Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 0537 609

so much smaller and/or so different from other hydrocarbyl groups, then the R D t Bu species might not have been such an outlier. Therefore, instead of contrasting the effects of Ph with those of Me, let us contrast those of Ph with t Bu instead. The related difference quantity υ45(Ph, t Bu; R)

is very much more of a constant over the choice of R groups as seen from the values N.D., 186.1 š 1.9, 188.9 š 1.8, N.D. and 182.2 š 2.6 from the same five R groups. Accordingly, we ‘roughly’ define a universal υ45 Ph, t Bu ³ 186 š 2 kJ mol 1.

Consider now the following ‘disproportionation’ reaction involving benzophenone (52, R D Ph):

For the R groups of interest, this reaction 46 is exothermic by N.D., 10.3š 0.8, 10.8š 0.9, N.D. and 5.2 š 1.7 kJ mol 1. A ‘constant’ value, υ46(Ph) of 9 š 2 kJ mol 1, is ‘roughly’ consistent with all of the data.

Let us use the new υ quantities to make thermochemical estimates and fill in the above blanks and omissions in the data. To begin with, what do we find for the enthalpy of

us to derive enthalpies of formation and vapourization of liquid formaldehyde to be ca140 and 31 kJ mol 1, respectively.

The above logic does not determine how much resonance stabilization there is for alkyl phenyl ketones but rather asserts that any additional stabilization is largely independent of the alkyl group flanking the carbonyl. We now present two admittedly contradictory analyses. The first notices that the location of the ketone hardly affects enthalpies of formation of acyclic ketones. At least, that is the case for 2- and 3-hexanone for which the former is more stable than the latter by 1.8 š 1.4 kJ mol 1 for the liquid and 1.5 š 1.4 kJ mol 1 for the gas. Likewise, the standard enthalpies of formation of 2- and 3-pentanone differ by 0.8 š 1.3 and 1.1 š 1.3 kJ mol 1 for the liquids and gases, respectively. By contrast, the non-conjugated phenylacetone is 15.3 š 2.3 and 8.0 š 3.6 kJ mol 1 less stable than its conjugated isomer, propiophenone, in the liquid and gas phases, respectively. That the location of the carbonyl group has such a dramatic effect on the enthalpy of formation for the alkyl phenyl ketones and essentially none for the aliphatic species is suggestive of the effects of conjugation. Furthermore, the enhanced enthalpy of vapourization of propiophenone over phenylacetone is suggestive of the importance of dipolar resonance structures.

However, an alternative probe of conjugation energy in alkyl phenyl ketones is to

For example, for R1 D R2 D Me and R3 D Bu, we find reaction 47 to be endothermic by 4.0š2.1 and 6.9š1.9 kJ mol 1 for gaseous and liquid species, respectively. Both numbers

are small, and statistically can be equated. So doing suggests that the conjugation energy of interest is small and the importance of dipolar resonance structures quite negligible. On the other hand, that the value for the liquid can be considerably greater than for the gas suggests PhCOR may well be considerably more stabilized as liquid than the acyclic 2-hexanone.

C. Alkyl Acetophenones

There are two compounds in this category for which there are reported enthalpies of formation, the 2,4,5-trimethyl and the 2,4,6-trimethylacetophenone (53a and 53b). From these data we may derive that the latter is more stable than the former in the liquid phase by 15.0 š 5.2 kJ mol 1 and by 15.9 š 5.9 kJ mol 1 as gases. That the difference is essentially independent of phase vindicates our decision above to use enthalpy-of- formation values in the liquid phase. But is the relative stability plausible? Consider replacing the COMe group by COOH in the above compounds. The difference of the gasphase enthalpies of formation123 of the related 2,4,5-trimethyl and 2,4,6-trimethylbenzoic acids is of comparable magnitude but with the opposite sign, 11.8 š 1.8 kJ mol 1. Save to suggest experimental error, we fail to understand why the relative isomer stabilities for trimethylacetophenones and trimethylbenzoic acids are so different124.

R2

Me

(53a) R1 = H, R2 = Me

(53b) R1 = Me, R2 = H

D. Alkyl Benzophenones

In this section, we will discuss compounds with the generic formula, PhCOC6H4 p-R (54) where R D H, Me, Et, iPr and t Bu. For most of these compounds, the available enthalpy-of-formation data from the literature is only for the liquid. We have the same options as in the first subsection. The simplest difference quantity we can define is υ48(PhCOC6H4, Ph; R),

R

(54)

where implicitly the substituent is on the 4-position of the benzophenone. The derived values are 65.3 (cf Pilcher and coworkers131), N.D., 52.0 š 2.3, 77.5 š 2.4 and64.9 š 2.5 kJ mol 1. We fail to understand why these values are so un-constant given their small error bars, and thus we are very hesitant to assign any universal value for the related υ48(PhCOC6H4, Ph).

XI. POLYCARBONYL COMPOUNDS

So far all of the discussion in this chapter has been devoted to the thermochemistry of compounds with one CDC double bond or one CDO double bond. What about compounds with more than one double bond? In fact, there have been ‘Patai’ volumes, and associated thermochemistry chapters, devoted to compounds containing at least one apiece of these types of double bonds4 and to more than one CDC double bond8. We know of no such chapter and volume published, in press or in preparation that deals with compounds with more than one CDO double bond. The thermochemistry of such species is briefly discussed in this section.

We start with ˛-dicarbonyl compounds, R1COCOR2. Data are sparse: enthalpies of formation are seemingly limited to R1 D R2 D H, Me and Ph, and R1 D Me, R2 D H. It was recently noted125 that the formal gas-phase reaction

is very close to thermoneutral for the hydrogen and methyl cases. For these three sets of R groups above, this reaction 49 is endothermic by 5.2 š 1.1, 5.1 š 1.5 and 3.7 š 4.8 (for R1 D R2 D H; R1 D R2 D Me; R1 D Me, R2 D H respectively). For the case with R1 and R2 both equal to phenyl, the endothermicity has increased to 17.9 š 5.1 kJ mol 1. This suggests considerable phenyl/carbonyl repulsion, and thus it would be useful to know the enthalpy of formation of the mixed R1 D Ph, R2 D H and Me compounds. What about for the liquid phase? The only compound for which all of the necessary data have been directly measured is R1 D R2 D Me. In this case, the endothermicity has increased to 17.8 š 1.1 kJ mol 1 for the liquid! R1 D R2 D Ph, using a temperature-uncorrected enthalpy of fusion of the diketone, results in an endothermicity of 44 kJ mol 1. And for the mixed Me, H case using the roughly estimated enthalpy of formation of liquid formaldehyde from a previous section, an endothermicity of 23 kJ mol 1 is found. These findings suggest that the enthalpy of vaporization of ˛-diketones is significantly reduced compared to monoketones a conclusion suggested by the analysis in Reference 126, and that the near-additivity of the vapourization substituent constants for numerous COX (see Reference 127) is sorely violated here. But is there any destabilization associated just with the adjacent carbonyl groups? To test this, by analogy with the question of the magnitude of stabilization8 associated with adjacent carbon carbon double bonds, one may consider the enthalpies of the formal gas phase reactions

for the four cases of R1 D R2 D H, Me and Ph, and R1 D Me, R2 D H. These reactions 50 are endothermic by 36.4 š 1.1, 24.7 š 1.9, 43.3 š 7.9 and 27.2 š 4.9 kJ mol 1. Indeed, all of these reactions are significantly endothermic the diphenyl most of all and so we may conclude that ˛-diketones are significantly destabilized.

What about ˇ-diketones where we mean species that explicitly have the CO C CO substructure, and not the hydroxyenones4 with the C(OH)DCH CO substructure that usually ‘answer to this name’. We now consider what is probably the best known of the

ˇ-diketones, MeCOCH2COMe, and resist calling it either acetylacetone or even pentane- 2,4-dione128. Nonetheless, there are measurements129 of the enthalpy of formation of this minority, as written130, tautomer 55a as well as 55b. For the liquid and gas, the enthalpies of formation are 416.3 š 1.1 and 374.4 š 1.3 kJ mol 1, respectively. To discern any destabilization associated with the two carbonyl groups being quite close, we consider the energetics of reaction 51 by analogy to the above reaction 50,

MeCOCH2CH2Me C MeCH2CH2COMe !MeCOCH2COMe

C MeCH2CH2CH2Me (51)

Some amount of destabilization is seen, 4.8 š 2.0 and 8.3 š 2.1 kJ mol 1, respectively, for the liquid and gaseous ˇ-diketone. Not surprisingly, there is less destabilization for the ˇ-diketone than for ˛-diketones.

Cyclohexane-1,3-dione is an interesting, but problematic, ˇ-diketone. Calculational, (cited) spectroscopic and calorimetric studies131 show that it exists as the hydroxyenone 56a in the intermolecularly hydrogen-bonded solid and is most stable as the diketone 56b in the gas. From the experimentally measured enthalpy of formation of this dione at335.6 š 1.6 kJ mol 1, we find that reaction 52

2[cycloCH2 5CO] ! [cycloCH2 6] C cyclo-[1, 3-(CH2)4 CO 2] 52

is exothermic by 6.8 š 3.5 kJ mol 1. We would have thought that this mimics the endothermic reaction 51 and fail to see any mechanism for stabilization of cyclohexane- 1,3-dione. Let us now turn to the 4,4- and 5,5-dimethyl derivatives of cyclohexane-1,3- dione. Consider the ‘transmethylation’ reaction 53

cyclo-[ CH2 5CMe2] C cyclo-[1, 3-(CH2)4 CO 2] ! [cycloCH2 6]

C cyclo-[x, x-CMe2-1, 3-(CH2)3 CO 2] (53)

From measured enthalpies of formation, we find this reaction to be exothermic by 7.3 š 4.0 kJ mol 1 for the 4,4-dimethyl isomer and endothermic by 9.5 š 3.6 kJ mol 1 for its 5,5-isomer. Why the 4,4-isomer should be stabilized and the 5,5-isomer destabilized relative to the unmethylated diketone is enigmatic. So is the nearly 17 kJ mol 1 difference in their enthalpies of formation132.

We now discuss -diketones. Pilcher and coworkers131 show cyclohexane-1,4-dione (57) to have a gas-phase enthalpy of formation of 332.6 š 1.2 kJ mol 1, i.e. 3.0 š 2.0 kJ mol 1 less stable than its 1,3-isomer. This, too, is quite inexplicable but is reproduced by their quantum chemical calculations, and by the relative stability of species 58a and 58b, the related bis-exomethylenecyclohexanes133. The only other -diketone for which we have enthalpy-of-formation data is solid 1,4-diphenylbutane-1,4-dione, 59.

The difference between this value and solid 1,4-diphenylbutane is 122.8 š 1.4 kJ mol 1. This value is reasonable, especially given some aryl-ketone stabilizing interaction.

We now note the calorimetric measurements of the enthalpies of combustion of the isomeric 1,15and 1,16-cyclotricontanediones (60a and 60b, respectively) or, more precisely, a solid-phase mixture of the two134. From the reported values, we deduce that the formal dimerization reaction

is exothermic by ca 10 kJ mol 1. Given ambiguities as to the nature of the sample, and intrinsic complications of solids, the near-thermoneutrality is encouraging135.

What about polyketones where poly means 3 or more? Experimental data are lacking. Nonetheless, the enthalpy of formation136 of a putative gas-phase polymer [61, i.e. (CH2CO)n] was discussed from which a cumulative error experimental, conceptual and calculational of but 8 kJ mol 1 was found from that predicted assuming thermoneutrality for reaction 55

This suggests very little additional destabilization arising from an ‘array’ of ˇ-diketones. Equivalently, there is insignificant enthalpic consequence of ε- or greater separation on the energetics of ketones, a finding consonant with the energetics of reaction 54.

XII. IMINES

A. Definition and Organization

By the word ‘imine’ we remind the reader that we mean a species containing at least one carbon nitrogen double bond that is not part of an aromatic ring and attached only

to hydrocarbyl groups. We now proceed through the various imines that qualify by the aforementioned definition. Because the measurements of the enthalpies of formation of the various imines have involved so many distinct techniques and remain so confused and conflicted, the organizing principle of this section is one of increasing carbon number. Whenever possible, we will attempt retrospective cross-referencing, i.e. later compounds may refer to earlier ones. As such, the treatment of imines is perhaps relatively longer than anticipated. To date we lack rules and regularities for understanding these species that allow for sweeping generalities and/or edifying examples.

B. Methylenimine

We start with formally the simplest species, CH2NH, which has been also been called formalidimine. Perhaps because it is the simplest imine, it has been studied by a variety of unconventional and non-calorimetric techniques. To illustrate the complexities we will devote more time and space than otherwise yet, we are still unable to say from these experiments137: ‘Formerly, it was confusing. Now it is not.’ The first measurement138 of its enthalpy of formation involved bracketing the energy of gas-phase hydride transfer to HCNHC and resulted in a value of 110 š 13 kJ mol 1. The next study139 involved the ionization and appearance energies of CH2NH relative to azetidine (62a), thereby resulting in 88 š 13 kJ mol 1. From various radical and radical ion processes, the enthalpies of

formation of the isomeric CH3NHž and CH2NH2ž were determined, and from the latter, that of CH2NH was deduced140 to be 105 š 6 kJ mol 1. In what was anticipated to be the

simplest and most direct study141, radical ion fragmentation from primary amine radical cations and proton transfer bracketing from CH2NH2C resulted in the dissonant value of 69š8 kJ mol 1 being obtained. Finally, in a more accurately measured appearance energy reprise142 of Reference 139 [and also using azetidine as well as the alternative precursor, propargylamine (62b)], a value of 82 š 13 kJ mol 1 was reported. It is disconcerting that over a 40 kJ mol 1 range of values has been reported for the enthalpy of formation of CH2NH. Is it perhaps too pessimistic to wonder what values would be obtained for most other gas-phase species if they were to be scrutinized as closely as this 5-atom molecule? Most of the species below have been studied but once and so such exquisite comparison and extensive doubts will not be exercised. It is too much an exercise in masochism to do so. We will be pleased with approximate self-consistency of the results.

NH

HC CCH2 NH2

(62a) (62b)

C. Methylated Methylenimines

There are two isomers, CH2NMe and MeCHNH. We know of but one measurement for the enthalpy of formation for the N-methylated isomer, that found in Reference 141, where a value of 44 š 8 kJ mol 1 was reported. Even if we were to feign knowledge of CH2NH, a simple estimation based on change of an N H to an N Me bond would be hard to achieve. The enthalpy-of-formation change going from MeNH2 to Me2NH is C4.4 š 0.9 kJ mol 1 while that from Me2NH to Me3N is 5.1 š 1.0 kJ mol 1. These are ‘sp3’ nitrogens. What about ‘sp2’ nitrogens? Admitting now that the enthalpy of formation of PhNHMe is uncertain two values of 85 and 95 kJ mol 1 have been offerred143 the change from PhNH2 to this secondary amine is either ca 2 or C8 kJ mol 1 and from

We would think that the following formal reactions are approximately thermoneutral:

Accepting the enthalpies of formation given by Jackman and Packam150 for all three imines results in a value of 40 š 14 kJ mol 1, considerably lower than experiment148. In principle, clarification of this discrepancy could be achieved by related studies on other 1-azacycloalkenes. However, except for an unsuccessful attempt148 for the corresponding 1-azacyclohexene (alternatively, 1-piperideine or 3,4,5,6-tetrahydropyridine) 65 its trimer fails to monomerize we do not know of any such study.

F. ‘Simple’ Acyclic Aldimines

This class of compounds, generically RCHDNR1, is surprisingly well-represented in the thermochemical literature although superficial reading does not appear to support this conclusion. For example, our archive presents but two enthalpies of combustion, both for

iPrCHDNBun, and having chosen the later one, results in an enthalpy of formation of the liquid imine149 of 132.8 š 3.4 kJ mol 1. To convert the earlier value to that for the gas phase, we invoke a protocol for the enthalpy of vapourization (cf Reference 128) that extends our standard approach for hydrocarbons. We then assume that an imine nitrogen contributes the same as a pyridine nitrogen, i.e. 12.1 kJ mol 1. The resulting gas-phase enthalpy of formation is ca 80 kJ mol 1 with a plausible š6 kJ mol 1 uncertainty.

Almost 40 years ago150, the enthalpy of hydride transfer to a collection of N-substituted aldimines R1CHDNR2 was reported and compared with the reaction enthalpy of the same (complex metal) hydride with the corresponding amine R1CH2NHR2; reactions 65 and 66, respectively.

From

(a)the difference of the measured hydride reaction numbers,

(b)group increment estimates of enthalpies of formation of product gaseous amines and

(c)the difference of the vapourization enthalpies for corresponding imines and amines

[i.e. Hv imine Hv amine ] set equal to 2.1 kJ mol 1,

the following gas-phase enthalpy of formation values were deduced151: MeCHDNPrn,5; EtCHDNEt, 0; EtCHDNPrn, 34; EtCHDNPri, 40; iPrCHDNEt, 31 kJ mol 1. Alternatively, we may derive the quite similar values below by use of