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Supplement F2: The Chemistry of Amino, Nitroso, Nitro and Related Groups.

Edited by Saul Patai Copyright 1996 John Wiley & Sons, Ltd.

ISBN: 0-471-95171-4

CHAPTER 8

Thermochemistry of amines, nitroso compounds, nitro compounds and related speciesŁ

JOEL F. LIEBMAN

Department of Chemistry and Biochemistry, University of Maryland Baltimore County, 5401 Wilkens Avenue, Baltimore, Maryland 21228-5398, USA

Fax: 410-455-2608; e-mail: JLIEBMAN@UMBC2.UMBC.EDU

MARY STINECIPHER CAMPBELL

DX-2 Explosive Science Technology, MS C920, Los Alamos National Laboratory, Los Alamos, New Mexico 87545-0000, USA

Fax: 505-667-0500; e-mail: MSCAMPBELL@LANL.GOV

and

SUZANNE W. SLAYDEN

Department of Chemistry, George Mason University, 4400 University Drive, Fairfax, Virginia 22030-4444, USA

Fax: 703-993-1055; e-mail: SSLAYDEN@GMU.EDU

I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

338

A. Definition of Thermochemistry . . . . . . . . . . . . . . . . . . . . . . . . . .

338

B. Classes of Compounds to be Discussed . . . . . . . . . . . . . . . . . . . . .

339

II. CORRELATIONS OF ENTHALPIES OF FORMATION

 

OF ALKYL NITROGEN-CONTAINING COMPOUNDS . . . . . . . . . . .

339

III. ALIPHATIC AND ALICYCLIC MONOAMINES . . . . . . . . . . . . . . . .

343

A. Acyclic Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

343

B. Alicyclic Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

345

Ł Contribution from the US government. Not subject to copyright.

337

338

Joel F. Liebman, Mary Stinecipher Campbell and Suzanne W. Slayden

 

IV. ANILINES AND OTHER AROMATIC AMINES . . . . . . . . . . . . . . . .

348

 

A. Aniline and N-Alkylated Anilines . . . . . . . . . . . . . . . . . . . . . . . .

348

 

B. Primary Aromatic Amines Containing more than

 

 

One Benzenoid Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

349

 

C. Secondary and Tertiary Aromatic Amines . . . . . . . . . . . . . . . . . . .

350

V. POLYAMINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

350

 

A. Aliphatic Diamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

351

 

B. Alicyclic Diamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

352

 

C. Aromatic Diamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

353

 

D. Alicyclic Triamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

354

 

E. Tetramines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

355

VI. NITROSO COMPOUNDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

357

 

A. Nitrosobenzene and Its Methylated Derivatives . . . . . . . . . . . . . . . .

357

 

B. Nitrosobenzene and Amino Substitution . . . . . . . . . . . . . . . . . . . .

358

 

C. Nitrosoarenols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

358

 

D. Aliphatic Nitroso Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . .

360

VII. AROMATIC NITRO COMPOUNDS . . . . . . . . . . . . . . . . . . . . . . . .

361

 

A. The Roles of Resonance and Steric Effects: Molecular Families

 

 

and Reference States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

361

 

B. Nitrobenzenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

362

 

C. Nitroanilines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

364

 

D. Nitrotoluenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

366

VIII. NITRO COMPOUNDS AS EXPLOSIVES . . . . . . . . . . . . . . . . . . . . .

368

 

A. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

368

 

B. Experimental Determination of the Enthalpy of Formation

 

 

of Explosives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

369

 

