Amines, aliphatic ionization constants

Aromatic amines are considerably weaker bases than aliphatic amines. Compare, for example, values of for aniline and cyclohexylamine. The ionization constant for the conjugate acid of aniline is larger (the smaller the value of the weaker the base) than that for cyclohexylamine by a factor of 10 . 

Infrared spectra, of primary and secondary amines 17, 111-113 d-a Interaction 706 Intramolecular inversion, in nitroso-amide decomposition 449 in nitrous acid deaminations 469 Ionization constants, of aliphatic amino acids 221-223 of aminophenols 226 of aromatic amino acids 223-226 of nitrogen bases 227, 228... 

According to previous reviews, ammonia and amines also retard hydro-formylation. This statement has to be corrected according to more recent work [7, 130, 131, 1042]. Contrary to the statement mentioned, the hydro-formylation reaction is accelerated by small amounts of certain amines having an ionization constant of 10 [130], as e.g. pyridine, chinoUne, picoline, lutidine, aniline, toluidine, xylididine, methyl aniline and aliphatic and aromatic amides such as N,N-dimethyl formamide, N-methyl pyrroli-done and acetanilide. If larger amounts of these amines are used with HCo(CO)4 the effect becomes smaller. Very large amounts hinder the reaction (see table 3). 

Cyclic chain termination with aromatic amines also occurs in the oxidation of tertiary aliphatic amines (see Table 16.1). To explain this fact, a mechanism of the conversion of the aminyl radical into AmH involving the (3-C—H bonds was suggested [30]. However, its realization is hampered because this reaction due to high triplet repulsion should have high activation energy and low rate constant. Since tertiary amines have low ionization potentials and readily participate in electron transfer reactions, the cyclic mechanism in systems of this type is realized apparently as a sequence of such reactions, similar to that occurring in the systems containing transition metal complexes (see below). 

Another physical study which used flash photolysis relates directly on the MB/amine system. Kayser and Young (36) examined a more extensive series of amines, both aromatic and aliphatic, than Steiner (33). Their results are shown in Table 3. Excellent correlation was obtained between the amine ionization potential and the rate constant for MB quenching the slope of the logarithmic plot was -1.75 eV-1- This value is relatively small compared to some oxidizing excited states (e.g. hydrocarbons, -17 eV l (37)), but it is similar to the value observed for ketone triplet quenching by amines (-1.5 eV l (38)), and does indicate that the quenching interaction becomes more facile as... 

HAS are weaker quenchers of 02 than aromatic amines [208]. Extensive quenching studies were performed with aliphatic and heterocyclic amines [209]. Data obtained in the inhibition of the self-sensitized photo-oxidation of rubrene revealed that the quenching efficiency of unhindered amines may be correlated with their ionization potentials. A low ionization potential means a better quenching activity. Substitution on the a-carbon atom to nitrogen by alkyls reduces the quenching rate predicted from the ionization potential [208, 209]. The sensitivity of 02 quenching to steric effects may be exemplified with quenching rate constants kq(in dm3 mol was used as a standard. 

Aliphatic amines have been found to quench the fluorescence of acridine efficiently and a linear relationship has been demonstrated between the logarithms of the Stern-Volmer quenching constants and the ionization potentials of the amines. The photoreduction of the acridine occurs in 20% MejCOH-MeCN via an exciplex formed between the ground state of the amine and the acridine (ti, n ) state and in 20% McaCOH-CgHg via the (n,n ) state of the acridine. A quantitative study has been reported of the photoreactivity of 2-nitrophenazine with tertiary amines. Excitation to the state leads via a non-emitting... 

For simple aliphatic amines, like the methylamines, there is a linear inverse correlation between proton affinities and vertical IPs ". A low IP value should therefore indicate high proton affinity and vice versa. The proton affinity PA(B) of a molecule B is related to the homolytic bond energy D(B+—H) in the conjugate acid, as indicated by equation 8. If the homolytic bond dissociation energy is assumed to be constant for a particular functional group, e.g. N—H, the proton affinity wiU exhibit a linear correlation with the quantity IP(H) — IP(B), and such correlations have been reported for the proton affinities and the nitrogen lone-pair ionizations . ... 

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