Protonated cyclohexylamine

The equilibrium shown m the equation lies to the right =10 for proton transfer from the conjugate acid of aniline to cyclohexylamine making cyclohexylamine 1 000 000 times more basic than aniline... 

Thin self-supporting clay films (appropriate for IR measurement) readily take up organic amines such as cyclohexylamine with displacement of the major fraction of the intercalated water. For the Ua -exchanged sample the majority of the amine is present in the unprotonated form - there being insufficient Bronsted acidity generated by the interlayer cation. When Al + is the exchangeable cation, however, a major fraction of the intercalated amine becomes protonated (see Figure 2). 

For R = benzylamine (B) and R — cyclohexylamine (C) the amounts of BB and CC and of the BC cross product formed as the relative concentration of the reactants varies is shown in Figure 3. Clearly whilst for an approximately 50 50 reactant mixture the amount of CC formed has dropped to practically zero that of the cross product (BC) is still hi. The interpretation suggests that the less baisic atmine is more readily intercalated auid essentially remains urprotonated whilst the more basic amine, althou present in much smaller concentration, removes nearly all of the available protons. Hence the major reaction is between protonated B and non-protonated C, yielding BC (23,23). 

Figure 2. Infrared spectra of a Na" and exchanged clays following the uptake of cyclohexylamine. Absorptions at 1585 and 1525 cnT are characteristic of the neutral and protonated amine species, respectively.

The aniline molecule is thus stabilised with respect to the anilinium cation, and it is therefore energetically unprofitable for aniline to take up a proton it thus functions as a base with the utmost reluctance (p Ka = 4 62, compared with cyclohexylamine, pKa = 10-68).Thebase-weakening effect is naturally more pronounced when further phenyl groups are introduced on the nitrogen atom thus diphenylamine, Ph2NH, is an extremely weak base (pKa = 0-8), while triphenylamine, Ph3N, is by ordinary standards not basic at all. 

Streitwieser and Boerth studied the kinetic acidities of cycloalkenes with lithium cyclo-hexylamide (LiCHA) in cyclohexylamine for comparison with those of benzene and toluene66. The relative rates of deprotonation and the corresponding equilibrium pK values are tabulated in Table 12. These proton transfer transition states are stabilized by conjugation of the reacting C—H bond with the double bond. 

Amino substituents in acyclic derivatives have been discussed by Eggert and Djerassi (396), who emphasize structural and conformational effects, whereas Batchelor has investigated SCS(NH2) and nitrogen protonation shifts in methylated cyclohexylamines (424). The, 3C NMR spectra of amino acids have been compared with those of amines and carboxylic acids (425,426). The transmission mechanisms of amino, ammonium, trimethylammonium, acetamido, and di-acetamido groups have been examined by Faure and co-workers (427), the SCSs of nitro groups by Ejchart (400), and those of azido functions in steroids by Lukacs and co-workers (428). 

With the same methodology, it was possible to quantify the gas-phase exchange equilibria of the (/f)-enantiomer of the chiral amines (M/ ), sec-butylamine, 1-cyclohexylethylamine, a-methylbenzylamine, and 1-(1-naphthyljethylamine with protonated (S,S)-3 ([Uss] ) or protonated (R,R)-3 ([U/ / ] ). One among the selected amines was introduced together with a reference achiral amine (Mjef), i e., isopropylamine or cyclohexylamine, into the FT-ICR cell, where they react with [Uss] or to form the corresponding [Uss M/ ]" and [U/j/j M/ ]" adducts. The... 

Then, in 1973, Streitwieser reported that the polyarylmethane exchange rates measured in cyclohexylamine-cyclohexylamide correlate with equilibrium p/T values with a = 0.31.79 Apparently, when the proton is removed by cyclo-hexylamide, the polyarylmethanes have early transition states, just as the fluorenes did for proton removal by the weaker base methoxide.80 A short extrapolation of the Bronsted correlation led to a pKa for toluene of 40.9, about seven units higher than the value assigned in the MSAD scale. Furthermore, if we... 

The pathway of the Kabachnik-Fields reaction depends on the nature of the substrates. The amine and hydrophosphoryl compound form a complex in which either one of the partners may react with the carbonyl compound. Often, the basicity of the amine determines the reaction pathway. Weakly basic amines such as anilines, which can act as proton donors, favour the formation cf an imine, whereas alkylamines such as cyclohexylamines do not form imines ... 

Drastic anhydrous conditions appeared essential for such study since 2-benzyl-4,4-dimethyl-l,2-thiazetidin-3-one 1,1-dioxide 52 was easily hydrolyzed even with only traces of water. Heating compound 52 with cyclohexylamine afforded an intermediate salt which was protonated on silica gel to afford sulfonic acid 84 (Scheme 20) <1999HCA354>. 

There is an extra R.E. of 6 kcal but the basicity of aniline is also very much smaller than that of an aliphatic primary amine, such as cyclohexylamine. This special resonance does not exist any longer in the anilinium ion because the free pair has now become a bonding pair by proton addition. The base constant Kb is related to the (free) energy AF of the proton addition according to the expression AF = —RTlnKb. 

In a recent study [175] of the methoxide ion catalysed tritium exchange in methanol of weakly acidic fluorinated bicycloalkanes (pIT ca. 20), comparison of the rates of exchange for six compounds, with ion-pair pK values determined in cyclohexylamine containing caesium ions, gave a Bronsted plot with slope (a) within experimental error of unity. The isotope effects for proton compared with deuteron transfer (feH/feD = 1.2) are similar to those observed with chloroform. The charge on these polyfluorocarbanions may also be localized on carbon [175]. 

In spite of the elegant solution to the problem as represented in the structure of 206, it is rather far from being optimal. In fact, the presence of the naphthalene system in this base made the latter rather vulnerable to electrophilic attack directed at aromatic rings. Owing to this complication, 206 is used in syntheses less often than other sterically hindered bases like 4-methyl-2,6-di-tcrt-butylpyridine 207, ethyldiisopropylamine, 208 (Hunig s base) or ethyldi-cyclohexylamine 209. All of these reagents are now manufactured commercially and widely used in cases where it is essential to carry out a reaction with strong electrophiles under strictly non-acidic conditions with the removal of proton acids as they are formed. In fact, the preparation of 205 was succesfully carried out in the presence of 208. °... 

For example, when cyclohexylamine is treated with aqueous HCl, it is protonated, forming an ammonium salt. Because the ammonium salt is ionic, it is soluble in water, but insoluble in organic solvents. A similar acid-base reaction does not occur with other organic compounds like alcohols, which are much less basic. 

Abbreviations DMSO, dimethyl sulfoxide THF, tetrahydrofuran CHA, cyclohexylamine. Most acidities were measured at 25°C. Some are extrapolated values some are values from kinetic studies. Errors in some cases are several pK units. The farther the pK value is from 0-14, the larger the errors because of estimates and assmptions made when water is not the solvent. Values of pK s for the same substance in different solvents differ because of differences in solvation. Although the acids actual structures are listed in this Appendix, not all references do this. Thus, you may find lists of the pK values for organic amines that refer to the pK of the protonated amine rather than the amine itself. A good rule of thumb is that if the pK value given for an amine is less than 15, it must be the pK of the protonated amine rather than the amine itself. 

The results with cyclopropyl-substituted amines also reveal the operation of several effects. Cyclopropylamine (5) is significantly less basic than cyclohexylamine in solution , as expected due to -withdrawal from the protonated ammonium ion. However, there is also evidence for significant stabilization of the free amine by u-electron donation to cyclopropyl (see Section III.D) so the resultant low basicity is evidently due to a combination of these effects, which operate in the same direction. p-Cyclopropylaniline is somewhat more basic than the p-isopropyl analog S and this effect evidently arises from 7c-donation to the ring, stabilizing the protonated conjugate acid. The gas phase basicity of 5 is discussed in Section V.I. 

Arylamines have a larger positive AG° for protonation and are therefore less basic than alkylamines, primarily because of resonance stabilization of their ground state. Electrostatic potential maps show that lone-pair electron density is delocalized in aniline compared to cyclohexylamine but that the corresponding ammonium ions localize charge in the same way. 

Aniline is a much weaker base because its delocalized lone pair is more strongly held than the nitrogen lone pair in cyclohexylamine. The more strongly held the electron pair, the less able it is to abstract a proton. 

Again, the same two factors can be invoked to account for why protonated aniline is a stronger acid than protonated cyclohexylamine. 

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