C. Sensitivity of Explosives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

370

IX. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

371

I.INTRODUCTION

A.Definition of Thermochemistry

As has been our practice in other thermochemistry chapters in the ‘Patai series’1, we commence our discussion by defining key concepts. The word ‘Thermochemistry’ is taken here to mean ‘Enthalpy of Formation’2 (or, more colloquially, ‘Heat of Formation’), and will be written Hf in lieu of the more proper but much less widely usedfH°m. Enthalpy and energy will dominate our discussion; heat capacity, entropy and gibbs energy3 will be all but ignored. To maximize understanding of fundamental molecular phenomena requires us to minimize complications from intermolecular interactions and so we wish to present data for gas-phase species under the ‘standard’ conditions of 25 °C (298 K) and 1 atmosphere. However, most of the species in this chapter like the vast majority of organic compounds are ‘naturally’ liquid or solid under conventional laboratory and thermochemically idealized conditions alike. We will make use of ancillary quantities such as experimentally measured phase-change enthalpies to ‘correct’ the state of the species. When these phase-change enthalpies are unavailable from experiment, enthalpies of vaporization will often be estimated4, and enthalpies of fusion5 measured at the melting point will be accepted without correction to 298 K. Enthalpies of sublimation are likewise taken from Reference 6 without any correction. We will also often find it necessary to use enthalpies of formation without any correction to the gaseous state because both measurements and reliable estimated phase-change enthalpies are absent.

8. Thermochemistry of amines, nitroso, nitro compounds and related species 339

B. Classes of Compounds to be Discussed

The two primary classes of species to be discussed in this chapter are amines, namely those compounds of the generic formula RR1R2N, where Ri is either hydrogen or hydrocarbyl and nitro compounds, RNO2. Not all types of amines, nitro compounds and their derivatives will be discussed here. No mention will be made of the theromochemistry of imines, N-amino and N-acyl derivatives; discussion of mutiply bonded nitrogen, hydrazines and amides belongs elsewhere. The thermochemistry of enamines will also be omitted, having been the subject of a recent review7, and derivatives of pyrrole and indole will be ignored because of their aromaticity. Discussions of nitro compounds are limited to nitro aromatic compounds. Following the precedent of the earlier relevant chapter in the Patai series8 we also include a brief treatment of nitrites, RONO, and nitrates, RONO2 to accompany our discussion of nitro compounds. Nitroso compounds will also be discussed.

II. CORRELATIONS OF ENTHALPIES OF FORMATION OF ALKYL

NITROGEN-CONTAINING COMPOUNDS

It has been firmly established that thermochemical properties of a homologous series of compounds show a linear dependence on the number of carbon atoms in the alkyl group1,9 11. Especially useful is equation 1 which expresses the standard molar enthalpies of formation of a homologous series as a function of the total number of carbon atoms in the compound, nc.

Hf°(l) D ˛lÐnc C ˇl

(1a)

Hf°(g) D ˛gÐnc C ˇg

(1b)

Hf°(l,g) D ˛Ðnc C ˇ

(1c)

In general, the function is unreliable12 for nc D 1. Ideally, the constants in equation 1 are derived from several members of the homologous series for which nc 4. Only occasionally, however, have enough experimental data been available to fulfill this condition. In the present case, few nitrogen-containing compounds for which there are gaseous enthalpy-of-formation data possess more than four carbon atoms in a hydrocarbyl substituent. Nonetheless, even restricted data sets may yield useful information. The numerical values in Table 1 were produced by applying the method of weighted least squares to the measured enthalpy of formation data for alkyl amines, nitrites, nitrates and nitro compounds.

The slopes, ˛g, for various n-alkyl substituted homologous series are commonly com-

pared to the ‘universal’ slope (methylene increment) of

 

20.6 kJ mol 1

calculated for

the n-alkanes

9

 

 

the methylene

 

. An unanswered question is whether for ‘large enough’ nc

increment should be identical for all functionalized alkanes. In previous studies it is shown that the slopes vary, although not too widely (ca š2 kJ mol 1), and there is no discernible relationship between the functional group and ˛. With three exceptions, the slopes reported in Table 1 are in line with those calculated for other functional group series.

The slope for the gaseous 2-nitroalkanes is unusually steep, rivaled only by an identical slope for di-n-alkylsulphoxides1. There are two other indications that at least one of the two data points may be incorrect. Generally, isomerization of an n-alkyl to a sec-alkyl or tert-alkyl substituent results in a slightly less negative slope, but for the nitroalkanes the opposite trend is seen in Table 1. Also, the slopes for liquid and gaseous phases must be different for any homologous series of compounds (a manifestation of the increasing enthalpy of vaporization), but for the 2-nitroalkanes, they are essentially identical.

