Amines proton affinities

Amine Proton Affinity (Order) BF3 (Order) A/-/° for amine—BMe adduct formation (kJ/mol) BMej (Order) B(f-Bu)3 (Order)... 

Thus, the values calculated for effective polarizability at the nitrogen atom for a series of 49 amines carrying only alkyl groups was correlated directly with their proton affinities, a reaction that introduces a positive charge on the nitrogen atom by protonation (Figure 7-7) [40. ... 

Once amines that also cany heteroatoms were included in the study, a dataset of 80 proton affinities was obtained. For those alkyl amines the inductive effect as quantified by residual electronegativity had also to be taken into account, A simple... 

Figure 7-7. Equations for the calculation of proton affinities (PA) of simple alkyl amines and of heteroatom-substituted alkyl amines.

The significance of the values calculated for the effective polarizability was first established with physical data, among them relaxation energies derived from a combination of X-ray photoelectron and Auger spectroscopy, as well as N-ls ESCA data53, 54). From our point of view, however, the most important applications of effective polarizability are to be found in correlating chemical reactivity data. Thus, the proton affinity (PA) of 49 unsubstituted alkylamines comprising primary, secondary and tertiary amines of a variety of skeletal types correlate directly with effective polarizability values (Fig. 22). 

The proton affinities (PA) of two restricted subsets of amines were correlated directly with inductive and polarizability effect parameters, respectively (Figs. 19 and 22). These can be combined with data on other hetero-substituted amines to give a set of 80 amines of different skeletal and substitution types (e.g. Fig. 24). In this and all other systems (below), a residual electronegativity value, %l2, (Eq. 5) derived from those of the atoms of the first, 1, and second, 2, sphere neighbors of the nitrogen atom is preferred as a measure of the inductive effect49). 

Fig. 24. Example amines included in the proton affinity study (Eq. 6)...

R = H, X = S, A = Et3N and Py). In solution the former is mainly in an ionic form the latter exists as a complex. The basicity of the amine is assumed to be important. Triethylamine is a stronger base than pyridine and the ionic form is stabilized. When the proton affinity is weak, the basicity in relation to the B(III) atom, a Lewis acid, plays an important role. This involves an equilibrium shift toward the complex. This assumption is confirmed by an easy displacement of pyridine by triethylamine. The reverse process demands more severe conditions. In the NMR spectra of the triethylamine complex the signal is shifted from 22 to 42 ppm as pyridine is added. The absence of signals of two separate forms is evidence in favor of their fast interconversion. The chemical shift of the signal in 3IP spectra is 22 ppm (EtOH), 26 ppm (Py, DMFA), and 42 ppm (EtOH, Py) for complexes with triethylamine and pyridine. 

As seen in Table V, there is a clear dependence of the equilibrium position on the basicity of amines, excluding triethylamine. However, it is necessary to take into account not only the proton affinity of the amine, but also the ability of the amine to form a dative bond with a boron atom. The equilibrium position also depends on the structure of the phosphorus-... 

It is well known that alkyl substitution changes the basicity of amines. However, solvation effects lead to an anomalous order of basicities in solution (NH3 tertiary amine < primary amine < secondary amine). From gas-phase proton affinity data the intrinsic effects of alkyl substituents can be evaluated and a quite regular order (NH3 < primary amine < secondary amine < tertiary amine) is obtained91. 

However, in more complicated amines, this straight correlation is violated. The bicyclic tertiary amine l-azabicyclo[4.4.4]tetradecane (22) and the acyclic tertiary amine n-Bu3N have nearly the same first IP (7.84 and 7.90 eV, respectively), but the proton affinity of the bicyclic amine is 20 kcal mol 1 lower than that of the acyclic52. On the other hand, for other bridge-head tertiary amines like l-azabicyclo[2.2.2]octane (quinuclidine, 20) and l-azabicyclo[3.3.3]undecane (manxine, 21) the expected relation between proton affinities and IP values is observed. The extraordinary properties of l-azabicyclo[4.4.4]tetradecane (22) are caused by its unusual conformation the nitrogen lone-pair is directed inward into the bicycle where protonation is not possible. In the protonated form, the strained out-conformation is adopted. This makes it the least basic known tertiary amine with purely saturated alkyl substituents. Its pKa, measured in ethanol/water, is only +0.693. Strain effects on amine basicities have been reviewed by Alder88. 

Figure 8.12 Proton affinity of B-H hydrogens in borane amines and nitrogen atoms in amines versus number of CH3 groups. (Reproduced with permission from ref. 22.)...

Compounds with a high HOMO and LUMO (Figure 5.5c) tend to be stable to selfreaction but are chemically reactive as Lewis bases and nucleophiles. The higher the HOMO, the more reactive. Carbanions, with HOMO near a, are the most powerful bases and nucleophiles, followed by amides and alkoxides. The neutral nitrogen (amines, heteroaromatics) and oxygen bases (water, alcohols, ethers, and carbonyls) will only react with relatively strong Lewis acids. Extensive tabulations of gas-phase basicities or proton affinities (i.e., —AG° of protonation) exist [109, 110]. These will be discussed in subsequent chapters. 

From the temperature dependence of the equilibrium constant for proton exchange between some deuterated and undeuterated primary and secondary amines, monitored by high-pressure mass spectrometry, the reaction enthalpy, or difference in proton affinity, could be measured.101 Protonation of the deuterated amine is favored by 0.2kcalmol-1, varying with structure by 0.1 kcal mol-1 but with no obvious pattern. However, the equilibrium, at least for CH3CD2NHCH3, appears to be entropy driven, not enthalpy. 

The proton affinity results reflect changes in polarizability and inductive effects with the substituent group R. Comparison of proton affinities in the nitrile and primary amine series (Figure 10) reveals that the magnitude of these effects is linearly related in the two series but larger for the nitriles. A least-squares fit to the data is given by equation 26... 

Fig, 5. Gas phase proton affinities of pyridine, phosphabenzene and arsabenzene compared to selected amines, phosphines and arsines... 

In the substrates, special attention has been devoted to zeolites and fullerenes [34, 38, 39, 44], in the reactions to acid-base equilibria and the influence of hardness and softness on both sides of the equilibrium (acidity of carboxylic acids [45], alkyl [46] and halogenated alcohols [47], hydrides [48] and recently hydrofullerenes [49], basicity of amines [50, 51] and proton affinities of amino acids [52]). For reviews of these studies, see [18, 53, 54],... 

Mui et al.36 report a comparative experimental - theoretical study of amines on both the Si(001)-(2x 1) and the Ge(001)-(2x 1) surface. Both substrates were modeled by X9H12 (X = Si, Ge) clusters, utilizing DFT at the BLYP/6-31G(d) level of theory. For both, the Si and the Ge substrate, formation of a X-N dative bond (X = Si, Ge) is the initial step of the reaction between the considered amine species and the semiconductor surface. Flowever, while primary and secondary amines display N-H dissociation when attached to Si(001)-(2 x 1), no such trend is observed for the Ge counterpart of this system. This deviating behavior may be understood in terms of the energy barrier that separates the physisorption from the chemisorption minimum, involving the cleavage of an H atom. For dimethylamine adsorption, this quantity turned out to be about 50% higher for the Ge than for the Si surface. The authors relate this characteristic difference between the two substrates to the different proton affinities of Si and Ge. 

Now the ratio Kb for aniline/iq, for prim, aliph. amine amounts to 4.6 io 10/3 to 5 io 4 — 1.5 to 0.9 io 6. This indicates a difference in AF of 8.4 to 7.gkcal/mole (T= 298° K). Since there is no reason to assume any essential difference in AS, the proton affinity of aniline, A 7, will also be 8 kcal lower, which is practically equal to the above-mentioned extra R.E. deduced from the heat of combustion. 

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