Indian Journal of Chemistry Vol. 44A, April 2005, pp. 661-666

Solid-state reaction between p-benzoquinone and p-aminophenol

--.!! _B Singh

Department of Chemistry. University of Gorakhpur, Gorakhpur 273 009, India Email: dr_n_b_singh @rediITmail.com -

Received 13 Decell/ber 2004; revised 9 February 2005

Kineti cs of solid-state reaction between p-benzoquinone and p-aminophenol has been studied at different temperatures. When the reactants are kept in contact in glass capillaries. the reaction rate increases with temperature and energy of activation is found to be much lower than the heat of sublimation of p-benzoquinone. It has been found that rate constants when the reactants are in contact and when they are separated by air gaps of different lengths are not the same. The resu lts show that the diffu sion of p-benzoquinone molecul es towards p-aminophenol occurs via surface migration. Kineti cs of reaction between p-benzoquinone (vapour) and p-aminophenol (solid) has also been studied by gravimetric method. The energy of act ivati on determined by thi s method is found to be much hi gher th an that for surface mi gration, indicating that the molecu les of p-benzoquinone enter into the crystal lattice of p-aminophenol forming the reacti on product in I: I molar rati o. Elemental analysis shows that stoi chiometry of the reaction products obtained from solution and solidstate reacti on is the same. UV -vis and infrared spectral studies indicate that the reaction product obtained from solution may be charge transfer complex stabi li zed by hydrogen bond. PMR. X-ray diffraction and DTA studi es show that the reaction products obtained from the two different ro[;tes are di ffe rent.

IPC Code: In t. Cl7 C07 50104

Organic solid state reactions have been known for we ll over a century. A large number of such reactions show ing considerable synthetic value and technological importance have been act ive ly in vestigated )·5. In recent years , one of the major break through in organic solid state chemistry is the crystal engineering which now includes design strategies for a variety of supramolecular and organized structures. A major focus of interest in organic solid state chemistry is now utilization of the crystalline state to lock molecu les into orientations favouring a desi red reaction . Taking crystal structures into consideration, solid state reactions between qui nones and phenols or ami nes have been studied extensivel/·7

. However, reactions between quinone and aminophenols have not been investi gated so far. Solid state react ions between p-benzoquinone and p-aminophenol have been studied in detail and described here.

Materials and Methods p-Benzoquinone was prepared as described in

literature8. The melting point was found to be 115°C.

p-Aminophenol was purified by repeated distillation under reduced pressure. The melting point of pure sample of p-aminophenol was 186°C.

Reaction product in solid state p-Aminophenol (solid) was mixed with p­

benzoquinone (solid) in an agate mortar in such a way

that stoichiometric ratio of the two components was in I: I. The start of the reaction was indicated by a colour change (black). The mixture was thoroughly mixed and kept at 30°C for a week. It was occasionall y mixed during this period . The reaction product was washed several times with benzene to remove unreacted components and the colour of the final product was black. The product was dried and kept in a desiccator.

Reaction product from solution The saturated solutions of p -aminophenol and p­

benzoquinone in acetone were mixed and the solution was concentrated for five min . After this , equal amount of benzene was added . The solution was kept at 30°C for 16 days , after which, black coloured crystals were obtained. The melting point of the product was 170°C. When the melt was heated up to 225°C, there was a colour change and when cooled, it regained the original colour.

Kinetics of solid state reaction in capillary (reactants in contact)

The detailed method for studyi ng the kinetics of the reaction is described elsewhere9

. A pyrex glass capillary having inner diameter (0.186 cm) and length about 5 cm was used for this purpose. One end of the capillary was sealed with the help of sealing wax. Half of the capillary was filled with p-aminophenol of

662 INDIAN J CHEM, SEC A. APRIL 2005

particle size below 100 mesh. For uniform packing, each capillary was tapped for 5 min. The surface was smoothened with the help of a thin glass rod. The remaining half of the capillary was filled with p­benzoquinone (particle size <100 mesh) in such a way that the two components came in close contact. After filling the capillary, the other end was also sealed with sealing wax and kept in an incubator maintained at a constant temperature (±1°C). At the junction of the two components, the reaction started with a colour change (black). The kinetics was followed by measuring the thickness of the reaction product at different intervals of time with the help of a travelling microscope at different temperatures (40, 50, 55, 60 and 70°C).

