Enthalpies of combustion of 2,2',6,6'-tetraethylazobenzene N,N-dioxide, 2,4,6--tri(1,1-dimethylethyl)nitrosobenzene, and 2,4,6-tri(1,1-dimethylethyl)nitrobenzene

Article abstract of DOI:10.1006/jcht.1995.0151

The standard (p^o = 0.1 MPa) molar enthalpies of combustion at the temperature T = 298.15 K were measured by static-bomb calorimetry for crystalline 2,2',6,6'-tetraethylbenzene N,N-dioxide, <2,6-(C2H5)2C6H3N(O)>2; 2,4,6-tri(1,1-dimethylethyl)nitrosobenzene, 2,4,6-3C6H2NO; and 2,4,6-tri(1,1-dimethylethyl)nitrobenzene, 2,4,6-3C6H2NO2.The standard molar enthalpies of sublimation at T = 298.15 K of the tri(1,1-dimethylethyl) compounds were measured by microcalorimetry.

.The standard molar enthalpy of decomposition of <2,6(C2H5)2C6H3N(O)>2 to form 2,2-(C2H5)2C6H3NO(g) at T = 298.15 K was measured by microcalorimetry: Δ_decH_m^o/(kJ*mol^-1) = (200.6 +/- 5.6).Application of group-additivity schemes applied to nitrosobenzene and nitrobenzene derivatives shows that 2,4,6-tri(1,1-dimethylethyl)nitrosobenzene is unstrained whereas the corresponding nitrocompound shows considerable strain in accord with an X-ray structure analysis demonstrating that steric hindrance prevents dimerization of the nitrosoderivative,

Full text of DOI:10.1006/jcht.1995.0151

M-3173  

J. Chem. Thermodynamics 1995, 27, 1433–1440  

Enthalpies of combustion of  

2,2',6,6'-tetraethylazobenzene N,N-dioxide,  

2,4,6-tri(1,1-dimethylethyl)nitrosobenzene,  

and 2,4,6-tri(1,1-dimethylethyl)nitrobenzene  

W. E. Acree, Jr., S. G. Bott, Sheryl A. Tucker,  

Department of Chemistry, University of North Texas, Denton, TX 76203-0068,  

U.S.A.  

Maria D. M. C. Ribeiro da Silva, M. Agostinha R. Matos,  

Departamento de Quımica, Faculdade de Ciencias, Universidade do Porto,  

P-4050 Porto, Portugal  

and G. Pilcher  

Department of Chemistry, University of Manchester, Manchester M13 9PL,  

U.K.  

(Received 21 July 1995)  

The  

temperature  

crystalline  

standard  

(p° = 0.1 MPa)  

298.15 K were measured by static-bomb calorimetry for  

2,2'6,6'-tetraethylazobenzene N,N-dioxide, {2,6-(C₂H₅)₂C₆H₃N(O)·}₂;  

2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO; and 2,4,6-  

molar  

enthalpies  

of  

combustion  

at  

the  

T

=

2,4,6-tri(1,1-dimethylethyl)nitrosobenzene,  

tri(1,1-dimethylethyl)nitrobenzene, 2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO₂. The standard molar  

enthalpies of sublimation at T=298.15 K of the tri(1,1-dimethylethyl) compounds were measured  

by microcalorimetry.  

g

−D_cH_m°(cr)/(kJ·mol⁻¹  

)

D_crH_m°/(kJ·mol⁻¹  

)

{2,6-(C₂H₅)₂C₆H₃N(O)·}₂  

2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO  

2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO₂  

11585.522.6  

11019.323.7  

10956.423.2  

—

91.023.2  

81.421.8  

The standard molar enthalpy of decomposition of {2,6-(C₂H₅)₂C₆H₃N(O)·}₂ to form  

2,6-(C₂H₅)₂C₆H₃NO(g) at 298.15 K was measured by microcalorimetry:  

_decH_m°/(kJ·mol⁻¹) = (200.6 2 5.6). Application of group-additivity schemes applied  

to nitrosobenzene and nitrobenzene derivatives shows that 2,4,6-  

T

=

D

tri(1,1-dimethylethyl)nitrosobenzene is unstrained whereas the corresponding nitrocompound  

shows considerable strain in accord with an X-ray structure analysis demonstrating that steric  

hindrance prevents dimerization of the nitrosoderivative. 7 1995 Academic Press Limited  

0021–9614/95/121433+08 $12.00/0  

7 1995 Academic Press Limited  

1434  

W. E. Acree, Jr. et al.  

1. Introduction  

Crystalline aromatic C-nitroso compounds can be either white or green: when  

white the compounds are dimeric and when green, monomeric.  

Crystal-structure determinations of nitrosobenzene,⁽¹⁾ 4-bromonitrosobenzene,⁽²⁾ and  

2,4,6-tribromonitrosobenzene,⁽³⁾ show these white compounds to be dimeric whereas  

for 4-iodonitrosobenzene,⁽⁴⁾ and 4-dimethylaminonitrosobenzene,⁽⁵⁾ they show these  

green compounds to be monomeric. Previous thermochemical studies showed that  

substitution of the electron-donating dimethylamino group para to the nitroso group  

results in exceptional stabilization of the monomer, making formation of the dimer  

energetically unfavourable.⁽⁶⁾  

The white dimeric forms melt to give green liquids and the vapour at high  

temperatures and low pressures will decompose to the monomer, thus previous  

thermochemcial measurements on 2,2',4,4',6,6'-hexamethylazobenzene N,N-dioxide  

and 2,2',6,6'-tetramethylazobenzene N,N-dioxide led to values for the enthalpies of  

formation of 2,4,6-trimethylnitrosobenzene and of 2,6-dimethylnitrosobenzene in the  

gaseous state.⁽⁷⁾ It is to be expected that increasing the size of the groups ortho to the  

nitroso group would eventually result in prevention of dimerization.  

In this paper we report the enthalpy of combustion of 2,2',6,6'-tetraethylazobenzene  

N,N-dioxide:  

a

the  

white crystalline solid, and the enthalpy of its decomposition to  

gaseous monomer: also the enthalpies of combustion of  

2,4,6-tri(1,1-dimethylethyl)nitrosobenzene:  

and of 2,4,6-tri(1,1-dimethylethylnitrobenzene:  

together with the enthalpies of sublimation, to investigate whether the green  

monomeric nitroso compound fails to dimerize due to stabilization of the monomer  

or due to steric hindrance in formation of the dimer.  

