A study on the reaction between N-substitutedp-phenylenediamines and ozone: experimental results andtheoretical aspects in relation to their antiozonant activity

Franco Cataldo *

Soc. Lupi Chemical Research Institute, Via Casilina 1626/A, 00133 Rome, Italy

Received 8 June 2001; received in revised form 5 September 2001; accepted 6 September 2001

Abstract

The following p-phenylenediamines (PPD): N ,N ,N 0,N 0-tetramethyl-p-phenylenediamine (TMPPD), N ,N 0-dimethyl-

butyl-p-phenylenediamine (6PPD), N ,N 0-diaryl-p-phenylenediamine (DPPD), tris-(N -dimethylpentyl-p-phenylene-diamine)-N 0,N 0,N 0-1,3,5-triazine (6PPDTZ), have been oxidized under the action of O3 in diluted solutions. In all cases

the radical cation or semiquinone radical was the first derivative formed by monoelectronic oxidation of the substrate.

The radical cation has been studied by electronic spectroscopy and the electronic spectral changes of all mentioned PPD

has been followed as function of the ozonation time. The results have been discussed in the frame of the antiozonant

properties of these PPD which are used as antiozonant agents in diene rubber protection. It is shown that the an-

tiozonant activity of each PPD considered correlates with the free enthalpy of formation of the respective radical cation.

The lowest is the free energy of formation of a PPD radical cation and the highest is the antiozonant activity in a diene

rubber compound. � 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Ozone; p-Phenylenediamine (PPD); Radical cation; Semiquinone radical electronic spectroscopy; Antiozonant; Diene

rubber; Protection

1. Introduction

N ,N -substituted p-phenylenediamines (PPD) are the

most widely used antidegradant and antiozonants agents

for diene rubber [1,2]. Although other classes of anti-

ozonants are well known [2], and some of them find

some special application mainly in niche sectors, the

PPDs are by far the most effective and widely used

agents.

The general reaction mechanism of ozone with ali-

phatic and aromatic amines has already been proposed

and reviewed by Bailey [3] and by Rakovsky and Zaikov

[2]. In the present paper we would like to report our

experimental results obtained by electronic spectroscopy

concerning the ozonation of some commercially avail-

able PPD and of N ,N ,N 0,N 0-tetramethyl-p-phenylenedi-amine (TMPPD). The interpretation of the experimental

results has led us to clarify the early stage of reaction of

PPD with O3 which involve the formation of PPD rad-

ical cation followed by the formation of the quinone-

diimine (QDI) derivative.

Once clarified this mechanism, we show that by the-

oretical calculations it is possible to predict the reactivity

with O3 of any substituted PPD on the basis of the free

enthalpy of formation of the radical cation of each PPD

molecule.

2. Experimental

2.1. Materials and experimental procedure

In the present study, ozone has been generated elec-

trochemically [4], as in all our previous studies [5–10]

European Polymer Journal 38 (2002) 885–893

www.elsevier.com/locate/europolj

* Tel.: +39-06-205-5084; fax: +39-06-205-0800.

E-mail address: cdcata@flashnet.it (F. Cataldo).

0014-3057/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0014-3057 (01 )00248-8

and the PPD used were commercial grades N ,N 0-di-

methylbutyl-p-phenylenediamine (6PPD), N ,N 0-diaryl-

p-phenylenediamine (DPPD), tris-(N-dimethylpentyl-

p-phenylenediamine)-N 0,N 0,N 0-1,3,5-triazine (6PPDTZ).

The latter PPD is a relatively new and non-staining anti-

ozonant commercialized under the trade name of Du-

razone � by Uniroyal Chemicals. Only TMPPD

di-hydrochloride has been purchased from Aldrich.

The ozonation reactions were conducted in analytical

grade CHCl3 from Fluka or in distilled water solution

by bubbling the ozonized oxygen into the PPD solu-

tions. The average ozone content by weight over oxygen

was about 10% and was determined iodometrically [4].