Kinetics when the reactants are separated by air gaps in the capillary

In this experiment, the glass capillaries were filled with p-aminophenol. Uniform packing was done for 5 min by tapping. The surfaces were smoothened with the help of a thin glass rod. p-Benzoquinone was filled in each capillary in such a way that the air gaps of different lengths were created between the two reactants. The capillaries were then sealed at both the ends and kept in an incubator maintained at a constant temperature. In all the capillaries, product layer was formed at the surface of p-aminophenol indicating that p -benzoquinone is the diffusing species. The studies were performed at temperatures 60°C.

Gravimetric studies The detailed method for studying the reaction is

described elsewhere 10. In order to understand the inner penetration of p-benzoquinone (vapours) into the crystal lattice of p-aminophenol (solid), the gravimetric studies were performed. p -Aminophenol was kept in glass tube fitted with B-19 standard glass joint. The tube was tapped for 5 min in order to have a uniform packing and the surface was smoothened with the help of a glass rod. p-Benzoquinone was fill ed in another glass tube fitted with B-19 standard glass joint. The two tubes were connected and kept in an incubator at a constant temperature. The kinetics was followed by noting the change in weight of the tube containing p-aminophenol at different intervals of time. The experiments were performed at 35, 45, 50 and 60°C.

X-ray dim'action studies The powder X-ray diffraction patterns of p­

benzoquinone, p-aminophenol and the reaction

product obtained from solution and solid state reactions were recorded with the help of a Rigaku, Rotaflex Diffractometer using CuKo. radiation at USIC, New Delhi.

Differential thermal analysis The differential thermal analysis of p-

benzoquinone and p-aminophenol and the reaction products obtained from solution and solid state reaction were recorded with the help of Rigaku Thermal Analyzer at USIC, New Delhi .

Electronic spectra The electronic spectra of p-benzoquinone and p­

aminophenol and the reaction product obtained from solution were recorded in methanol wi th Hitachi U-2000 spectrophotometer.

IR spectra The lR spectra of p-benzoquinone . p -aminophenol

and the reaction products obtained from solution and solid state reaction were recorded with the help of Perkin-Elmer spectrophotometer in Nujol mull at USIC, New Delhi.

Elemental analysis C, Hand N in product obtained from solid state

reaction and solution were estimated at Institut fur Organische Chemie der RWTH, Aachen, Germany.

NMR spectra The NMR spectra of the products and the

components were recorded in DMSO with the help of NMR spectrometer VXR 300/Sw at Institut fur Organische Chemie der RWTH, Aachen, Germany.

Results and Discussion When p -benzoquinone was kept in contact to p­

aminophenol in glass capillary, the start of the reaction was indicated with the formation of a coloured boundary at the junction of the two reactants. It was observed that the thickness of the product layer increased towards the side of p­aminophenol indicating that p-benzoquinone diffuses towards p-aminophenol. Kinetics of the reaction was studied at different temperatures and the kinetic data obeyed Eq. (1):

~= k. t" .. .. (I )

where ~ is the thickness of the product layer at any time t, k and n are constants. k is associated with diffusion coefficient and varies with the temperature

SINGH: SOLID-STATE REACTION BETWEEN p-BENZOQUINON E AND p-AMINOPHENOL 663

and hence known as apparent rate constant. The validity of Eq. (1) was tested by plotting log ~ vs log t. From the intercept and slope of the lines, the values of k and n, respectively were calculated, as given in Table 1. The values of k were found to increase with temperature whereas n remained constant. A plot of log k vs liT gave straight lines showing the validity of Arrhenius equation. The energy of activation was calculated from the slope of the line, as given in Table I. The value of energy of activation is very low (23.2 kJ mole· l) and much smaller as compared to the heat of sublimation of p-benzoquinone (63 kJ/mol)ll. Such a low value of energy of activation indicates that some easier mechanism is involved during the course of the reaction. This suggests that the di ffusion of p­benzoquinone molecules towards p -aminophenol might be occurring via surface migration.