D_fH°(nitrosobenzene and nitrobenzene derivatives,g)  

1435  

2. Experimental  

2,2',6,6'-tetraethylazobenzene N,N-dioxide was prepared by oxidation of  

2,6-diethylaniline using sodium tungstate plus hydrogen peroxide as described by  

Stowell and Lau,⁽⁸⁾ and purified by four crystallizations from (benzene+hexane).  

2,4,6-Tri(1,1-dimethylethyl)nitrosobenzene was prepared by oxidation of 2,4,6-  

tri(1,1-dimethylethyl)aniline with two equivalents of peroxybenzoic acid in  

dichloromethane at the temperature 273 K,^{(9, 10)} and purified by sublimation in vacuo.  

2,4,6-Tri(1,1-dimethylethyl)nitrobenzene (Aldrich) was recrystallized four times from  

methanol. Elemental analyses were in agreement with expected values: mass fractions  

for C₂₀H₂₆N₂O₂: found: C, 0.7344; H, 0.0807; N, 0.0863; calculated: C, 0.7359; H,  

0.0803; N, 0.0858, for C₁₈H₂₉NO: found: C, 0.7841; H, 0.1055; N, 0.0516; calculated:  

C, 0.7849; H, 0.1061; N, 0.0509, and for C₁₈H₂₉NO₂; found: C, 0.7431; H, 0.0998; N,  

0.0472; C, 0.7418; H, 0.1003; N, 0.0481. The densities of the samples and the average  

ratios of the mass of carbon dioxide produced by the sample to that calculated from  

its mass with uncertainties of twice the standard deviation of the mean were:  

C₂₀H₂₆N₂O₂: r/(g·cm⁻³) = 1.35, (1.0000 2 0.0001); C₁₈H₂₉NO₂: r/(g·cm⁻³) = 0.85  

(estimated), (0.999820.0001); C₁₈H₂₉NO₂: r/(g·cm⁻³) = 0.85 (estimated),  

(1.000120.0004).  

Hexadecane (Aldrich, Gold Label) stored under nitrogen was used in some  

experiments as an auxiliary combustion aid.  

2,4,6-Tri(1,1-dimethylethyl)nitrobenzene was further characterized by X-ray  

crystallography using an Enraf-Nonius CAD-4 diffractometer.⁽¹¹⁾ The results showed  

that this molecule crystallizes in the hexagonal space group Pꢀ62c, but with a  

considerable degree of disorder. Due to this disorder, a refined model for the structure  

could not be derived but sufficient information was obtained to render some  

clarification of the overall structure. The disorder was characterized as both  

intramolecular and intermolecular. The benzene ring lies on a mirror plane (z=1/4),  

perpendicular to a six-fold inversion axis (1/3, 2/3, 1/4). Every carbon position  

therefore, is bonded to a 1/6 occupancy nitro group and a 1/2 occupancy  

1,1-dimethylethyl moiety. Within the resolution of the model, the nitrogen and the  

quaternary carbon atoms coincide. Six points of electron density connect to this  

coincident point, corresponding to two coincident C/O ‘‘atoms’’ and four 1/4  

occupancy carbons, all of which lie out of the aromatic plane. Thus the nitro group  

adopts an orientation perpendicular to the benzene ring, and the 1,1-dimethylethyl  

groups adopt one of two orientations that place the carbons at dihedral angles of −p/6,  

p/2, and −5·p/6 with respect to the ring. The intermolecular disorder consists of each  

O/C position lying on a three-fold axis (2/3, 1/3, z; and 0, 0, z). The overall structure,  

therefore, consists of numerous peaks of partial and/or mixed occupancies. Germane  

to this discussion, however, is the fact that all peaks could be located through both  

direct and Fourier-difference methods (if not refined), and none of the attached group  

peaks lies in the same plane as the aromatic ring.  

The enthalpy of combustion of 2,2',6,6'-tetraethylazobenzene N,N-dioxide  

was measured using the Manchester static-bomb calorimeter,^{(12, 13)} whereas  

2,4,6-tri(1,1-dimethylethyl)nitrosobenzene and 2,4,6-tri(1,1-dimethylethyl)nitrobenzene  

1436  

W. E. Acree, Jr. et al.  

were measured using the Porto static-bomb calorimeter.^{(14, 15)} The energy equivalent of  

the Manchester bomb calorimeter was determined from the combustion of benzoic  

acid (NBS SRM 39i) having a massic energy of combustion under standard bomb  

conditions of (2643423) J·g⁻¹. From 12 calibration experiments, o(calor) =  

(15525.4020.47) J·K⁻¹ where the uncertainty quoted is the standard deviation of  

the mean for an average mass of water added to this calorimeter of 2897.0 g. The Porto  

bomb calorimeter was calibrated using benzoic acid (Bureau of Analysed Samples,  

CRM 190p) having a massic energy of combustion under standard bomb  

conditions of (26431.823.7) J·g⁻¹. From nine calibration experiments, o(calor)=  

(15911.221.5) J·K⁻¹ for an average mass of water added to this calorimeter of  

3119.6 g.  

The experimental conditions were similar for both calorimeters. Samples, in pellet  

form, were ignited at T=(298.15020.001) K in oxygen at a pressure p=3.04 MPa  

with a volume 1 cm³ of water added to the bomb. The electrical energy for ignition  

was determined from the change in potential difference across a capacitor when  

discharged through the platinum ignition wire. For the cotton-thread fuse, empirical  

formula CH_1.686O_0.843, −D_cu°=16250 J·g⁻¹.⁽¹⁶⁾ For the combustion auxiliary aid,  

hexadecane, from separate measurements, −D_cu° = (47156.921.2) J·g⁻¹.  