340 Joel F. Liebman, Mary Stinecipher Campbell and Suzanne W. Slayden

TABLE 1. Constants from the least-squares analysis of equation 1 for nitrogen-containing alkanesa (kJ mol 1)

 

 

H°(l)

 

 

H°(g)

 

 

f

 

 

 

f

Homologous series

 

˛l

ˇl

 

˛g

ˇg

Primary amines

27.1

20.0

22.4

2.8

n-RNH2 [C2-C4]

sec-RNH2 [C3, C4]

25.2

36.7

21.1

20.5

Secondary amines

25.4

3.4

20.8

 

n-R2NH [C2-C4]

9.14

Tertiary amines

25.4

 

22.7

 

n-R3N [C2-C4, C6, C8-C10 (l); C2-C3 (g)]

23.8

43.6

Diamines

22.5

30.3

20.4

 

1,2-(NH2)2R [C3-C4]

7.6

Nitrites

31.8

55.2

22.2

55.5

n-RONO [C3, C4 (1); C2-C4 (g)]

sec-RONO [C3-C4]

23.9

92.7

19.2

75.9

tert-RONO [C4-C5]

19.2

129.1

20.1

91.1

Nitro compounds

23.8

96.1

21.1

60.2

n-RNO2 [C2-C5 (l); C2-C4 (g)]

sec-RNO2 [C3, C4, C10 (l); C3, C4 (g)]

24.4

107.6

24.7

64.8

Dinitro compounds

22.9

100.5

 

 

 

1,1-(NO2)2R [C2, C3, C5]

 

 

 

1,ω-(NO2)2R [C3-C4]

36.2

96.2

 

 

 

Nitrates

24.1

142.2

19.8

114.5

n-RONO2 [C2, C3]

a In the least-squares analyses of equation 1, the individual enthalpies were weighted inversely as the squares of the experimental uncertainty intervals, except for the nitrites for which some errors were not reported.

Omitting the liquid-phase enthalpy of formation for 2-nitrodecane gives a slope of 27.2, which is somewhat larger. This third enthalpy of formation for the liquid phase probably mitigates the effect of the incorrect value in the regression analysis.

There are problems with the enthalpy of formation data for the n-alkyl and tert-alkyl nitrite families. The values used in this work were taken from the review by Batt and Robinson8, but only the value for methyl nitrite appears in Pedley. Batt and coworkers determined all (except ethyl and t-pentyl nitrite) the other liquid-phase enthalpies of formation and of vaporization, except the enthalpy of vaporization for isopropyl nitrite was calculated. Although the gaseous enthalpy-of-formation data for the other series in Table 1 form reasonably straight lines, it is evident from inspection of a graphical plot that at least one of the data points for the n-alkyl nitrites is incorrect. The ˛g of equation 1, calculated using only the more recent n-propyl and n-butyl nitrite data, is 26.8 which is much too steep to be credible, while ˛g calculated from ethyl and n-propyl nitrites is 17.5, also an incredible value. The liquid-phase slope in Table 1, determined from only the n-propyl and n-butyl nitrites, is also much steeper than any other known slope. The slope calculated from only the gaseous ethyl and n-butyl nitrites is 22.2, essentially identical to ˛g, the slope of the best straight line in Table 1. Interpolating a value for the enthalpy of formation of n-propyl nitrite gives 123.5 kJ mol 1 which is slightly outside the reported error for the experimental value of 118.8 š 4.2 kJ mol 1. Finally, a comparison of the gaseous enthalpies of formation for the 1-nitroalkanes with the corresponding nitrites shows that the values for the ethyl and n-butyl derivatives are virtually identical while those of the

8. Thermochemistry of amines, nitroso, nitro compounds and related species 341

n-propyl compounds differ by 5 kJ mol 1. The value derived above for n-propyl nitrite now compares with the experimental value of 123.8 kJ mol 1 for 1-nitropropane. Thus, the enthalpy of formation for n-propyl nitrite is most likely the incorrect value.

