1,8-naphthalic acid and anhy-
dride.a
under the short heating times employed; this thus indicates its
suitability as a potential dehydrating agent. It must, however,
be noted that the cyanamide sample used in this study is known
to contain an acidic stabilizing agent. It is, however, expected
that such an agent will not interfere with the dehydrating
properties of cyanamide, due, in part, to its very low
concentration.
Assignments of the FT-IR vibrational bands of cyanamide
are summarized in Table IV. Since cyanamide residues show
no vibrations in the region 1850 to 1650 cmꢀ1, the clear
identification of the carboxylic acid and eventual anhydride
carbonyl stretch bands is possible.
When heating a sample of phthalic acid in the presence of
cyanamide for 30 seconds at various temperatures, formation of
phthalic anhydride, as determined by the presence of the
carbonyl symmetrical and anti-symmetrical stretch bands at
1849 and 1755 cmꢀ1, respectively, is evident at 110 8C and
above (190 8C in the absence of dehydrating agent), thus
confirming the catalytic dehydrating action of cyanamide
leading to anhydride formation.
The reaction of carboxylic acids with carbodiimides,
illustrated in Fig. 6, is believed to initially involve the
protonation of the carbodiimide, followed by the subsequent
attack of the acid anion to form the O-acylisourea derivative
(Fig. 6, I). The latter may either rearrange to form the
corresponding N-acylurea (Fig. 6, II), or may be further
protonated, leading to a subsequent attack by a second
carboxylate ion, resulting in the simultaneous formation of
both the corresponding N0-substituted urea (Fig. 6, IV) and
acid anhydride (Fig. 6, V). Which product is formed is known
to depend on the nature of the reagents and the conditions
employed.2,30,31 The formation of N-acylurea (II) from the O-
acylisourea (I) is thought to take place via a cyclic electronic
displacement, leading to the migration of the acyl group from
the oxygen to the nitrogen.2,31 However, in the cases where the
attachment of the second proton to form the intermediate form
III overtakes the oxygen to nitrogen migration, the formation of
the anhydride results.30 Thus, the ease of cyclization and the
nucleophilicity of the acid anion are predominant factors in
dictating the nature of the final product.30,31,51,53 The reaction
of phthalic acid, for which the carboxyl groups are so placed as
to facilitate intramolecular anhydride formation, has been
reported to result, with a variety of carbodiimides, in anhydride
formation solely.2,30 Results presented here indeed confirm the
formation of the anhydride, while no trace of N-acylurea (Fig.
6, II) can be detected.
It was found that addition of a small amount of hydrochloric
acid to the phthalic acid–cyanamide mix resulted in the
lowering of the temperature at which anhydride formation was
first observed, as can be seen from the band intensities
tabulated in Table II. This effect was not observed upon
addition of a small amount of deionized water to the mix, thus
refuting the possibility of increased reactivity, as indicated by
the lower anhydride formation temperature, being linked to the
presence of a greater amount of moisture in the mix. The
increased acidity is expected to have an influence on steps
where a proton is involved, i.e., in the steps leading to the
formation of compounds I and III (Fig. 6), via the increase in
concentration of the reactive protonated carbodiimide deriva-
tives. Results presented here thus confirm that the protonated
cyanamide species is indeed involved in the reaction
mechanism.
1,8-Naphtalic
anhydride
1,8-Naphthalic acid
Assignment
ꢁꢁꢁ
3098
W
W
W
m(PhH)b,c
m(PhH)b,c,d,e
m(PhH)b,c,e
m(OH)d,e
3054
2964
2873
2695
w
w
s, br
w
3075
3038
ꢁꢁꢁ
ꢁꢁꢁ
overtone and combination m(C–OH)
and d(COH)d
2640
2546
2487
1954
w
w
vw
vw
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
1983
W
overtone and combination x(PhH)
(summation bands)
1907
1858
1778
ꢁꢁꢁ
vw
vw
vw
1938
1897
1799
1769
W
W
W
S
ms(C¼O)d,e
mas(C¼O)d,e
mas(C¼O)d,e
m(CC)d,e
ꢁꢁꢁ
1734 vs
1680
1612
1595
1576
1509
ꢁꢁꢁ
vs
m
w
s
ꢁꢁꢁ
1629
ꢁꢁꢁ
W
1579
1511
M
M
m(CC)b,e
m(CC)b,e
m
1477 vw
1460 vw
1438
1406
1385
1457
1433
1417
1370
1357
1358
1281
1268
ꢁꢁꢁ
vw
m
m
w
w
w
m(CC)b
M
M
W
d(CCC)e
x(PhH)e
dip(PhH)b
m(CC)b
1368 vw
1355
1301
1265
1231
1214
1191
1173
ꢁꢁꢁ
W
S
m(CC)b,e
x(PhH)e
dip(PhH)b
s
vw
W
M
M
M
M
1213
1200
ꢁꢁꢁ
vw
m
dip(PhH)b,e
x(PhH)e
m(C–O)e
1156
ꢁꢁꢁ
m
1120
ꢁꢁꢁ
S
m(C–O)e
1092
1077
1020
vw
w
w
1076
1006
M
S
d(CCC)e
d(CCC)b
a All frequencies are given in cmꢀ1. Relative intensities: s, strong; m, medium;
w, weak; vw, very weak; vs, very strong; and br, broad. Vibrational modes: m,
stretching; d, deformation (all kinds of); and x, wagging; subscripts: s,
symmetrical; as, anti-symmetrical; and ip, in-plane.