Corrections for nitric-acid formation were based on −59.7 kJ·mol⁻¹ for the molar  

energy of formation of 0.1 mol·dm⁻³ HNO₃(aq) from N₂, O₂, and H₂O(l).⁽¹⁷⁾ The  

amount of substance used in each experiment was determined from the total mass of  

carbon dioxide produced after allowance for that formed from the cotton-thread fuse  

and hexadecane. For each compound (1u/1p)_T at T=298.15 K was assumed to be  

−0.2 J·g⁻¹·MPa⁻¹, a value typical for most organic solids. For each compound, D_cu°  

was calculated by the procedure given by Hubbard et al.⁽¹⁶⁾ The molar masses used for  

the elements were those recommended by the IUPAC Commission.⁽¹⁸⁾  

The standard molar enthalpies of sublimation of 2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO  

and 2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO₂ and the enthalpy of decomposition of  

{2,6-(C₂H₅)₂C₆H₃N(O)·}₂ were measured by the ‘‘vacuum sublimation’’  

drop-microcalorimetric method.⁽¹⁹⁾ Samples (of mass about 5 mg) of each compound  

TABLE 1. Typical combustion results (p°=0.1 MPa)  

{2,6-(C₂H₅)₂C₆H₃N(O)·}₂ 2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO 2,4,6-{CH₂C(CH₃)₂}₃C₆H₂NO₂  

m(CO₂,total)/g  

m(cpd)/g  

m(hexadecane)/g  

m(fuse)/g  

2.75718  

1.02087  

—

0.00281  

2.33701  

14.8  

1.79155  

0.62118  

—

0.00306  

1.56401  

16.5  

1.69334  

0.32944  

0.25473  

0.00355  

1.53647  

DT_ad/K  

o_f /(J·K⁻¹  

)

16.6  

Dm(H₂O)/g  

0.0  

36316.5  

−0.4  

24907.4  

28.1  

−0.4  

24469.0  

26.6  

1.0  

8.3  

12012.1  

57.7  

37531.3  

−DU(IBP)/J  

DU(HNO₃)/J  

DU(ign)/J  

59.0  

1.1  

18.6  

—

45.6  

1.1  

10.1  

—

49.7  

DU_S/J  

−mD_cu°(hexadecane)/J  

−mD_cu°(fuse)/J  

−D_cu°(cpd)/(J·g⁻¹  

)

35453.4  

39955.4  

D_fH°(nitrosobenzene and nitrobenzene derivatives,g)  

1437  

contained in a small thin glass capillary tube sealed at one end were dropped at room  

temperature into the hot reaction vessel in the Calvet High-Temperature  

Microcalorimeter and then removed from the hot zone by vacuum sublimation. The  

observed standard molar enthalpies of sublimation {H_m°(g,T)−H_m°(cr,298.15 K} were  

T

corrected to T=298.15 K using D_{298.15 K}H_m°(g) estimated by a group method based on  

the values of Stull et al.⁽²⁰⁾ The observed enthalpy of decomposition of  

{2,6-(C₂H₅)₂C₆H₃N(O)·}₂(cr), {2H_m°(monomer,g,T)−H_m°(cr,dimer,298.15 K)}, was  

corrected to T=298.15 K in a similar fashion. For this decomposition, the calorimeter  

was held at a sufficiently high temperature and this temperature was varied to ensure  

that the decomposition was complete. The microcalorimeter was calibrated in situ by  

making use of the reported molar enthalpy of sublimation of naphthalene, C₁₀H₈:  

(72.5120.01) kJ·mol⁻¹.⁽²¹⁾  

3. Results  

Results for a typical combustion experiment on each compound are given in table 1:  

Dm(H₂O) is the deviation of the mass of water added to the calorimeter from the  

mass assigned for o(calor); DU_S is the correction to the standard state; the  

remaining terms are as previously described.⁽¹⁶⁾ As samples were ignited at  

T=(298.15020.001) K:  

DU(IBP)=−{o(calor)+c_p(H₂O, l)·Dm(H₂O)+o_f}DT_ad+  

DU(ign); where DT_ad is the calorimeter temperature change corrected for heat  

exchange and the work of stirring. The individual values of −D_cu° together with the  

mean and its standard deviation are given in table 2. Table 3 lists the derived standard  

molar enthalpies of combustion and of formation in the condensed and gaseous  

states. In accordance with normal thermochemical practice, the uncertainties  

assigned to the standard molar enthalpies of combustion and formation are twice the  

overall standard deviation of the mean and include the uncertainties in calibration  

and in the auxiliary quantities used. To derive D_fH_m° from D_cH_m° the standard molar  

enthalpies of formation: for H₂O(l): −(285.8320.04) kJ·mol⁻¹ and for CO₂(g):  

−(393.5120.13) kJ·mol⁻¹ were used.⁽²²⁾  

TABLE 2. Values of −D_cu° at T=298.15 K (p°=0.1 MPa)  

{2,6-(C₂H₅)₂C₆H₃N(O)·}₂  

2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO  

2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO₂  

−D_cu°/(J·g⁻¹  

39939.4  

39963.5  

39937.3  

39944.2  

)

35456.9  

35443.7  

35453.4  

35455.3  

35465.7  

35463.9  

37558.1  

37551.9  

37538.7  

37556.3  

37544.9  

37536.1  

37548.2  

37546.4  

37531.3  

39965.2  

39955.4  

−ꢀD_cu°ꢁ/(J·g⁻¹  

39950.825.0  

)

35456.523.2  

37545.823.0  

1438  

W. E. Acree, Jr. et al.  

TABLE 3. Derived standard molar values at T=298.15 K (p°=0.1 MPa)  

g

−D_cU_m°(cr)  

−D_cH_m°(cr)  

D_fH_m°(cr)  

D_crH_m°  

D_fH_m°(cr)  

kJ·mol⁻¹  

kJ·mol⁻¹  

kJ·mol⁻¹  

kJ·mol⁻¹  

kJ·mol⁻¹  

{2,6-(C₂H₅)₂C₆H₃N(O)·}₂  

2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO  

2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO₂  

11574.322.6  

11003.823.7  

10942.123.2  

11585.522.6  

11019.323.7  

10956.423.2  

−0.523.8  

−208.424.4  

−271.324.0  

200.625.6 ^a  

91.023.2  

81.421.8  

−117.425.4  

−189.924.4  

^a Corresponds to dimer(cr)=2·monomer(g).  