One of the two enthalpies of formation for the tertiary alkyl nitrites is most probably incorrect. As seen in Table 1, not only is the slope for the liquid-phase data less negative than that for the gas-phase data, but it is much less steep than all other liquid-phase data of which we know. Part of the discrepancy is due to the inverted order of the

archival enthalpies of vaporization: t-butyl nitrite D 34.4 kJ mol 1 and t-pentyl nitrite D 33.5 kJ mol 1.

We would like to extrapolate equation 1 to generate enthalpy-of-formation values for the higher homologs in each of the nitrogen-containing families. The constancy of ˛ and ˇ values for restricted or expanded data sets can be explored using homologous series for which there are extensive data, such as n-alkanes, n-alkanols, n-alkanethiols and n-alkyl chlorides and bromides. For each series, constants were calculated from all experimental enthalpies of formation for nc 4 and from experimental enthalpies of formation for nc D 2 4 only, the most commonly encountered restricted data set. When the slopes for both sets in a series are compared, the change is variable, ranging from almost no change for the n-alkanols (0.2 kJ mol 1) to a quite large change for the n-alkyl bromides (2.3 kJ mol 1)12. In all cases the smaller data set produces a slope which is more negative and an intercept which is less negative. Whatever the uncertainty about the accuracy of the enthalpy of formation derived from an extrapolation using the smaller data set, it is probable the result is at least a lower limit.

Examination of plots of the enthalpies of formation of the various nitrogen-containing homologous series in the gaseous or liquid phase reveals several features. Selected data are shown in Figure 1. The enthalpies of formation become more negative as the oxygen content of the functional group increases (the nitrite and nitro families are nearly co-linear). Compared to the corresponding hydrocarbons, the enthalpies of formation of amines are less negative and the enthalpies of formation of the vicinal diamino alkanes are yet less negative. The enthalpies of formation of all the oxygen nitrogen functionalized alkanes are more negative than the corresponding alkanes and the enthalpies of formation of the 1,ω-dinitro alkanes are more negative than either (there are seemingly no gas-phase

Enthalpy of Formation (kJmol 1)

10.0

30.0

50.0

70.0

90.0

110.0

130.0

150.0

170.0

190.0

n-RNH2

n-RNO2 n-RONO2

1

2

3

4

Number of Carbon Atoms

FIGURE 1. Enthalpies of formation of nitrogen-containing compounds (g)

342 Joel F. Liebman, Mary Stinecipher Campbell and Suzanne W. Slayden

data). Among primary, secondary and tertiary amines containing the same total number of carbons as n-alkyl groups, primary amines have the most negative and tertiary amines the least negative enthalpy of formation. Amines thus behave similarly to alcohols and ethers in that alcohols have more negative heats of formation than the isomeric ethers.

It is readily apparent that the enthalpy of formation of the methyl derivative in each of the n-alkyl series in Table 1 deviates from the otherwise apparently linear relationship established by the higher members of the series12. This ‘methyl effect’ is well-known, if little understood. In every case here, the experimental methyl compound enthalpy is less negative than that derived from the appropriate regression constants. It has been shown that the magnitude of the methyl deviation generally increases as the electronegativity of the attached heteroatom increases in both the gaseous and liquid phases13,14. In the present case, the primary, secondary and tertiary methyl amines deviate less than nitromethane, which deviates less than methyl nitrate and methyl nitrite. (If the gaseous enthalpies of formation of only n-propyl and n-butyl nitrite are considered, the methyl nitrite deviation from the extrapolated line is 1.2 kJ mol 1 in the ‘wrong’ direction, a further indication of the incorrectness of one of these values.) The increase in deviation with oxygen content and with proximity of oxygen to methyl accords with both our understanding of an electronegativity effect on the methyl deviation as well as our prior experience with sulphur and sulphur oxygen containing compounds1.

Because the enthalpies of formation of a homologous series correlate with the number of carbon atoms according to equation 1, the enthalpies of formation of any one series must correlate with the enthalpies of formation of any other series with like nc, as in

equation 2.

 

 

 

 

 

 

 

 

 

H° (RZ)

D

˛

z,z0

H° RZ0

 

C

ˇ

z,z0

2

f

 

f

 

 

 

An informative comparison for the nitrogen-containing compounds is with the corresponding hydrocarbons, either R H or R CH3.