When p-benzoquinone and p-aminophenol were separated by air gaps of different lengths, the reaction started at the surfaces of p-aminophenol with the formation of black coloured product. Again , the kinetic data obeyed Eq. (1). In this case, k and n are referred to as kl and 111 and the values are given in Table 2. The variation of rate constant kl with the length of air gap (d) is given by Eq.(2).

kl = A exp (-pel) .... (2)

where A and p are constants. The validity of Eq. (2) is tested by plotting log kl against d, where straight line is obtained. The values of k and kl for the case (i) when the reactants are in contact, and (ii) when separated by air gaps and extrapolated to d = 0 are not same (Table 2). The value of k in the former case is higher than in the latter one. One reason for this difference may be that for large air gaps, vapour phase diffusion is the dominant process and as the air gap is decreased, diffusion by surface migration becomes more and more significant.

It is to be noted that high vapour pressure is advantageous for high reactivity but at the same time volatility is not the only factor affecting the rate. Patil et at. G reported that methylquinone and methyl­hydroquinone showed only very slow formation of a coloured layer whereas methylquinone reacted relatively rapidly with the un substituted hydroquinone and also the unsubstituted quinone reacted readily with methylhydroquinone. Had it been a vapour phase controlled diffusion of p-benzoquinone, the rate constant would have been independent of the length of the air gap.

The process of diffusion of p-benzoquinone into the crystal lattice of p-aminophenol can be understood by gravimetric studies. The kinetic data obeyed Eq.(3).

.. . (3)

where 6. W is the increase in weight of p-aminophenol at any time t, Wo is the initial weight of p­aminophenol and k2 is a rate constant. The validity of Eq. (3) is tested by plotting 6. WIWo vs t, where straight lines are obtained. The values of k? are given in Table 3.

From Arrhenius plot, the energy of activation was found to be 61.2 kJ/mo\. The energy of activation for diffusion into the crystal lattice of p-aminophenol is much higher than the value obtained for surface migration (when the reactants are in contact in capillary).

In kinetic study (in the capillary), measurements are made by noting the thickness of the product layer

Tabl e I - Effect of temperature on the kinetic parameters o f Eq . I in the system p-aminophenol and p -benzoquinone. when

the reactants were kept in contact in capillary (Particle size < I 00 mesh)

Temperature (± 1°C) k X 102 (cm/h) n E (kJ mo!" ')

40 4 .89 0.20

50 6.76 0 . 11 23 .2

60

70

8.31

9.55

0. 11

0 . 11

Table 2 - Effect of air gaps on the kinetic parameters of Eq . I in the syste m p -aminophenol and p-benzoquinone (Temperature 60± 1°C; Particle size < I 00 mesh)

*d(cm) k,x 102 (cm/h) n,

0.400 4 .07 0 . 15

0 .650 3. 16 0.23

0.980

1.560

2.13

1.34

0 .30

0.38

*when d=O. on extrapolation: k, = 6.31 X 10.2 (cm/h); k(contact) = 8.31 x 10.2 (cm/h)

Table 3 - Kinetic parameters of Eq. (3) inner diffusion of p­benzoquinone (vapour) in p-aminophenol (solid)

(gravimetric study)

Temperature (± I 0c) k2 X 103 Wi ) E (kJ mo!" ')

35 1.2

45

50

60

1.9

2.5

5.2

61.2

664 INDIAN J CHEM, SEC A, APRIL 2005

which is indicated by change in colour at the surface of the phenol. This observation simply gives an idea about the migration of p -benzoquinone molecules over the surface of p-aminophenol. However, gravimetric studies give an idea about the inner penetration of p-benzoquinone molecules into the crystal lattice of p-aminophenol. The present system can be considered as interaction between host and guest molecules. p-Benzoquinone may be considered as a guest molecule whereas p-aminophenol as host molecule. In order that the guest will enter into the crystal lattice of the host, the dimension of the two must be comparable. From the bond angles and bond lengths, the dimensions of p-benzoquinone

(4.lx5 .2A2) and p -aminophenol (4.1x6.3A2

) are calculated. It is obvious that the dimensions of the host and the guest molecules are comparable.