4. Discussion  

From the molar enthalpies of formation and decomposition of  

{2,6-(C₂H₅)₂C₆H₃N(O)·}₂: D_fH_m°{2,6-(C₂H₅)₂C₆H₃NO,g}=(100.123.4) kJ·mol⁻¹ is  

derived. In table 4, the observed D_fH_m°(g) values for C₆H₅NO derivatives are compared  

with those calculated using the Cox scheme,⁽²³⁾ in which each group is associated with  

a characteristic increment in D_fH_m°(g) when substituted into the benzene ring. Cox  

proposed corrections for steric hindrance between neighbouring groups but no such  

corrections have been made here. To derive the calculated values, an average increment  

for substitution of the NO group into C₆H₆ of (122.422.2) kJ·mol⁻¹ was used together  

with the following values of D_fH_m°(g)/(kJ·mol⁻¹): C₆H₆, (82.620.7);⁽²⁴⁾ 1,3-(CH₃)₂C₆H₄,  

(17.3 2 0.8);⁽²⁴⁾  

1,3,5-(CH₃)₃C₆H₃, −(15.9 2 1.3);⁽²⁴⁾  

1,3-(C₂H₅)₂C₆H₄,  

−(21.8 2 [2.5]);⁽²⁵⁾ C₆H₅N(CH₃)₂, (100.5 2 4.7).⁽²⁴⁾ D_fH_m°(g)/(kJ·mol⁻¹) for  

CH₃C(CH₃)₂C₆H₅: −(22.621.2),⁽²⁴⁾ was used to estimate D_fH_m°(g)/(kJ·mol⁻¹) of  

1,3,5-{CH₃C(CH₃)₂}₃C₆H₃: −(233.023.7). From the comparison in table 4 it is clear  

TABLE 4. Observed and calculated D_fH_m°(g) values for nitrosobenzene derivatives  

DD_fH_m°  

D_fH_m°(g)/(kJ·mol⁻¹  

)

kJ·mol⁻¹  

observed  

calculated  

C₆H₅NO  

209.828.0 ⁽⁷⁾  

139.821.6 ⁽⁷⁾  

107.421.9 ⁽⁷⁾  

100.123.4  

205.022.3  

139.722.3  

106.522.6  

100.62[3.3]  

−110.624.3  

222.925.2  

4.828.3  

0.122.8  

0.923.2  

2,6-(CH₃)₂C₆H₃NO  

2,4,6-(CH₃)₃C₆H₂NO  

2,6-(C₂H₅)₂C₆H₃NO  

2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO  

4-(CH₃)₂NC₆H₄NO  

−0.52[4.7]  

−6.826.9  

−37.925.7  

−117.425.4  

185.022.3 ⁽⁶⁾  

TABLE 5. Observed and calculated D_fH_m°(g) values for nitrobenzene derivatives  

DD_fH_m°  

D_fH_m°(g)/(kJ·mol⁻¹  

)

kJ·mol⁻¹  

observed  

calculated  

4-CH₃C₆H₄NO₂  

2,6-(CH₃)₂C₆H₃NO₂  

2-(C₂H₅)C₆H₄NO₂  

4-(C₂H₅)C₆H₄NO₂  

2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO₂  

31.023.8 ⁽²⁴⁾  

8.621.6 ⁽⁷⁾  

35.321.1  

2.221.2  

14.821.4  

−4.324.0  

6.422.0  

−3.626.7  

−7.426.7  

58.225.8  

11.226.6 ⁽²⁴⁾  

7.426.6 ⁽²⁴⁾  

14.821.4  

−189.924.4  

−248.123.8  

D_fH°(nitrosobenzene and nitrobenzene derivatives,g)  

1439  

that within the limits of uncertainty, 2,4,6-tri(1,1-dimethylethyl)nitrosobenzene has an  

expected enthalpy of formation thus showing neither strain nor stabilization energy.  

A similar exercise for C₆H₅NO₂ derivatives is presented in table 5: to derive  

the calculated values the following additional D_fH_m°(g)/(kJ·mol⁻¹) values were  

used: C₆H₅NO₂, (67.520.6);⁽²⁴⁾ C₆H₅CH₃, (50.420.6);⁽²⁴⁾ C₆H₅C₂H₅, (29.921.1).⁽²⁴⁾  

From the comparison in table 5 it is clear that 2,4,6-tri(1,1-dimethylethyl)nitrobenzene  

exhibits very large steric-strain energy.  

If 2,4,6-tri(1,1-dimethylethyl)nitrosobenzene were to dimerize, it would seem  

reasonable to assume that the steric-strain energy in the dimer would be at least  

twice that shown by the nitroderivative, i.e. 1116 kJ·mol⁻¹. As D_m(N.N) in  

C₆H₅N(O)·N(O)·C₆H₅ is (91.128.5) kJ·mol⁻¹,⁽⁷⁾ it is clear that steric hindrance  

prevents formation of the dimer of 2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO in contrast to the  

case of 4-N(CH₃)₂C₆H₄NO where exceptional stabilization of the monomer prevents  

dimerization.⁽⁶⁾  

The determination of D_m(N-O) in  

RNO₂ = RNO + O, requiring D_fH_m°(O,g) = (249.17 2 0.10) kJ·mol⁻¹.⁽²²⁾ For  

marked  

a

nitrocompound is D_rH_m°(g) for  

2,4,6-{CH₃C(CH₃)₂}₃C₆H₂NO₂, D_m(N-O) = (321.827.0) kJ·mol⁻¹,  

a

reduction from that in C₆H₅NO₂ of (391.528.0) kJ·mol⁻¹.⁽⁷⁾ This shows the very large  

effect of steric hindrance of the two 1,1-dimethylethyl groups ortho to the nitro group as  

the X-ray analysis has shown that the nitro group has been forced from its preferred  

orientation planar with the benzene ring to be perpendicular to the ring, hence  

eliminating any stabilization effect of p-electron delocalization between the nitro group  

and the benzene ring.  

M.D.M.C.R.S. and M.A.R.M. thank Junta Nacional de Investigacao Cientıfica e  

Tecnologica for the research project (STRDA/C/CEN/519/92) and for financial  

support to the research group of Centro de Investigacao em Quımica of Universidade  

do Porto (CIQ/L5).  