A typical plot of equation 2 is shown for RNH2 versus R CH3 in Figure 2. As also observed from plots of equation 1, the methyl group is destabilizing relative to the n-alkyl group and the sec-alkyl groups are stabilizing. We know from our data compendia that t-butyl amine and neopentane are more stable than their respective isomers and we can now observe that fact graphically. Only for the case where Z is a relatively electropositive

Enthalpy of Formation of RNH2 (gas, kJ mol1)

20

 

 

 

 

Me

 

40

 

 

Et

 

 

 

60

 

 

n-Pr

 

 

 

 

 

 

 

 

 

80

 

n-Bu

 

 

 

 

 

 

i-Pr

 

 

 

100

 

i-Bu

 

 

 

 

 

 

 

 

 

120

 

sec-Bu

 

 

 

 

t-Bu

 

 

 

 

 

 

 

 

 

 

 

140

165

155 145

135 125 115 105

95

85

75

175

Enthalpy of Formation of RCH3 (gas, kJ mol1)

FIGURE 2. Enthalpies of formation of alkyl amines vs alkyl methanes

8. Thermochemistry of amines, nitroso, nitro compounds and related species 343

atom such as Li is the effect of branching in R on enthalpies of formation reversed15, in keeping with the differences in bond polarity Rυ ZυC and RυC Zυ . The data point for R D isobutyl lies on the line for R D n-alkyl. Plotting equation 2 for alcohols or other larger data sets shows that this is a general result. This provides a method for deriving the enthalpies of formation of primary isoalkyl compounds for which experimental data are lacking. For example, the gaseous enthalpies of formation for isobutyl nitrate and 2-methyl-1-nitropropane are calculated to be 201 and 151 kJ mol 1, respectively. In a plot of equation 2 for primary R ONO versus RCH3 (R D Et, n-Pr, n-Bu, i-Bu), the best straight line does not include the n-propyl nitrite. Again it appears that this is a discrepant enthalpy of formation. Calculating an enthalpy of formation for n-propyl nitrite

(g) from the constants derived from equation 2 (obviously excluding R D n-Pr) gives a value of ca 123 kJ mol 1. This compares favorably with the value derived above from equation 1.

A plot of secondary amines, RR0 NH, versus the corresponding hydrocarbon, RR0 CH2, is instructive. Compared to the straight line established by the symmetrical di-n-alkyl species (R, R0 D Et, n-Pr, n-Bu), the data point for the dimethyl species is above the line and hence the amine appears destabilized. The di-isopropyl, isopropyl tert-butyl, and di-tert-butyl species appear stabilized, as expected. However, among the latter three, the di-tert-butyl species is not stabilized nearly as much as either the di-isopropyl or the mixed isopropyl tert-butyl species, and the mixed secondary/tertiary species is slightly less stabilized than the di-secondary, suggesting that the steric effect of the tertiary group is important. The overall effect when R D Me and R0 D tert-Bu is destabilizing compared to the n-alkyl line. Evidently the destabilizing effect of methyl (combined perhaps with a destabilizing steric effect) dominates over a stabilizing tertiary group.

III. ALIPHATIC AND ALICYCLIC MONOAMINES

As part of recent reviews of the thermochemistry of oxygenand sulphur-containing functional groups1, we found investigation of the energies associated with ‘exchange’ particularly useful16. For example, recall the ‘ether/methylene exchange’ energy quantity υ3 defined by

υ3(R, R1) υ3(g, RCH2R1, ROR1) D Hf(g, RCH2R1) Hf(g, ROR1)

3

and also the analogously defined ‘thioether/methylene’, ‘alcohol/methyl’ and ‘thiol/methyl exchange’ quantities. The rationale for this procedure is readily apparent from equation 1. The exchange quantity υ Hf for two compounds from homologous series with identical slopes and with the same nc equals the difference of the intercepts (υˇ), that is, a constant. For the oxygenand sulphur-containing compounds and the corresponding alkanes, the slopes differ only slightly and the approach is justified. Inspection of Table 1 shows that some of the nitrogen-containing families have values considerably different from the alkanes and thus the individual exchange quantities are expected to differ accordingly. However, because there are limited data available for nitrogen-containing compounds belonging to homologous series and because we need to establish a basis for comparing more structurally complex compounds in later sections, the exchange analysis will be performed and the limitations noted.