Once the mechanism of diffusion during the course of solid state reaction is established, it is essential to know the nature of the reaction product. C, Hand N va lues in the reaction products obtained by the reaction between p-benzoquinone and p -aminophenol in solid state and solution respectively are:

Cobs= 65.325%, Hobs = 5.649%, and Nobs = 6.432%, and C obs= 65.045 %, H obs = 5.791 %, and N obs = 5 .995%.

The theoretical values for 1: I complex are Clhco= 66.350%, Hlheo= 5.104%, and Nlheo = 6.445%. The results indicate that reaction occurs in 1:1 molar ratio in both solid state and solution.

The powder X-ray diffraction patterns of p­benzoquinone, p-aminophenol and the reaction products obtained from solution and solid state reaction are recorded. These results show that the reaction products obtained from solid state reaction and solution are different. However, some additional lines due to unreacteJ components are observed in the case of the product obtained from solid state reaction. In solution, the molecules are free to move and may have a number of conformations whereas in the solid state the movements are restricted and hence limited conformations are possible. As a result of this, the reaction products obtained from solid state reaction and solution may be different.

DT A patterns of reaction products and components are given in Fig. 1. DT A pattern of the reaction product obtained from acetone shows two endothermic peaks at 170.6 and 223.5°e. The first pea is due to melting of the product whereas the

second peak may be due to thermochromic effect. The reaction product obtained by solid state reaction shows two exothermic peaks at 107.9 and 297.2°C and endothermic peaks at 132 and I58 .3°e. DTA patterns of the two products are entirely different.

( mierclv) 20

o

- 20

400

o

-400

400

o

-400

100

o

50

50

100

107· 9

OTA

Q

\50

y 148'7

b

100 \50 200

170"

C

200 300

197·2-

~ I'fS ' J

d 100 200 lOa 400

- 100 O~----~~--~~----~----~~

Temperature 0 C

Fig. 1- DTA of: (a) p -benzoquinone, (b) p-aminophenol, (c) reaction product obtained from solution, and (d) reaction product obtained from solid state reaction

SINGH: SOLID-STATE REACTION BETWEEN p-BENZOQUINONE AND p-AMINOPHENOL 665

Q o

a

b

Wavelength (nm) Fig. 2 - UV and visible spectra in methanol: (a) p-benzoquinone, (b) p-aminophenol, (c) I: 1 mixture of p-benzoquinone and p­aminophenol immediately, (d) after 2 min, (e) after 5 min, (f) after 8 min, and (g) after 10 min

Thus, it appears that the two products have different structures.

When the solution of p-aminophenol was mixed with p-benzoquinone in 1: 1 molar ratio in methanol, a new absorption band at 456 nm was obtained. The intensity of the band increased with time and shifted to longer wavelength (Fig. 2). The results show that as soon as the two components come in contact, they form some new compound or charge-transfer complex.

The IR spectra of p-benzoquinone and p­aminophenol and their products obtained from solution and solid state reaction were recorded. The IR spectra of p-benzoquinone show a peak at 1680 cm- I due to Dc=o stretching vibration. The IR spectra of p-aminophenol show a number of peaks between 3200-3700 cm-I due to DNH2 and DOH vibrations. The IR spectra of the reaction products obtained by solid state reaction and solution are almost identical. The only difference is that the intensities of the peaks in the case of product obtained by solid state reaction are lower as compared to those obtained from solution. This simply indicates that the reaction products are

same and the reaction is incomplete in solid state. As a result of complexation when p-aminophenol reacted with p-benzoquinone, bands due to DNH2 and DOH vibrations are broadened and shifted to lower wave number (3100-3400 cm -I). Further, the Dc=o band of p-benzoquinone is shifted to lower wave number (1560 cm- I

) in the complex. The results show that the C=O group of p-benzoquinone is hydrogen bonded with the hydrogen atom of the amino and hydroxyl group of p-aminophenol. Thus, one can say that a charge transfer molecular complex between p­aminophenol and p-benzoquinone stabilized by hydrogen bonding is formed in the solution. Formation of quinhydrone by the interaction of p­benzoquinone and p-dihydroxybenzene in solution and solid state is a well known example of charge transfer interaction stabilized by hydrogen bondingl2. Thus, on the basis of spectral studies and stoichiometry, following structure may be assigned to the reaction product obtained from solution. During solid state reaction, it is possible that the reaction product having same chemical formula may have different structure.

Further, the I3C_NMR and PMR spectra of p­benzoquinone, p-aminophenol and their reaction product obtained from solution were recorded. The CMR spectra of p-benzoquinone shows signal at 187.591 ppm due to C-atom attached to hydrogen atoms. The CMR spectra of p-aminophenol shows three signals one at 148.235 ppm due to C-atom attached to -OH group, the second signal at 140.383 ppm due to C-atom attached to -NH2 group and the third signal at 115.371 ppm due to C­atoms attached to hydrogen atoms. The CMR spectra of the reaction product show three signals, one at 149.603, the other at 125.432 and the third at 115.589 ppm. If we compare the spectra with that of p-aminophenol, then we find that the signal due to C-atom attached to -OH group is shifted downfield and the signal of C-atom attached to -NH2 group is shifted upfield indicating that the

666 INDIAN J CHEM, SEC A, APRI L 2005

sUIToundings are changed which may probably be due to hydrogen bonding.

The PMR spectra of p-benzoquinone shows a signal at 6.84 ppm due to hydrogen atoms of p­benzoquinone. The PMR spectra of p-aminophenol indicate a signal at 8.48 ppm due to phenolic hydrogen and a broad qUaItet signals for amino and aromatic protons at around 6.53 ppm. Nitrogen-14 has a spin number 1=1 and thus possesses a quadrupole moment. As a result of this, the signals for H nuclei attached to N-atom are generally broadened due to quadrupole relaxation effects . The PMR spectra of the products obtained from sol ution and from solid state reaction are different and the spectra of the product obtained from solution is little complicated. The PMR spectra of the product obtained from solid state reaction show singlet signal at 6 .54 ppm due to hydrogen of p-benzoquinone. This signal is slightly upfield as compared to the signal due to free p-benzoquinone (approx. 6.84 ppm). In addition , there are signals at around 8.67 ppm and quartet signal at around 6.4 ppm which may be due to p-aminophenol moiety. The PMR spectra show the presence of

hydrogen bonding in the product. Thus it a lso supports the proposed structure.

From the overall results, one can conclude that the reaction products obtained by solid state reaction and solution are different.

References I Desiraju G R, Elldeavour, 8 ( 1984) 201. 2 Ramamurthy V & Venkatesan K, Chelll Rev, 87 (1987) 433. 3 Singh N B. Pathak A & Frohlich R, AI/st J Chelll. 5 (2003 )

329. 4 Toda F. SYII Lell , 8 ( 1993)303. 5 Singh N B, Singh R J & Singh N P. Tetrahedroll. 50 (22)

( I 994) 6441. 6 Patil A O. Curtin D Y & Paul I C, J Am Choll Soc. 106

(1984) 348. 7 Singh N B & Singh R J, Solid State Reactivity, 8 ( 1990) 11 5. 8 Vogel A I, A Text Book oj Practical Orgall ;c Chemisll),. 4lh

edn , (ELBS & Longman, London). 1978 , pp .788. 9 Rastogi R P, Bassi P S & Chaddha S L. J Phys Cheill . 66

(1962) 2707. 10 Rastogi R P, Bassi P S & Chaddha S L, J Phys Chen/. 67

(1963) 2569. II Cox J D & Pilcher G, Th ermochelllist ry oj Org(lIl ic &

Orgallolll etallic COlllpol/llds (Academic Press. New York). 1970.

12 Singh N B & Singh H C. J Solid State Chem. 38 ( 198 1) 211.