Ozone was produced at a rate of 3:43� 10�7 mol/min in

all cases. The solution volume used in each ozonation

study was 25 or 50 ml as specified case by case in the

text. After opportune intervals of ozonation times the

electronic spectra have been recorded on a Shimadzu

UV160A spectrophotometer.

2.2. Theoretical calculations

The theoretical calculations concerning the free

enthalpy of formation of the molecules considered in

this work have been conducted by using AM1 method.

Also the free enthalpy of formation of the radical cation

of each molecule has been calculated with the same

method, by extracting one electron from one nitrogen

atom.

3. Results and discussion

In our previous works [7,8] which we have reviewed

recently [11,12], we have shown that long unsubstituted

and substituted polyene chains generate radical cations

as one of the first elementary reaction act with ozone.

We have proposed a new electrophilic mechanism ofScheme 1.

Fig. 1. Electronic spectra of TMPPD. (A) Pure TMPPD–2HCl

1:77� 10�4 M in H2O. (B) Same as in A after 5 s (� � �) and after

300 s (––) ozonation. (C) TMPPD–2HCl 5:65� 10�4 M in H2O

from bottom to top respectively after 25, 65 and 120 s ozona-

tion. (D) TMPPD 4:22� 10�5 M in CHCl3: the curve with the

peak at 264 nm is the original solution. The solid line with

peaks at 565 and 614 nm is after 120 s ozonation and the dotted

line is the spectrum after the discolouration from blue to pink.

On top of these two curves the other more intense curves were

recorded after 240 s ozonation (––) and after colour fading to

pink (� � �).

886 F. Cataldo / European Polymer Journal 38 (2002) 885–893

conjugated addition across conjugated double bonds

when ozone or other electrophiles [13,14] cause the

addition reaction. Moreover, we have shown that it is

possible to generate C60 fullerene radical cations in su-

peracid solutions [15].

In the case of PPD, the formation of a radical cation

as the first elementary reaction step is a very well known

experimental fact when the oxidizing agent is bromine in

minute amounts [16]. In fact a stable radical cation,

known as Wurster’s complex or salt is formed [16]. Of

course many other different oxidizing agents have been

tested and the reaction has been found [17,18] to be

second order overall, but first order with respect to the

PPD and the oxidant. The values of the second order

rate constants for the formation and the decay of the

PPD radical cation were found to be in line with the

oxidation potentials of the oxidant [17,18]. We have

shown in a previous paper [9] that chlorine (E0 ¼ 1:36 V)and ozone which are stronger oxidizing agents than

bromine (in fact E0 O3=O2 ¼ 2:07 V while for E0 for

Br2=Br� ¼ 1:065 V) are able to form the radical cation

of PPD, provided that they are used in small amounts

over the organic substrates.

As shown in Scheme 1, the formation of a radical

cation from a PPD derives from the one-electron oxi-

dation of PPD. Once formed the radical cation is rela-

tively stable but could be further oxidized to a QDI

derivative by loosing another electron. It is interesting to

note that the reaction between QDI and unreacted PPD

leads to the formation of two radical cation molecules.

This means that until in the reaction system remain a

certain amount of unreacted PPD molecules, we will

have a buffering effect which hinder the oxidation of the

radical cation to the QDI. Infact each QDI formed is

reduced back to the radical cation by the oxidation to a

radical cation state of another PPD molecule.

3.1. About the ozonation of TMPPD

In Fig. 1A is reported the spectrum of TMPPD–

2HCl (about 1:77� 10�4 M) in water. It is characterized

by three maxima respectively at about 200 (e ¼ 13; 180),254 (e ¼ 13; 720) and 319 nm (e ¼ 2250). These three

bands correspond to the three bands of unsubstituted

PPD ([19], spectrum D9/99) respectively at 197 (e ¼33; 500), 241 (e ¼ 9300) and 305 nm (e ¼ 1900). These

bands are generated from p ! p� transitions.

According to Baude notation [20] the band at shorter

wavelength is called E1, followed by E2 and by the B

(benzenoid)-band at longer wavelength. The intense E1-

band at shorter wavelength derives from an allowed

transition whereas the other two bands result from

forbidden transitions in the highly symmetrical benzene

molecule. In the B-band there is a contribution derived

from the n–p interaction of the nitrogen atoms of the

PPD with the p electrons of the benzene ring. When O3

is passed in the solution of TMPPD–2HCl in water, a

new band is observed (Fig. 1B and C). This new band

shows a double peak at 562 and 610 nm and is due to the

radical cation of the TMPPD [16,21] and is generated

since the early reaction stages of O3 with the amine [21].

The formation of the TMPPD radical cation is accom-

panied also by the parallel growth of the TMPPD B-

band at 320 nm (see Figs. 1B, C and 2). In fact, the

320 nm band under the action of O3 undergoes an

hypschromic shift to 300 nm and increases its inten-

sity, reaching a maximum and decreasing (see Fig. 2),

following the same trend of the other bands assigned to

Fig. 2. Ozonation of TMPPD-2HCl 1:77� 10�4 M in H2O: evolution of the optical density of the bands associated to TMPPD (200

and 254 nm) and to its radical cation (319, 561 and 609 nm).

F. Cataldo / European Polymer Journal 38 (2002) 885–893 887

the radical cation in Fig. 2. This behaviour clearly sug-

gest that also this band is related to the formation of the

radical cation and the electronic structure of the radical

cation has an hyperchromic effect on the B-band of

TMPPD and the PPD in general.

From the literature [16] it is known that e560 ¼ 11,500

while e610 ¼ 11,000. By using these data and the changes

in concentration of PPD as function of ozonation time,

and knowing the approximate ozone amount bubbled

into the reaction mixture it is possible to estimate the

molar ratio between the most interesting chemical spe-

cies involved. In Table 1 we have summarized our results

which show that the highest molar ratio of radical cation

formation over the ozone supplied occurs in the early

seconds of ozonation due to the monoelectronic oxida-

tion act which leads to the radical cation. For instance

the maximum molar ratio between the concentration of

the radical cation and ozone is reached after 5 s of re-

action (see Table 1) and it appears that there are more

than four radical cation molecules for each O3 molecules

(under the assumption that all O3 bubbled into the so-

lution has reacted). This fact gives an idea of the excel-

lent scavenging effect of PPD towards O3. After 5 s of

reaction, 1/24 of all TMPPD molecules has already been

oxidized and transformed into the radical cation (Table

1). Of course the radical cation is just an intermediate

which in the case of TMPPD is relatively stable and its

further oxidation is for a certain period hindered by the

reducing effect of the unreacted TMPPD as discussed in

Section 1 (see Scheme 1).

However, as stated by Bailey [3], other competitive

reactions occur in substituted tertiary PPD such as the

alkyl side chain oxidation, which involves the proton

abstraction mechanism. This reaction is reported to be

slow and to lead, in the case of TMPPD, to N ,N 0-di-

methyl-p-quinonediimine and other products, as can be

deduced from the ozonation of N ,N 0-dimethylaniline [3].

It is also interesting to note that when the band at

about 300 nm reaches its maximum concentration, the

Table 1

Concentration molar ratio of the species involved in TMPPD

ozonation

Time (s) Radical ca-

tion/ozone

(molar

ratio)

TMPPD/

Ozone

(molar

ratio)

TMPPD/Radical

cation (molar

ratio)

0 0 0.0

5 4.4 104 23.8

15 2.5 35 13.7

60 1.4 7 5.1

120 1.1 3 2.9

180 0.8 1.68 2.0

300 0.6 0.69 1.2

480 0.3 0.25 0.9

720 0.05 0.15 0.5

1200 0 0.07 0.0

Fig. 3. Electronic spectra of 6PPD. (A) Pure 6PPD 7:5� 10�5

M in CHCl3. (B) Same as in A after 15 s ozonation. (C) Same as

in A after 30 s ozonation. (D) Same as in A after 60 s ozonation.