A. Acyclic Amines

We distinguished between alcohols and ethers and between thiols and thioethers, and so we will consider the three relevant exchange quantities for amines, one apiece for primary, secondary and tertiary species. For each exchange reaction, all species are in the

344

Joel F. Liebman, Mary Stinecipher Campbell and Suzanne W. Slayden

 

same physical state.

 

 

 

υ4( prim/ R) υ4

(RMe, RNH2) D Hf(RMe) Hf(RNH2)

(4)

 

υ5(sec/ R, R1) υ5

(RCH2R1, RNHR1) D Hf(RCH2R1) Hf(RNHR1)

(5)

υ6(tert/ R, R1, R2) υ6

(RR1R2CH, RR1R2N) D Hf(RR1R2CH) Hf(RR1R2N) (6)

For the exchange increments involving oxygen and sulphur, the individual hydrocarbyl or Ri groups were divided into four classes: methyl, primary, secondary and tertiary. Although perhaps desirable for conceptual understanding or numerical accuracy, we lack the data to differentiate amines into all of the obvious classes. For example, we have no enthalpy-of-formation value for a secondary amine with affixed primary and secondary alkyl groups such as ethyl isopropyl amine. Likewise, we lack data for most tertiary amines except for trimethyl and tri-n-alkyl amines. While there are plentiful data for the liquid tri-n-alkyl amines, there are no data for most of the corresponding hydrocarbons. The presence of aromatic rings interacting with the amino functionality introduces new complexities and discussion of these species is deferred to another section of this chapter.

Numerical values for the various amine/hydrocarbon difference quantities, υ4( prim/ R), υ5(sec/ R, R1) and υ6(tert/ R, R1, R2) calculated using all available gas-phase data, and some estimated data, are shown in Table 2. υ4( prim/ prim) is derived from the Hf data for ethylamine, 1-propylamine and 1-butylamine; from isopropyl and sec-butylamine, we find υ4( prim/ sec); and finally, for υ4( prim/ tert), tert-butylamine is the sole representative species. The monotonic increase of the four υ4 values is smaller than the regular ca 10 kJ mol 1 increase for the corresponding alcohol/methyl exchanges, but is certainly greater than the essentially unvarying thiol/methyl exchanges1.

Consider now υ5(sec/ R, R1). All four exchange energies in which R D R1 can be calculated. There are three values for which the substituent groups are primary: Et, n-Pr and n-Bu. The data available for secondary R groups are limited to i-Pr while those for tertiary R groups are limited to t-Bu. Increased branching in the alkyl group attached to nitrogen makes the exchange energy more positive in the order dimethyl < di-primary < di-secondary just as it does for oxygen and sulphur ethers. Unlike those ethers though, for which the exchange value further increases with di-tertiary substitution, the exchange energy for the di-tertiary amine decreases considerably. The strain energy of di-t-butyl species has confused and bemused us before1. We know of no relevant amine data for several of the remaining possible combinations of υ5(sec/ R, R1). Interestingly, all of the υ5(sec/ R, R1) except R D R1 D t-Bu are roughly consistent with a constant second

TABLE 2. Enthalpy-of-formation differences between gaseous primary, secondary and tertiary amines and the corresponding hydrocarbons (kJ mol 1)

R

 

Methyl

Primary

Secondary

Tertiary

 

 

 

 

 

 

υ4

( prim/ R)

60

56

50

47

υ5

(sec/ R,R1)

86

 

 

78

Methyl

?

?

Primary

 

73

?

?

Secondary

 

 

58

59

Tertiary

 

 

 

70

υ6

(tert/ R,R1,R2)

111

 

 

 

Methyl, methyl

93

 

 

Primary, primary

 

 

 

8. Thermochemistry of amines, nitroso, nitro compounds and related species

345

difference, υυ7(R, R1), of ca 38 š 4 kJ mol 1.