b
Assignment from Ref. 28.
Assignment from Ref. 29.
Assignment from Ref. 26.
Assignment from Ref. 27.
c
d
e
nitrogen. It has been proposed that the driving force for most of
the reactions involving the use of carbodiimides is the very
powerful saturating ability of the C¼N bond, joined to, in the
case of dehydration, the very stable product.32 In this instance,
the suggested catalytic pathway involving cyanamide, as
illustrated in Fig. 6, results in the formation of a urea.
The stability of aqueous solutions of cyanamide has aroused
the interest of many workers.14,35–46 However, its stability in
crystalline form has been less extensively studied.47–50 Golov
et al.47 reported the stabilizing effect of some acids and
solvents when heating cyanamide. They reported times of
polymerization, leading to both melamine and dicyandiamide
formation, from 5 hours at 80 8C to just 12 minutes at 110 8C;
Baughen,48 however, indicates that cyanamide polymerizes
when heated to 150 8C. Differential scanning calorimetry
analysis performed in this study, and shown in Fig. 7, confirms
cyanamide’s stability over the temperature range required and
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Volume 60, Number 12, 2006

FIG. 5. Thermograms of (A) 1,8-naphthalic acid, (B) 1,8-naphthalic anhydride, (C) phthalic anhydride, (D) phthalic acid, and (E) phthalic acid reheat. All
conditions as described in the Experimental section.
The effects of cyanamide on the thermal dehydration of 1,8-
naphthalic di-carboxylic acid were also investigated. As for the
case of phthalic acid, it was found that cyanamide did act as a
dehydrating agent and lowered the temperature of anhydride
formation; in this case, 1,8-naphthalic anhydride was detected
to form significantly from 30 seconds at 130 8C (compared to
180 8C in the absence of cyanamide).
Dicyandiamide as a Dehydrating Agent. As for cyana-
mide, dicyandiamide is known to exist in two tautomeric
forms, in equilibrium with each other.30 The suggested
FIG. 6. Carbodiimide aided thermal dehydration of a di-carboxylic acid.
APPLIED SPECTROSCOPY
1409

FIG. 7. Thermograms of (A) cyanamide, and (B) dicyandiamide. All conditions as described in the Experimental section.
dehydrating action of dicyandiamide, as illustrated in Fig. 6, is
thus similar to that of cyanamide.
Differential scanning calorimetry analysis of dicyandiamide,
shown in Fig. 8, confirms its stability over the temperature
range required and thus refutes polymerization, which might
have hindered its dehydrating action.
Vibrational assignments of the FT-IR bands are summarized
in Table IV. Since dicyandiamide does not give rise to
significant absorbance in the 1850 to 1750 cmꢀ1 region, any
acid anhydride bands can be clearly identified.
Upon heating a sample of phthalic acid in the presence of
dicyandiamide for 30 seconds at various temperatures,
formation of phthalic anhydride from dehydration of phthalic
acid, as determined by the presence of the carbonyl
symmetrical and anti-symmetrical stretch bands at 1849 and
1755 cmꢀ1, respectively, is evident at and above 130 8C.
The temperature at which, in the presence of dicyandiamide,
anhydride first forms (130 8C) is higher than that in the
presence of cyanamide (110 8C). Presumably, this effect can be
attributed to the difference in reactivity of the corresponding
carbodiimides of both additives. Indeed, the presence of an
electron-withdrawing or -donating substituent on the carbodii-
mide results in a lesser or greater concentration of the active
protonated forms.31,54 Furthermore, the efficiency of the
respective carbodiimides at aiding anhydride formation is
dependent on the concentration of the protonated species.
Therefore, the reactivity of the carbodiimides is linked to the
electron density on the nitrogen atoms. It is assumed that, in the
case of dicyandiamide, the dipole of the amidine group leads,
due to its latent polarity, to its central carbon atom bearing a
slight positive charge. This, in turn, leads, through inductive
effects, to the amidine group being electron withdrawing.
Hence, the electron density on the amidine substituted nitrogen
of dicyandiamide is lower than that found on either nitrogens in
cyanamide. As a result, it is presumed that dicyandiamide’s
amidine substituted nitrogen is less prone to protonate. It is
interesting to note that cyanamide and dicyandiamide have
similar pKa values (ꢀ0.6 for cyanamide55 and ꢀ0.3 for
56
dicyandiamide ). It is presumed that, due to the high
equilibrium constant for tautomerization, the pKa values are
those of the cyano tautomer rather than the carbodiimide
tautomer. However, it suggests that the presence of the amidine
substituent does not greatly influence the electron density on
the cyano nitrogen. Thus, in their carbodiimide form, the pKa
values of the primary imino group of cyanamide and
dicyandiamide could also be expected to be similar. It would
therefore be expected that both compounds should exhibit
similar reactivity. Furthermore, in the case of dicyandiamide,
the unsubstituted nitrogen would be expected to be less
sterically hindered and more basic than that bearing the
amidine group. It would thus be expected that this particular
nitrogen would be involved in the formation of the reactive
protonated carbodiimide derivative. Therefore, it would be
TABLE IV. Vibrational assignments of cyanamide and dicyandiamide.a
Cyanamide
Dicyandiamide
Assignment
m(NH)
mas(NH2)
overtone d(NH2)
ma(NH2)
ꢁꢁꢁ
3368
w
m
w
s
3318
ꢁꢁꢁ
S
3327
3238
3138
ꢁꢁꢁ
3238
3104
2723
2215
ꢁꢁꢁ
S
M
M
S
overtone d(NH2)
ꢁꢁꢁ
2202
2161
1862
1627
1583
1564
1501
1252
1140
1070
1005
928
s
s
m(C[N)
ꢁꢁꢁ
w
s
m
m
s
m
w
m
w
s
overtone ms(N–C–N)
d(NH2)
1568
ꢁꢁꢁ
S
m(C¼N)
mas(N–C–N)
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
m(C–NH2)
1136
ꢁꢁꢁ
W
mas(C–N–C)
m(C–NH2)
ms(N–C–N)
x(NH2)
ꢁꢁꢁ
ꢁꢁꢁ
864
ꢁꢁꢁ
Vw
ꢁꢁꢁ
795
735
m
m
m
m
w
m(C–N–CN)
ꢁꢁꢁ
x(NH2) and x(NH)
dip(N–C[N)
ꢁꢁꢁ
720
668
548
667
563
Vw
W
doop(N–C[N)
a
All frequencies are given in cmꢀ1. Relative intensitis: s, strong; m, medium;
w, weak; and vw, very weak. Vibrational modes: m, stretching; d, deformation
(all kinds of); and x, wagging; subscripts: s, symmetrical; as, anti-
symmetrical; oop, out-of-plane; and ip, in-plane.
FIG. 8. Infrared spectra of (A) cyanamide and (B) dicyandiamide in the 3500
to 1400 cmꢀ1 region.
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FIG. 10. Tautomerism of the cyanate ion; (I) cyanate form; (II) isocyanate
form.
amidine group of dicyandiamide, cyanamide more readily
forms the reactive protonated species; therefore, cyanamide is
subsequently found to be more reactive, and thus promotes
anhydride formation at lower temperatures.
The same effect was observed when heating 1,8-naphthalic
acid in the presence of dicyandiamide; in this particular case,
significant anhydride formation was detected from 140 8C,
compared to 130 8C in the presence of cyanamide and 180 8C
in the absence of dehydrating agent.
FIG. 9. Representation of the nucleophilic attack of a carboxylate anion on the
carbodiimide central carbon atom; the pi-orbitals are shown in order to illustrate
charge delocalization and thus the stabilizing effect through pi-orbital
conjugation in the carbodiimide molecule.
Sodium Cyanate as a Dehydrating Agent. The reaction of
carboxylic acids with isocyanates to form acid anhydrides, and
the dehydrating properties of the aforementioned isocyanates
leading to the formation of anhydrides otherwise difficult to
prepare, was reported early on in the literature.60–62 Isocyanates
have also been proposed as heterocumulenes, which are more
reactive than their carbodiimides counterparts.32 They thus
have the potential of being dehydrating agents of choice when
considering the thermal formation of anhydrides from
carboxylic acids.
Cyanates are known to isomerize readily to their isocyanate
form, as illustrated in Fig. 10. It is presumed that the isocyanate
tautomer, rather than the cyanate tautomer, is involved in
reported cyanate reactions, due to its high reactivity. It must
also be noted that, due to the instability of cyanic acid (and its
tautomer isocyanic acid), the sodium salt was used in this
study.
The isocyanate-aided dehydration of carboxylic acids to
yield anhydrides, as illustrated in Fig. 11, is known to proceed
through the addition of the isocyanate to the acid, forming a
mixed anhydride (Fig. 11, I) of the carbamic and carboxylic
acid. This derivative is then thought to disproportionate into
two pure anhydrides, with the carbamic anhydride eliminating
carbon dioxide to yield urea (Fig. 11, IV). Alternatively, carbon
dioxide can be eliminated from the mixed anhydride, forming
the corresponding amide (Fig. 11, II).61,62
expected that both cyanamide and dicyandiamide should
exhibit similar reactivity.
Mock et al.,54 who studied the substituent effect in the
addition of carboxylic acids to arylcarbodiimides, also
observed that the carbodiimide reactivity was influenced by
substituent effects. They found that mono-aryl-substituted
carbodiimides bearing either indifferent or electron-donating
substituents are more reactive than those possessing inductive-
ly electron-withdrawing substituents. They explained these
findings in terms of stereoelectronic effects and orbital
orientation. They suggested that, for strictly kinetic reasons,
and so that the O-acylisourea may experience continuous pi
bonding as the central carbon atom changes configuration from
sp to sp2 hybrid, approach of the nucleophile towards the
protonated carbodiimide must be focused on the central carbon
atom’s p-orbital, which suffers extended conjugated overlap
with the nitrogen substituents, as represented in Fig. 9. Others
have also found that the substituents on the carbodiimides have
important repercussions on the electron density localized on the
carbodiimide moiety. Glaser et al.57,58 found this was apparent
when studying the dipole and quadrupole moments of
carbodiimides. They found that for cyanamide itself (in the
carbodiimide tautomeric form), the dipole and quadrupole
moments were related to the N–H bond polarity. Thamassebi,60
in his study of the substituent effects on the stability of
carbodiimides, found that the substituents were able to stabilize
or destabilize the carbodiimides through an electronic effect.
Effectively, he found that substituents that have unsaturated
bonds conjugated with the carbodiimide moiety can serve as pi
acceptors and thus delocalize the carbodiimide pi-electron
density. Others also confirmed through their findings delocal-
ization of the electron density over the whole molecule.35,42
The difference in reactivity between cyanamide and dicyan-
diamide observed here is presumably due to one of the
following: (1) the amidine substituted nitrogen is the nitrogen
atom from which, following protonation, the reactive species
forms; in accordance with Mock et al.,54 the amidine
substituent on dicyandiamide can be alleged to be of stabilizing
effect on the O-acylisourea formation, and this also implies that
protonation must take place on the sterically hindered amidine
substituted nitrogen; (2) the electron density on the unsub-
stituted nitrogen is affected by the inductive effect of the
amidine substituents and thus this substitution has a deactivat-
ing effect. In either case, the ability of the amidine group to
disperse charge through resonance forms leads to its having an
electron-withdrawing effect, to which the observed difference
in reactivity between cyanamide and dicyandiamide can be
attributed. Due to the electron-withdrawing nature of the
Differential scanning calorimetry performed on sodium
cyanate confirms its stability over the temperature range
required. Furthermore, FT-IR analysis shows that no vibrations
of sodium cyanate give rise to significant absorbance in the
1900 to 1600 cmꢀ1 range, thus allowing for the carbonyl
symmetrical and anti-symmetrical stretches of the anhydride
(1849 and 1755 cmꢀ1, respectively) and the acid (1662 cmꢀ1
)
to be clearly identified in the mix. Extensive assignments of
cyanate vibrations have been published in the literature63–71
and are thus not reiterated here.
Upon heating phthalic acid in the presence of sodium
cyanate, anhydride formation is detected after 30 seconds at or
above 120 8C (refer to Table II). However, the intensity values
of the bands at 1849 and 1755 cmꢀ1 associated with the
presence of phthalic anhydride are relatively weak, suggesting
that only a small amount of anhydride is formed. Heating to
130, 140, and 150 8C (30 seconds) does not result in an
increase in the intensity of those particular bands, thus
suggesting that no further anhydride was formed. At higher
temperatures (160 8C and above), the anhydride carbonyl
stretch bands are seen to disappear altogether. This disappear-
ance is associated with the appearance of bands notably at 1773
APPLIED SPECTROSCOPY
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FIG. 11. Isocyanate-aided dehydration of a di-carboxylic acid.
and 1717 cmꢀ1. It has been reported in the literature that, at
adds onto one of the anhydride’s carboxyl carbon atoms, while,
most probably in a concerted manner, the anhydride oxygen
adds onto the electron-deficient cyanate central carbon atom.
Oxygen to nitrogen migration then follows, associated with
carbon dioxide elimination, resulting in the formation of
phthalimide (Fig. 12).
higher temperatures, the anhydride previously formed may
react with the resulting urea, yielding the amide.61 Further-
more, the reaction of phthalic anhydride with urea, leading to
the formation of phthalimide, is well documented in the
literature.72–91 The bands observed at 1773 and 1717 cmꢀ1 are
thus presumably associated with vibrations of phthalimide (Fig.
12); the imide carbonyl symmetrical and anti-symmetrical
stretches at 1773 and 1717 cmꢀ1, respectively, are indeed in
good accordance with vibrations reported in the literature.92 It
is, however, interesting to note that urea was also formed as
part of the dehydration mechanism when using cyanamide as
the dehydrating agent. However, no phthalimide can be
detected under the conditions employed in that particular
instance. It must therefore be presumed that phthalimide
formation, as observed in the presence of sodium cyanate,
results from a different mechanistic pathway than simple
addition of urea to the anhydride previously formed, as
proposed in the literature. This is also further supported by the
fact that little phthalic anhydride is seen to form, yet significant
conversion of the acid to phthalimide is observed from just 30
seconds at 160 8C. Therefore, phthalimide formation must
result from one of the following: (1) dehydration of the amide
II; (2) reaction of the anhydride formed with sodium cyanate
itself; or (3) reaction of sodium cyanate or urea with the mixed
anhydride intermediate.
Sodium cyanate assisted the thermal dehydration of 1,8-
naphthalic acid as expected. Anhydride formation was detected
from 30 seconds at 110 8C (150 8C in the absence of a
dehydrating agent), while no imide formation was observed.
Six-membered ring anhydrides, such as 1,8-naphthalic anhy-
dride, have been reported as being less reactive than their five-
membered ring counterparts, presumably due to the lower ring
strain.93,94 Therefore, the difference in reactivity between 1,8-
naphthalic and phthalic acids further confirms the involvement
of the anhydride species in the formation of the cyclic imide.
Presumably, the electron-rich nitrogen of the cyanate anion
CONCLUSION
The heterocumulenes cyanamide and dicyandiamide have
proven to be effective activating agents in lowering the
temperature at which thermal carboxylic acid dehydration leads
to cyclic anhydride formation. It was observed that cyanamide
lowered anhydride formation temperature to a greater extent
than dicyandiamide. It is presumed that this difference can be
attributed to electronic effects by which the amidine group
deactivates, through the inductive effect, and lowers the
electron density on the nitrogen atom(s), changing the
availability of the carbodiimide moiety to nucleophilic attack.
However, the actual mechanistic implications are still unclear;
two possible mechanisms have been presented, involving either
of the carbodiimide nitrogen atoms.
Sodium cyanate was seen to have poor catalytic effects on
phthalic acid under the conditions used. Presumably, its high
reactivity leads to its adding onto the anhydride first formed,
resulting in the formation of phthalimide. However, in the case
of 1,8-naphthalic acid, for which the anhydride is less reactive
due to the lower ring strain, no cyclic imide forms and sodium
cyanate acts as an efficient dehydrating agent, leading to a
decrease in the temperature at which thermal dehydration of the
acid leads to anhydride formation.
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