REFERENCES  

1. Dieterich, D. A.; Paul, I. C.; Curtin, D. Y. J. Am. Chem. Soc. 1974, 90, 6372.  

2. Darwin, S.; Hodgkin, D. Nature 1950, 166, 827.  

3. Fenimore, C. P. J. Am. Chem. Soc. 1950, 72, 3226.  

4. Webster, M. S. J. Chem. Soc. 1956, 2841.  

5. Rꢀomming, CH. R.; Talberg, H. J. Acta Chem. Scand. 1973, 27, 2246.  

6. Acree, W. E., Jr.; Tucker, S. A.; Pilcher, G.; Toole, G. J. Chem. Thermodynamics 1994, 26, 85.  

7. Acree, W. E., Jr.; Tucker, S. A.; Pilcher, G.; Chowdhary, A.; Ribeiro da Silva, M. D. M. C.; Monte,  

M. J. S. J. Chem. Thermodynamics 1993, 25, 1253.  

8. Stowell, J. C.; Lau, C. H. J. Org. Chem. 1986, 51, 1614.  

9. Burgers, J.; Hoefnagel, M. A.; Verkade, P. E.; Visser, H.; Wepster, B. M. Rec. Trav. Chim. 1958, 77,  

491.  

10. Okazaki, R.; Hosogai, T.; Iwadare, E.; Hashimoto, M.; Inamoto, N. Bull. Chem. Soc. Japan 1969, 42,  

3611.  

11. Mason, M. R.; Smith, J. M.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1993, 115, 1971.  

12. Bickerton, J.; Pilcher, G.; Al-Takhin, G. J. Chem. Thermodynamics 1984, 16, 373.  

13. Kirchner, J. J.; Acree, W. E., Jr.; Pilcher, G.; Li Shaofeng J. Chem. Thermodynamics 1986, 18, 793.  

14. Ribeiro da Silva, M. D. M. C.; Ribeiro da Silva, M. A. V.; Pilcher, G. J. Chem. Thermodynamics 1984,  

16, 1149.  

15. Ribeiro Da Silva, M. A. V.; Ribeiro da Silva, M. D. M. C.; Pilcher, G. Rev. Port. Quim. 1984, 26, 163.  

1440  

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16. Hubbard, W. N.; Scott, D. W.; Waddington, G. Experimental Thermochemistry. Vol. 1. Chap. 5.  

Rossini, F. D.: editor. Interscience: New York. 1956.  

17. The NBS Tables of Chemical Thermodynamic Properties. J. Phys. Chem. Ref. Data 1982, 11, Supplement  

no. 2.  

18. IUPAC J. Phys. Chem. Ref. Data 1993, 22, 1571.  

19. Adedeji, F. A.; Brown, D. L. S.; Connor, J. A.; Leung, M.; Paz-Andrade, M. I.; Skinner, H. A.  

J. Organometallic Chem. 1975, 197, 221.  

20. Stull, D. R.; Westrum, E. F., Jr.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds.  

Wiley: New York. 1969.  

21. Kruif, C. G. de; Kuipers, T.; Miltenburg, J. C. van; Schaake, R. C. F.; Stevens, G. J. Chem.  

Thermodynamics 1981, 13, 1081.  

22. Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA Key Values for Thermodynamics. Hemisphere:  

New York. 1989.  

23. Cox, J. D. A method of estimating the enthalpies of formation of benzene derivatives in the gas state. NPL  

Report CHEM83–June 1978.  

24. Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds. 2nd edition.  

Chapman and Hall: London. 1986.  

25. Prosen, E. J.; Johnson, W. H.; Rossini, F. D. J. Res. Natl. Bur. Stand. 1946, 36, 455.  

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Full text of DOI:10.1006/jcht.1995.0151

M-3173
J. Chem. Thermodynamics 1995, 27, 1433–1440
Enthalpies of combustion of
2,2',6,6'-tetraethylazobenzene N,N-dioxide,
2,4,6-tri(1,1-dimethylethyl)nitrosobenzene,
and 2,4,6-tri(1,1-dimethylethyl)nitrobenzene
W. E. Acree, Jr., S. G. Bott, Sheryl A. Tucker,
Department of Chemistry, University of North Texas, Denton, TX 76203-0068,
U.S.A.
Maria D. M. C. Ribeiro da Silva, M. Agostinha R. Matos,
Departamento de Quımica, Faculdade de Ciencias, Universidade do Porto,
P-4050 Porto, Portugal
and G. Pilcher
Department of Chemistry, University of Manchester, Manchester M13 9PL,
U.K.
(Received 21 July 1995)
The
temperature
crystalline
standard
(p° = 0.1 MPa)
298.15 K were measured by static-bomb calorimetry for
2,2'6,6'-tetraethylazobenzene N,N-dioxide, {2,6-(C2H5)2C6H3N(O)·}2;
2,4,6-{CH3C(CH3)2}3C6H2NO; and 2,4,6-
molar
enthalpies
of
combustion
at
the
T
=
2,4,6-tri(1,1-dimethylethyl)nitrosobenzene,
tri(1,1-dimethylethyl)nitrobenzene, 2,4,6-{CH3C(CH3)2}3C6H2NO2. The standard molar
enthalpies of sublimation at T=298.15 K of the tri(1,1-dimethylethyl) compounds were measured
by microcalorimetry.
g
−DcHm°(cr)/(kJ·mol−1
)
DcrHm°/(kJ·mol−1
)
{2,6-(C2H5)2C6H3N(O)·}2
2,4,6-{CH3C(CH3)2}3C6H2NO
2,4,6-{CH3C(CH3)2}3C6H2NO2
11585.522.6
11019.323.7
10956.423.2
—
91.023.2
81.421.8
The standard molar enthalpy of decomposition of {2,6-(C2H5)2C6H3N(O)·}2 to form
2,6-(C2H5)2C6H3NO(g) at 298.15 K was measured by microcalorimetry:
decHm°/(kJ·mol−1) = (200.6 2 5.6). Application of group-additivity schemes applied
to nitrosobenzene and nitrobenzene derivatives shows that 2,4,6-
T
=
D
tri(1,1-dimethylethyl)nitrosobenzene is unstrained whereas the corresponding nitrocompound
shows considerable strain in accord with an X-ray structure analysis demonstrating that steric
hindrance prevents dimerization of the nitrosoderivative. 7 1995 Academic Press Limited
0021–9614/95/121433+08 $12.00/0
7 1995 Academic Press Limited

1434
W. E. Acree, Jr. et al.
1. Introduction
Crystalline aromatic C-nitroso compounds can be either white or green: when
white the compounds are dimeric and when green, monomeric.
Crystal-structure determinations of nitrosobenzene,(1) 4-bromonitrosobenzene,(2) and
2,4,6-tribromonitrosobenzene,(3) show these white compounds to be dimeric whereas
for 4-iodonitrosobenzene,(4) and 4-dimethylaminonitrosobenzene,(5) they show these
green compounds to be monomeric. Previous thermochemical studies showed that
substitution of the electron-donating dimethylamino group para to the nitroso group
results in exceptional stabilization of the monomer, making formation of the dimer
energetically unfavourable.(6)
The white dimeric forms melt to give green liquids and the vapour at high
temperatures and low pressures will decompose to the monomer, thus previous
thermochemcial measurements on 2,2',4,4',6,6'-hexamethylazobenzene N,N-dioxide
and 2,2',6,6'-tetramethylazobenzene N,N-dioxide led to values for the enthalpies of
formation of 2,4,6-trimethylnitrosobenzene and of 2,6-dimethylnitrosobenzene in the
gaseous state.(7) It is to be expected that increasing the size of the groups ortho to the
nitroso group would eventually result in prevention of dimerization.
In this paper we report the enthalpy of combustion of 2,2',6,6'-tetraethylazobenzene
N,N-dioxide:
a
the
white crystalline solid, and the enthalpy of its decomposition to
gaseous monomer: also the enthalpies of combustion of
2,4,6-tri(1,1-dimethylethyl)nitrosobenzene:
and of 2,4,6-tri(1,1-dimethylethylnitrobenzene:
together with the enthalpies of sublimation, to investigate whether the green
monomeric nitroso compound fails to dimerize due to stabilization of the monomer
or due to steric hindrance in formation of the dimer.

DfH°(nitrosobenzene and nitrobenzene derivatives,g)
1435
2. Experimental
2,2',6,6'-tetraethylazobenzene N,N-dioxide was prepared by oxidation of
2,6-diethylaniline using sodium tungstate plus hydrogen peroxide as described by
Stowell and Lau,(8) and purified by four crystallizations from (benzene+hexane).
2,4,6-Tri(1,1-dimethylethyl)nitrosobenzene was prepared by oxidation of 2,4,6-
tri(1,1-dimethylethyl)aniline with two equivalents of peroxybenzoic acid in
dichloromethane at the temperature 273 K,(9, 10) and purified by sublimation in vacuo.
2,4,6-Tri(1,1-dimethylethyl)nitrobenzene (Aldrich) was recrystallized four times from
methanol. Elemental analyses were in agreement with expected values: mass fractions
for C20H26N2O2: found: C, 0.7344; H, 0.0807; N, 0.0863; calculated: C, 0.7359; H,
0.0803; N, 0.0858, for C18H29NO: found: C, 0.7841; H, 0.1055; N, 0.0516; calculated:
C, 0.7849; H, 0.1061; N, 0.0509, and for C18H29NO2; found: C, 0.7431; H, 0.0998; N,
0.0472; C, 0.7418; H, 0.1003; N, 0.0481. The densities of the samples and the average
ratios of the mass of carbon dioxide produced by the sample to that calculated from
its mass with uncertainties of twice the standard deviation of the mean were:
C20H26N2O2: r/(g·cm−3) = 1.35, (1.0000 2 0.0001); C18H29NO2: r/(g·cm−3) = 0.85
(estimated), (0.999820.0001); C18H29NO2: r/(g·cm−3) = 0.85 (estimated),
(1.000120.0004).
Hexadecane (Aldrich, Gold Label) stored under nitrogen was used in some
experiments as an auxiliary combustion aid.
2,4,6-Tri(1,1-dimethylethyl)nitrobenzene was further characterized by X-ray
crystallography using an Enraf-Nonius CAD-4 diffractometer.(11) The results showed
that this molecule crystallizes in the hexagonal space group Pꢀ62c, but with a
considerable degree of disorder. Due to this disorder, a refined model for the structure
could not be derived but sufficient information was obtained to render some
clarification of the overall structure. The disorder was characterized as both
intramolecular and intermolecular. The benzene ring lies on a mirror plane (z=1/4),
perpendicular to a six-fold inversion axis (1/3, 2/3, 1/4). Every carbon position
therefore, is bonded to a 1/6 occupancy nitro group and a 1/2 occupancy
1,1-dimethylethyl moiety. Within the resolution of the model, the nitrogen and the
quaternary carbon atoms coincide. Six points of electron density connect to this
coincident point, corresponding to two coincident C/O ‘‘atoms’’ and four 1/4
occupancy carbons, all of which lie out of the aromatic plane. Thus the nitro group
adopts an orientation perpendicular to the benzene ring, and the 1,1-dimethylethyl
groups adopt one of two orientations that place the carbons at dihedral angles of −p/6,
p/2, and −5·p/6 with respect to the ring. The intermolecular disorder consists of each
O/C position lying on a three-fold axis (2/3, 1/3, z; and 0, 0, z). The overall structure,
therefore, consists of numerous peaks of partial and/or mixed occupancies. Germane
to this discussion, however, is the fact that all peaks could be located through both
direct and Fourier-difference methods (if not refined), and none of the attached group
peaks lies in the same plane as the aromatic ring.
The enthalpy of combustion of 2,2',6,6'-tetraethylazobenzene N,N-dioxide
was measured using the Manchester static-bomb calorimeter,(12, 13) whereas
2,4,6-tri(1,1-dimethylethyl)nitrosobenzene and 2,4,6-tri(1,1-dimethylethyl)nitrobenzene

1436
W. E. Acree, Jr. et al.
were measured using the Porto static-bomb calorimeter.(14, 15) The energy equivalent of
the Manchester bomb calorimeter was determined from the combustion of benzoic
acid (NBS SRM 39i) having a massic energy of combustion under standard bomb
conditions of (2643423) J·g−1. From 12 calibration experiments, o(calor) =
(15525.4020.47) J·K−1 where the uncertainty quoted is the standard deviation of
the mean for an average mass of water added to this calorimeter of 2897.0 g. The Porto
bomb calorimeter was calibrated using benzoic acid (Bureau of Analysed Samples,
CRM 190p) having a massic energy of combustion under standard bomb
conditions of (26431.823.7) J·g−1. From nine calibration experiments, o(calor)=
(15911.221.5) J·K−1 for an average mass of water added to this calorimeter of
3119.6 g.
The experimental conditions were similar for both calorimeters. Samples, in pellet
form, were ignited at T=(298.15020.001) K in oxygen at a pressure p=3.04 MPa
with a volume 1 cm3 of water added to the bomb. The electrical energy for ignition
was determined from the change in potential difference across a capacitor when
discharged through the platinum ignition wire. For the cotton-thread fuse, empirical
formula CH1.686O0.843, −Dcu°=16250 J·g−1.(16) For the combustion auxiliary aid,
hexadecane, from separate measurements, −Dcu° = (47156.921.2) J·g−1.
Corrections for nitric-acid formation were based on −59.7 kJ·mol−1 for the molar
energy of formation of 0.1 mol·dm−3 HNO3(aq) from N2, O2, and H2O(l).(17) The
amount of substance used in each experiment was determined from the total mass of
carbon dioxide produced after allowance for that formed from the cotton-thread fuse
and hexadecane. For each compound (1u/1p)T at T=298.15 K was assumed to be
−0.2 J·g−1·MPa−1, a value typical for most organic solids. For each compound, Dcu°
was calculated by the procedure given by Hubbard et al.(16) The molar masses used for
the elements were those recommended by the IUPAC Commission.(18)
The standard molar enthalpies of sublimation of 2,4,6-{CH3C(CH3)2}3C6H2NO
and 2,4,6-{CH3C(CH3)2}3C6H2NO2 and the enthalpy of decomposition of
{2,6-(C2H5)2C6H3N(O)·}2 were measured by the ‘‘vacuum sublimation’’
drop-microcalorimetric method.(19) Samples (of mass about 5 mg) of each compound
TABLE 1. Typical combustion results (p°=0.1 MPa)
{2,6-(C2H5)2C6H3N(O)·}2 2,4,6-{CH3C(CH3)2}3C6H2NO 2,4,6-{CH2C(CH3)2}3C6H2NO2
m(CO2,total)/g
m(cpd)/g
m(hexadecane)/g
m(fuse)/g
2.75718
1.02087
—
0.00281
2.33701
14.8
1.79155
0.62118
—
0.00306
1.56401
16.5
1.69334
0.32944
0.25473
0.00355
1.53647
DTad/K
of /(J·K−1
)
16.6
Dm(H2O)/g
0.0
36316.5
−0.4
24907.4
28.1
−0.4
24469.0
26.6
1.0
8.3
12012.1
57.7
37531.3
−DU(IBP)/J
DU(HNO3)/J
DU(ign)/J
59.0
1.1
18.6
—
45.6
1.1
10.1
—
49.7
DUS/J
−mDcu°(hexadecane)/J
−mDcu°(fuse)/J
−Dcu°(cpd)/(J·g−1
)
35453.4
39955.4

DfH°(nitrosobenzene and nitrobenzene derivatives,g)
1437
contained in a small thin glass capillary tube sealed at one end were dropped at room
temperature into the hot reaction vessel in the Calvet High-Temperature
Microcalorimeter and then removed from the hot zone by vacuum sublimation. The
observed standard molar enthalpies of sublimation {Hm°(g,T)−Hm°(cr,298.15 K} were
T
corrected to T=298.15 K using D298.15 KHm°(g) estimated by a group method based on
the values of Stull et al.(20) The observed enthalpy of decomposition of
{2,6-(C2H5)2C6H3N(O)·}2(cr), {2Hm°(monomer,g,T)−Hm°(cr,dimer,298.15 K)}, was
corrected to T=298.15 K in a similar fashion. For this decomposition, the calorimeter
was held at a sufficiently high temperature and this temperature was varied to ensure
that the decomposition was complete. The microcalorimeter was calibrated in situ by
making use of the reported molar enthalpy of sublimation of naphthalene, C10H8:
(72.5120.01) kJ·mol−1.(21)
3. Results
Results for a typical combustion experiment on each compound are given in table 1:
Dm(H2O) is the deviation of the mass of water added to the calorimeter from the
mass assigned for o(calor); DUS is the correction to the standard state; the
remaining terms are as previously described.(16) As samples were ignited at
T=(298.15020.001) K:
DU(IBP)=−{o(calor)+cp(H2O, l)·Dm(H2O)+of}DTad+
DU(ign); where DTad is the calorimeter temperature change corrected for heat
exchange and the work of stirring. The individual values of −Dcu° together with the
mean and its standard deviation are given in table 2. Table 3 lists the derived standard
molar enthalpies of combustion and of formation in the condensed and gaseous
states. In accordance with normal thermochemical practice, the uncertainties
assigned to the standard molar enthalpies of combustion and formation are twice the
overall standard deviation of the mean and include the uncertainties in calibration
and in the auxiliary quantities used. To derive DfHm° from DcHm° the standard molar
enthalpies of formation: for H2O(l): −(285.8320.04) kJ·mol−1 and for CO2(g):
−(393.5120.13) kJ·mol−1 were used.(22)
TABLE 2. Values of −Dcu° at T=298.15 K (p°=0.1 MPa)
{2,6-(C2H5)2C6H3N(O)·}2
2,4,6-{CH3C(CH3)2}3C6H2NO
2,4,6-{CH3C(CH3)2}3C6H2NO2
−Dcu°/(J·g−1
39939.4
39963.5
39937.3
39944.2
)
35456.9
35443.7
35453.4
35455.3
35465.7
35463.9
37558.1
37551.9
37538.7
37556.3
37544.9
37536.1
37548.2
37546.4
37531.3
39965.2
39955.4
−ꢀDcu°ꢁ/(J·g−1
39950.825.0
)
35456.523.2
37545.823.0

1438
W. E. Acree, Jr. et al.
TABLE 3. Derived standard molar values at T=298.15 K (p°=0.1 MPa)
g
−DcUm°(cr)
−DcHm°(cr)
DfHm°(cr)
DcrHm°
DfHm°(cr)
kJ·mol−1
kJ·mol−1
kJ·mol−1
kJ·mol−1
kJ·mol−1
{2,6-(C2H5)2C6H3N(O)·}2
2,4,6-{CH3C(CH3)2}3C6H2NO
2,4,6-{CH3C(CH3)2}3C6H2NO2
11574.322.6
11003.823.7
10942.123.2
11585.522.6
11019.323.7
10956.423.2
−0.523.8
−208.424.4
−271.324.0
200.625.6 a
91.023.2
81.421.8
−117.425.4
−189.924.4
a Corresponds to dimer(cr)=2·monomer(g).
4. Discussion
From the molar enthalpies of formation and decomposition of
{2,6-(C2H5)2C6H3N(O)·}2: DfHm°{2,6-(C2H5)2C6H3NO,g}=(100.123.4) kJ·mol−1 is
derived. In table 4, the observed DfHm°(g) values for C6H5NO derivatives are compared
with those calculated using the Cox scheme,(23) in which each group is associated with
a characteristic increment in DfHm°(g) when substituted into the benzene ring. Cox
proposed corrections for steric hindrance between neighbouring groups but no such
corrections have been made here. To derive the calculated values, an average increment
for substitution of the NO group into C6H6 of (122.422.2) kJ·mol−1 was used together
with the following values of DfHm°(g)/(kJ·mol−1): C6H6, (82.620.7);(24) 1,3-(CH3)2C6H4,
(17.3 2 0.8);(24)
1,3,5-(CH3)3C6H3, −(15.9 2 1.3);(24)
1,3-(C2H5)2C6H4,
−(21.8 2 [2.5]);(25) C6H5N(CH3)2, (100.5 2 4.7).(24) DfHm°(g)/(kJ·mol−1) for
CH3C(CH3)2C6H5: −(22.621.2),(24) was used to estimate DfHm°(g)/(kJ·mol−1) of
1,3,5-{CH3C(CH3)2}3C6H3: −(233.023.7). From the comparison in table 4 it is clear
TABLE 4. Observed and calculated DfHm°(g) values for nitrosobenzene derivatives
DDfHm°
DfHm°(g)/(kJ·mol−1
)
kJ·mol−1
observed
calculated
C6H5NO
209.828.0 (7)
139.821.6 (7)
107.421.9 (7)
100.123.4
205.022.3
139.722.3
106.522.6
100.62[3.3]
−110.624.3
222.925.2
4.828.3
0.122.8
0.923.2
2,6-(CH3)2C6H3NO
2,4,6-(CH3)3C6H2NO
2,6-(C2H5)2C6H3NO
2,4,6-{CH3C(CH3)2}3C6H2NO
4-(CH3)2NC6H4NO
−0.52[4.7]
−6.826.9
−37.925.7
−117.425.4
185.022.3 (6)
TABLE 5. Observed and calculated DfHm°(g) values for nitrobenzene derivatives
DDfHm°
DfHm°(g)/(kJ·mol−1
)
kJ·mol−1
observed
calculated
4-CH3C6H4NO2
2,6-(CH3)2C6H3NO2
2-(C2H5)C6H4NO2
4-(C2H5)C6H4NO2
2,4,6-{CH3C(CH3)2}3C6H2NO2
31.023.8 (24)
8.621.6 (7)
35.321.1
2.221.2
14.821.4
−4.324.0
6.422.0
−3.626.7
−7.426.7
58.225.8
11.226.6 (24)
7.426.6 (24)
14.821.4
−189.924.4
−248.123.8

DfH°(nitrosobenzene and nitrobenzene derivatives,g)
1439
that within the limits of uncertainty, 2,4,6-tri(1,1-dimethylethyl)nitrosobenzene has an
expected enthalpy of formation thus showing neither strain nor stabilization energy.
A similar exercise for C6H5NO2 derivatives is presented in table 5: to derive
the calculated values the following additional DfHm°(g)/(kJ·mol−1) values were
used: C6H5NO2, (67.520.6);(24) C6H5CH3, (50.420.6);(24) C6H5C2H5, (29.921.1).(24)
From the comparison in table 5 it is clear that 2,4,6-tri(1,1-dimethylethyl)nitrobenzene
exhibits very large steric-strain energy.
If 2,4,6-tri(1,1-dimethylethyl)nitrosobenzene were to dimerize, it would seem
reasonable to assume that the steric-strain energy in the dimer would be at least
twice that shown by the nitroderivative, i.e. 1116 kJ·mol−1. As Dm(N.N) in
C6H5N(O)·N(O)·C6H5 is (91.128.5) kJ·mol−1,(7) it is clear that steric hindrance
prevents formation of the dimer of 2,4,6-{CH3C(CH3)2}3C6H2NO in contrast to the
case of 4-N(CH3)2C6H4NO where exceptional stabilization of the monomer prevents
dimerization.(6)
The determination of Dm(N-O) in
RNO2 = RNO + O, requiring DfHm°(O,g) = (249.17 2 0.10) kJ·mol−1.(22) For
marked
a
nitrocompound is DrHm°(g) for
2,4,6-{CH3C(CH3)2}3C6H2NO2, Dm(N-O) = (321.827.0) kJ·mol−1,
a
reduction from that in C6H5NO2 of (391.528.0) kJ·mol−1.(7) This shows the very large
effect of steric hindrance of the two 1,1-dimethylethyl groups ortho to the nitro group as
the X-ray analysis has shown that the nitro group has been forced from its preferred
orientation planar with the benzene ring to be perpendicular to the ring, hence
eliminating any stabilization effect of p-electron delocalization between the nitro group
and the benzene ring.
M.D.M.C.R.S. and M.A.R.M. thank Junta Nacional de Investigacao Cientıfica e
Tecnologica for the research project (STRDA/C/CEN/519/92) and for financial
support to the research group of Centro de Investigacao em Quımica of Universidade
do Porto (CIQ/L5).
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