 

υυ7(R, R1) D υ4( prim/ R) C υ4( prim/ R1) υ5(sec/ R, R1)

7

There remain two acyclic secondary amines for which there are no experimental enthalpies of formation for the corresponding hydrocarbons: butyl isobutyl amine and diisobutyl amine. These missing enthalpies of formation are easily estimated using an accurate group additivity scheme17. A complete listing of individual υ5(sec/ prim, prim) values, including the newly estimated ones, is: Et, Et ( 74.7); Pr, Pr ( 71.6); n-Bu, n-Bu ( 71.6); Bu, i-Bu ( 65.8); and i-Bu, i-Bu ( 64.3). Hoping that the exchange quantities for the n-alkyl species are constant, we might think that υ5(sec/ Et, Et) is discrepant. However, after examining the exchange quantities and the enthalpies of formation of the isomeric dibutyl species we are led to conclude that the measured enthalpy of formation of di-n-butyl amine is probably incorrect. The isomerization of n-BuNH2 to i-BuNH2 is 6.9 kJ mol 1 and isomerization of n-butyl isobutyl amine to di-isobutyl amine is8.2 kJ mol 1. These values are typical for isomerization of butyl groups attached to Cl, OH, O , SH, S and SS , the largest of which is 9.2 kJ mol 1 for isomerization of thiols1. Thus, the isomerization value for dibutyl amine to butyl isobutyl amine of 14.4 kJ mol 1 is highly discrepant while one-half the isomerization value for di-n-butyl amine to di-isobutyl amine of 11.3 kJ mol 1 is only slightly less so. The exchange quantities for di-n-alkyl amines are probably not constant, but because most of the derivations in later sections are based on ethyl or propyl substructures, we choose υ(sec/ prim, prim) as the average of R D R1 D Et and n-Pr.

For the tertiary amines, the desired exchange values are available from experiment only for R D R1 D R2 D Me and R D R1 D R2 D Et. The gaseous enthalpy of formation for the hydrocarbon corresponding to tri-n-propyl amine has not been measured, but it may be reliably estimated17 as 251.0 kJ mol 1. A derived υ6(tert/n-Pr, n-Pr, n-Pr) is ca 90 kJ mol 1. Because υ6(tert/ Et, Et, Et) D 97 kJ mol 1, it is apparent that the exchange quantities for tertiary amines are not constant, as was surmised from the slopes reported in Table 1. Most of the derivations involving tertiary amines in later sections are based on an ethyl or propyl substructure and so an intermediate value of 93 kJ mol 1 is recommended.

Finally, because equation 8 is expected to be nearly thermoneutral for both X D N, CH in the absence of any special electronic or steric effects, υ5(sec/ R, R1) is approximately the average of υ4( prim/ R) and υ6(tert/ R, R1, R2) as can be verified for the relevant values in Table 2.

RXH2 C R3X ! 2R2XH 8

B. Alicyclic Amines

Let us now consider amines associated with saturated rings. We find that there are five distinct types of amines relevant to our discussion. The first type comprise primary amines with nitrogen affixed to a secondary carbon in a saturated ring. These are the cycloalkylamines with the generic formula (CH2)n 1 CHNH2 (1). The acyclic υ4( prim/ sec) equals 50 kJ mol 1 for acyclic secondary amines. For primary amines with secondary cycloalkyl groups and for n D 5 and 6, υ4(cyclic pri /sec) values of 51 and 50 kJ mol 1 are found. For n D 3, using experimentally determined enthalpies save an estimated Hv(CyprMe), a plausible difference of 53 kJ mol 1 is found. However, using the aforementioned protocol for the n D 4 case results in a difference of59 kJ mol 1. Substituted cyclobutane derivatives are usually anomalously stabilized18.

