Protonation and site of protonation of anilines. Hydration and site of protonation after hydration

The [C2D5]+ species forms an adduct in the gas phase with ethyl acetate, ethylbenzene, and p-ethyltoluene, which I subsequently unimolecularly eliminates C2H4 and C2D4 in approximately a 2: 1 ratio. These results indicate that the adduct is not a weakly bound ion-molecule complex but a bonded species in which the C2D5 group has become equivalent to the C2H5 group present in the molecule; the preference for elimination of C2H4 is due to an isotope effect. From observations of the relative loss of C2H4 and C2D4 from [ C 2~5 ] + adducts with other molecules containing a C2H5 group, it is concluded that the [C2D5]+ species attaches to the carboxyl group in ethyl benzoate, primarily to the ring in substituted phenetoles and in ethylphenols, primarily to the nitrogen in ethylpyridines, and primarily to the ring for N-ethyl-and p-ethyl-aniline. There is considerable interest as to the site of protonation and alkylation of organic molecules in the gas phase; this interest is prompted not only by questions of intrinsic molecular properties such as proton affinities but also by the question of the effect of the site of attack on ionic fragmentation reactions under chemical ionization conditions. In considering the site of protonation/alkylation it is important to distinguish between conditions of thermodynamic control that apply when equilibrium proton affinities are measured and kinetic control that characterizes protonation and alkylation under chemical ionization conditions.There now is a large body of equilibrium gas-phase proton affinity data (for a summary of data and methods of measurement, see ref. 1) including many bifunctional molecules. Although the thermodynamically favoured site of protonation has been elucidated from an investigation of substituent effects (2), for most bifunctional molecules, the site of protonation has been determined from correlations of gas-phase proton affinities with core binding energies. Thus, from correlations of the measured proton affinities of carboxylic acids and derivatives with O(1s) binding energies it has been established (3-5) that these compounds are protonated on the carbonyl oxygen. A -I similar study (6) has established that in the gas phase amides are i protonated on the oxygen, which is in agreement with solution phase results (7). The correlation of proton affinity data of substituted pyridines with N(l s) binding energies (8) shows that protonation occurs on the nitrogen, while a similar correlation I for substituted anilines (9) reveals a more complex behaviour where protonation on either the ring or a nitrogen depends on the substituent. The position of proton or cation attack under the conditions of chemical ionization is much less clear. Much of the evidence has arisen from collision-induced dissociation (CID) studies (10) of the protonated and cationized products. Thus, Maquestiau et al. (11) found that the CID mass spectra of ethylated pyrrole and protonated N-ethylpyrrole showed substantial differences and they concluded that neither ethylation nor protonatio...

supporting

confidence: 93%

Site of gas-phase ethyl ion attachment

Harrison¹

1986

“…One important benefit from the data in Table 4 is the clarification of the site of protonation that they provide. It has been shown that protonation of the nitrogen in aniline is more favorable (by -1 kcal/mol) than protonation of the ring (15,16). Without the present calculations for the phenyl phosphine it would not have been completely straightforward to decide whether ring or substituent protonation is more favorable.…”

Section: Resultsmentioning

confidence: 86%

A comparison of methyl and phenyl substituent effects on the gas phase basicities of amines and phosphines

Ikuta¹,

Kebarle²

1983

Self Cite

S. IKUTA and P. KEBARLE. Can. J. Chem. 61. 97 (1983). The proton affinities of phenyl phosphine and cyclohexylphosphinc were measured by determining the equilibrium constants of proton transfer equilibria with a pulsed electron beam high ion source pressure mass spectrometer. These proton affinities combined with values for methyl and phcnyl phosphines and the analogous amines provide an interesting comparison of the methyl and phenyl substituent effects on the basicities of phosphine and ammonia. Methyl substitution increases the basicity of both ammonia and phosphine; however, the increase is significantly larger for the phosphine. Phenyl substitution increases the basicity of ammonia and phosphine and the increase for phosphinc is very much larger. Calculations at the STO-3G, 4-3 IG, STO-3G*, and 4-31G* (* with d orbitals) for PH,, MePH,, PhPHZ, the protonated species, and the nitrogen analogues predict proton transfer reaction energies in good agreement with the experimental results. A shortening of the C-P bond is predicted for protonation of MePH, and particularly PhPH,, while a lengthening of the C-N bond is predicted for the corresponding nitrogen compounds. The much stronger increase in proton affinity of the phosphines caused by phenyl substitution is due to the stabilization of the phenyl phosphonium ion by a donation from the phenyl group to the empty orbitals of phosphorus in the PH: group; in contrast, in the amines, it is the free base anilinc which is stabilized by conjugation of the nitrogen lone pair with the aromatic ring. This stabilization of the free base is less important in phcnyl phosphine. The participating empty orbitals of phosphorus in the conjugation of phenyl with PH: in phenyl phosphonium arc mostly a * with some a d participation. The stabilization of the aniline free base contributes considerably more than the conjugation in the phosphonium ion, to the phenyl substituent difference for the amines and phosphines. Thc factors involved in the bigger substituent effect of methyl in the phosphines are somewhat similar to those for phenyl: stabilizftion of the methyl amine by conjugation of the nitrogen lone pair with empty orbitals of CH, and stabilization of the CH,PH, by hyperconjugation. An alternate description can be given in terms of hybridization chariges.S. IKUTA et P. KEBARLE. Can. J. Chem. 61, 97 (1983) On a mesure les affinitCs pour le proton de la phenylphosphine et de la cyclophosphine en determinant les constantes des Cquilibres de transfert du proton ii l'aide d'un spectromttre de masse a faisccau d'Clectrons pulses avec une source d'ions a haute pression. Ces affinitks pour le proton cornbintes aux valeurs dcs mCthyl-ct phdnyl-phosphines et des amines analogues fournissent une comparaison intkressante des effets des substituants mCthyle et phCnylc sur les basicitds de la phosphine et dc I'ammoniac. La substitution du rnCthyle augmente la basicite de la phosphine et de I'arnmoniac: I'augmentation est plus forte pour la phosphine. La substitution du phCnylc augmentc l...

“…(7,9). More recently, Meot-Ner and co-workers found a linear correlation between the bond dissociation energy of BH+---A and the difference between the proton affinities for proton donor B and that for proton acceptor A (10-12).…”

Section: Introductionmentioning

confidence: 99%

Relationship between gas-phase proton affinities of conjugate bases B's and free energies of hydration of conjugate acid ions BH⁺'s for B = alkyl-substituted H₂O and NH₃

Hiraoka¹

1987

KENZO HIRAOKA. Can. J. Chem. 65, 1258 (1987. An approximate linear relationship between the proton affinities of conjugate bases (B 's) and the free energies of hydration of conjugate acid ions (BH' 's) is found for B = alkyl-substituted H 2 0 and NH3. With increase in proton affinities, the free energies of hydration increase linearly with a slope of about 1 for analogous compounds. The charge delocalization in the protonated bases and the structure-making effect induced by the hydrophobic groups in the alkyl-substituted H 3 0 + and NH4' are crucial factors in the determination of the free energies of hydration.