(E) Same as in A after 90, 120, 180, 240, 360 s ozonation (re-

spectively from top to bottom) in CHCl3.

888 F. Cataldo / European Polymer Journal 38 (2002) 885–893

molar ratio between the still unreacted TMPPD and its

radical cation approximates to the unity (see Table 1 and

compare with Fig. 2), the TMPPD/O3 molar ratio is 0.7

and also the other radical cation bands at longer

wavelength reach their maximum. From Fig. 2 it is clear

that all TMPPD has been quite completely consumed

after 400 s of ozonation so that the ozone attack is now

directed to the oxidation of the radical cation whose

concentration in fact starts to decline. As shown in

Scheme 1 one of the reaction products derived from

further monoelectronic oxidation of the radical cation is

the QDI derivative.

The ozonation of a QDI structure involves both the

O3 attack to C@N double bonds as well as to C@C

bonds of a quinoid structure. The general guidelines of

these further degradation reactions have been discussed

by Bailey [3].

When the free base TMPPD is dissolved in CHCl3and ozonated, the blue colouration of the Wurster com-

plex can be observed demonstrating that also in non-

polar solvents the radical cation is formed under the

action of O3. In diluted CHCl3 solution the radical ca-

tion band appears as a doublet at 565 and 614 nm, as in

the case of water solution. In more concentrated CHCl3solutions (for instance at 4:22� 10�5 M of Fig. 1D) the

radical cation band of TMPPD appear more complex

with two additional features at 481 and 519 nm in ad-

dition to the other normal bands at 565 and 614 nm. In

these conditions, immediately after ozonation (120 s at

the usual O3 feeding rate) the solution appears blue but

after the interruption of the passage of O3, after 1 min,

the solution becomes pink and the two radical cation

peaks at 565 and 614 nm vanish in a band tail while the

other two bands at 481 and 519 nm remain almost un-

changed (see Fig. 1D). Further ozonation restores for a

short time the blue colour which turns back pink (Fig.

1D). These experimental facts suggest that in CHCl3 the

TMPPD radical cation is not stable free but it forms a

dimeric complex [22].

In 10% HCl solution TMPPD shows a unique maxi-

mum at 232 nm and a shoulder at 260 nm. When O3 is

bubbled into the solution for 30–60 s, the peak shifts to

246 nm and two new bands appear at 312 and 432 nm

(broad). By further ozonation the band at 246 nm shifts

back to 232 nm, hence this phenomenon indicate that in

the early ozonation stages there is the formation of a

charge-transfer complex between TMPPD and O3. The

band at 434 nm is due to the diprotonated radical ca-

tion. Protonation of amines typically causes the hypso-

chromic shift of their B-bands [20]. Thus, it is obvious to

expect the radical cation band of diprotonated TMPPD

lying at 434 nm instead of at 560 and 610 nm as it

happens in non-acidic media. After 60 s ozonation this

band is no more observable. Instead, what it has been

observed is the steady growth of the band at 312 nm as

function of the ozonation time and its slow shift to

306 nm. The position of this band is almost the same as

observed in non-acidic water solution as described pre-

viously and it is due to the radical cation.

3.2. About the ozonation of 6PPD

Contrary to the tertiary amine TMPPD, 6PPD is an

N-alkyl–N 0-aryl disubstituted secondary amine. It is the

most widely used antiozonant for tyre application to

protect the diene rubber [1]. In CHCl3 6PPD shows a

maximum at 290 nm (Fig. 3A). By using 25 ml of a

6PPD solution of 7:5� 10�5 M, after 15 s ozonation we

observe the growth of a new band at 388 nm with two

Fig. 4. Ozonation of 6PPD 7:5� 10�5 M in CHCl3: evolution of the optical density of the band associated to 6PPD (290 nm) and to its

radical cation (410 nm).

F. Cataldo / European Polymer Journal 38 (2002) 885–893 889

shoulders at 410 and 480 nm (Fig. 3B). On further ozo-

nation the shoulder at 410 nm becomes a peak and in-

cluded the band at 388 nm while the shoulder at 477 nm

undergoes a gradual batochromic shift to 483 nm (after

30 s, Fig. 3C) and to 500 nm (after 60 s, Fig. 3D). Ad-

ditional ozonation causes a broadening of the band at

410 nm which buries the shoulder at 500 nm (Fig. 3E).

The radical cation of pure unsubstituted PPD ap-

pears at 462–479 nm [16] and is accompanied by a

pronounced peak at 380 nm, as we have verified by

ozonating pure PPD in CHCl3. Thus it is obvious to

assign the band at 388 nm with the shoulder at about

480 nm to the 6PPD radical cation. In the early stages,

the ozonation proceeds with the accumulation of the

6PPDþ in the solution, thanks to the mechanism of

Scheme 1 which hinders the formation of the QDI de-

rivative. This is clearly illustrated in Fig. 4 where we can

observe that the rapid drop in the concentration of the

6PPD is compensated by the accumulation of the radical

cation. After 60 s the radical cation starts to be degraded

by O3. Among the products there is certainly the QDI

derivative which still absorb at 400 nm or at longer

wavelengths, but there are also several other derivatives

some of them have been identified with chromatographic

techniques [23].

3.3. About the ozonation of DPPD

DPPD is another well known antiozonant. It is a

diaryl-substituted PPD and it is a commercial mixture

of N ,N 0-phenyl–tolyl, N ,N 0-tolyl–tolyl and N ,N 0-diphe-

nyl-PPD. By itself, it is not a particularly effective an-

tiozonant in rubber compounds when used alone [1].

However, in combination with 6PPD it has sinergistic

effects and ensures long protection to the rubber good

[1].

In CHCl3, DPPD shows a shoulder at 246 nm and a

peak at about 300 nm (Fig. 5A). The ozonation of 25 ml

of a 7:2� 10�5 mol/l solution causes the rapid growth of

the radical cation band at 411 nm (Fig. 5B, C). This

band shifts slowly to 415 nm and simultaneously we

observe a reduction of the band at 300 nm. By plotting

the absorbance of these two bands as function of the

ozonation time (Fig. 6), we get a graph which is very

close to that obtained by ozonating 6PPD (compare

with Fig. 4). Thus the band assignment and interpreta-

tion is analogous. The ozonation of the DPPD can be

followed by the drop in optical density of the band at

300 nm. The band at 246 nm which is due to the aryl

rings of the N ,N 0-substitution survives to the ozonation

and in fact the optical density of this band remains

unchanged even after prolonged ozonation (see Fig.

5D).

By comparing the Figs. 4 and 6 which refer respec-

tively to 6PPD and DPPD solution of approximately

equal concentration it appears evident that the maxi-

mum concentration of the radical cation occurs in both

cases after about 60 s of ozonation. This is in line with

literature data which show that in CCl4 the ozonation

rate is equal for both PPDs [2]. However, the con-

sumption rate of the 6PPDþ appears faster than that of

DPPDþ.

3.4. About the ozonation of 6PPDTZ

The antiozonant 6PPDTZ is rather new and it is non-

staining [1]. The ozonation of a CHCl3 solution 5:45�10�5 mol/l causes a steady reduction of the band origi-

nally lying at 297 nm. This band shifts to 275 nm after

30 s ozonation. By following this band it is possible to

Fig. 5. Electronic spectra of DPPD. (A) Pure DPPD 7:2� 10�5

M in CHCl3. (B) Same as in A after 20 s ozonation. (C) Same as

in A after 35 s (––) and 50 s (� � �) ozonation. (D) Same as in A

after 80, 120, 150, 210 and 300 s ozonation (respectively from

top to bottom) in CHCl3.

890 F. Cataldo / European Polymer Journal 38 (2002) 885–893

follow the consumption of the 6PPDTZ (see Fig. 7). A

more concentrated solution of 6PPDTZ (1:19� 10�4

mol/l) shows also a B-band at 487 nm. Ozonation causes

the development of a series of bands in the same spectral

region having a features which recall that of ozonated

TMPPD in CHCl3. Four different bands appear re-

spectively at 466, 483, 503 and 525 nm with the latter

being the most intense (see Fig. 8). The position of this

band is compatible with the assignment to the radical

cation. The optical density of the 525 nm band is re-

ported as function of the ozonation time in Fig. 8 for the

1:19� 10�4 M solution. In the case of this molecule the

prolonged ozonation causes the solution to become

dark. It appears that ozone causes the formation of

polycondensation products. The ozone attack can be

directed not only to PPD groups but also to the triazine

Fig. 6. Ozonation of DPPD 7:2� 10�5 M in CHCl3: evolution of the optical density of the band associated to DPPD (300 nm) and to

its radical cation (414 nm).

Fig. 8. Electronic spectrum of 6PPDTZ 1:19� 10�4 M in

CHCl3. Detail of the radical cation band after 200 s ozonation.

Fig. 7. Ozonation of 6PPDTZ 5:45� 10�5 M in CHCl3: evolution of the optical density of the band associated to 6PPDTZ (297 nm)

and to its radical cation (525 nm), measured in the 1:19� 10�4 M in CHCl3 solution.

F. Cataldo / European Polymer Journal 38 (2002) 885–893 891

ring although in this case the reaction speed should be

slower than that with the PPD.

3.5. Discussion of the theoretical calculations and con-

cluding remarks

Although the ozonation of secondary amines leads to

the formation of nitroxide radicals as the main products

as shown by Razumovskii and Batashova (as cited in

Ref. [3]) the general interpretation of the ozonation of

the secondary aromatic PPD involve the formation of

the corresponding radical cations as the first reaction

derivative [3]. Our study has confirmed the correctness

of this interpretation.

The experimental results reported in the previous

sections have confirmed once again that all PPDs gen-

erate the respective radical cation as the first elementary

intermediate when oxidized with O3. It has also been

reported [2] that the most powerful antiozonant PPDs

lead to the formation of stable radical cation interme-

diates while the less effective generate unstable radical

cation intermediates.

This led us to run some calculations according to

AM1 as stated in the Experimental section. We have

calculated the free enthalpy of formation of several well

known PPDs reported in Table 2, the free enthalpy of

formation of the radical cation and the corresponding

free energy of reaction which led to the radical cation.

The most surprising thing we are showing in Table 2 is

that the free energy of formation of the radical cation of

each PPD considered correlates with the antiozonant

activity as experimentally observed in the rubber vul-

canizates [24] without making any consideration on the

molecular weight and diffusion rate constants. More in

detail, the lowest is the free energy of formation of a

radical cation, and the highest is its antiozonant activity.

As well known [1] N ,N 0-dialkyl-substituted PPD are

better antiozonants than N ,N 0-aryl–alkyl-PPD which in

their turn are better than N ,N 0-diaryl-PPD.

Thus, in principle, to design new antiozonants or to

predict their antiozonant activity it could be possible for

the future to apply the proposed approach which in-

volves the calculation of the free energy of formation of

the corresponding PPD.

Acknowledgements

My warm thanks to Mr. Paolo Mondini for the com-

putational calculations according to the AM1 method.

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Table 2

Antiozonant activity: correlations between calculations and experimental facts

Molecule Enthalpy of formation of

the PPD (kJ/mol)

Enthalpy of formation of the

radical cation (kJ/mol)

Antiozonant

activity (rating)

Fatigue crack

formation (rating)

Antioxidant

activity

(rating)

77PD �176 571 100 70 42

IPPD 126 774 95 100 70

6PPD 143 793 70 92 70

(6PPD)3-

triazine

196 963 60 75 60

DTPD 356 1025 50 65 70

DPPD 420 1094 35 50 70

DNPD 581 1254 0 0 100

892 F. Cataldo / European Polymer Journal 38 (2002) 885–893

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F. Cataldo / European Polymer Journal 38 (2002) 885–893 893