346 Joel F. Liebman, Mary Stinecipher Campbell and Suzanne W. Slayden

The second type of amine replaces a CH2 in the ring by a NH thereby transforming cycloalkanes (CH2)n (2) into their monoaza derivatives (CH2)n 1NH (3). This change would appear to be related to the acyclic υ5(sec/ prim, prim) of 73 kJ mol 1. For n D 3, 5, 6 and 719, the υ5(cyclic sec/ prim, prim) equals 73, 73, 76 and 73. We lack the n D 4 exchange energy. None of our analysis precludes polycyclic species and so we consider 3-azabicyclo[3.2.2]nonane (4) for which the enthalpy of formation is known to be 43.6 š 0.9 kJ mol 1. While we know of no thermochemical data for the parent hydrocarbon, 5, we need not be thwarted if we are willing to use estimates to guide us in other estimates. Let us use the enthalpy of formation of the 3-oxa derivative and the appropriate ether/methylene exchange energy from Reference 1. Hf(g, 5) is thus predicted to be 118 kJ mol 1. The resulting exchange energy of 74 kJ mol 1 for the polycyclic n D 7 species is consistent with both the monocyclic and acylic analogs.

The third type of amines are the C-alkylated derivatives of the preceding type of species. Their thermochemical data are surprisingly sparse. One example is that of 2- methylpiperidine, a ring-containing analog of the unrepresented acyclic υ5(sec/ prim, sec) class of amines. From its literature value of 84.4 š 1.0 kJ mol 1 and that of methylcyclohexane, we derive the desired exchange energy increment cyclic υ5(sec/ prim, sec) D68 kJ mol 1. The second example of this class of compounds is the highly toxic alkaloid 2-propylpiperidine (a.k.a. coniine). From Kharasch and Domalski, we find the nearly century-old enthalpy of formation for the liquid of 241 kJ mol 1. Combining this with an estimated Hv results in a value of 192 kJ mol 1 for the enthalpy of formation of the gas. This value is incompatible with the ca 121 kJ mol 1 value obtained by summing the enthalpy of formation of gaseous propylcyclohexane and υ5(cyclic sec/ prim, sec). Is the century-old measurement wrong or is the fault in our estimation approach? We also consider as an example of the third type of compound, trans-decahydroquinoline, with its year-old reported enthalpy of formation20 of 112.9 š 0.7 kJ mol 1. With this value and cyclic υ5(sec/ prim, sec) we would predict an enthalpy of formation of decalin, its hydrocarbon analog, of 181 kJ mol 1 in splendid agreement with its archival experimental value of 182.1 š 2.3 kJ mol 1. We hesitate to argue that new data are more accurate than old data, although that is a good general guideline for the selection of Hf values.

The fourth type of amine is the N-alkylated derivatives of monoazacycloalkanes. Our archives include the enthalpies of formation of four conceptually simple liquid species: the N-alkylated piperidines where R D Pr, Bu, Cype and Cyhx. To assess the accuracy of the experimental enthalpies, we can employ a multi-step procedure involving estimation of the enthalpies of vaporization of the piperidines and of the monosubstituted cyclohexanes (where necessary) using the same procedure as before21, then using the value of υ6(tert/ R, R1, R2) to ‘derive’ the enthalpy of formation of the monosubstituted cyclohexane and comparing the derived values for gaseous alkylcycloalkanes with experimental or semiexperimental values. Alternatively, we can confirm consistency of the piperidines by ascertaining how constant is the difference of the enthalpies of formation of liquid N- alkylated piperidine and alkylated cyclohexane22. Choosing the latter approach, we find the differences of the substituted piperidine and cyclohexane liquid enthalpies of formation to be 90, 91, 80 and 89 kJ mol 1. Three of the results are mutually consistent. Which, if any, is correct? Recall that our earlier logic suggested a ‘constant’ difference of enthalpies of formation of the gaseous species (υ6(tert/ R, R1, R2)) and a ‘constant’ difference of enthalpies of vaporization for arbitrary cases23. From these we can derive υ6(liquid, tert/ R, R1, R2) as ca 89 kJ mol 1. It is the cyclopentylpiperidine that is the outlier while the other three amines are in excellent agreement.

This success encourages us to consider the other monocyclic tertiary amines for which there are enthalpies of formation. In particular, consider N-(2-phenylethyl)azetidine (6),

Соседние файлы в папке Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups