Basicity of Substituted Arylamines

As noted previously, arylamines are generally less basic than alkylamines. Anilinium ion has pKg = 4.63, for instance, whereas methylammonium ion has piCg = 10.64. Arylamines are less basic than alkylamines because the nitrogen lone-pair electrons are delocalized by interaction with the aromatic ring electron system and are less available for bonding to H. In resonance terms, arylamines are stabilized relative to alkylamines because of their five resonance forms. 

Much of the resonance stabilization is lost on protonation, Jiovvcver,. so the energy difference between protonated and nonprotonated forms is higher for arylaniines than it is for alkyiamines. As a result, ary-lamines are less basic. i ig-Lire 24..-i illustrates the difference. 

Problem 24.6 Without looking at Table 24.2, rank the following compounds in order of ascending 

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There has been a review of the effects of high pressure on the substitution reactions of amines with haloaromatic compounds, including polyhalobenzenes.17 Nucleophilic substitutions by amines often proceed readily in dimethyl sulfoxide (DMSO). The pAa values, in DMSO, have been reported for some ammonium ions derived from amines widely used as nucleophiles in, S Ar reactions.18 Correlations have been established19 between the oxidation potentials and the basicities of some arylamine and diarylamine anions and the rate constants for their reactions with aryl halides in DMSO. 

The basicity order of substituted arylamines is the same as their reactivity order in... 

Arylamines contain two functional groups, the amine group and the aromatic ring they are difunctional compounds. The reactivity of the amine group is affected by its aryl substituent, and the reactivity of the ring is affected by its amine substituent. The sane electron delocalization that reduces the basicity and the nucleophilicity of an arylamine nitrogen increases the electron density in the aromatic ring and makes arylamines extremely reactive toward electrophilic aromatic substitution. 

Substituted arylamines can be either more basic or less basic than aniline, depending on the substituent. Electron-donating substituents, such as — CH3, -NH2, and -OCH3, which increase the reactivity of an aromatic ring toward electrophilic substitution (Section 16.4), also increase the basicity of the corresponding arylamine. Electron-withdrawing substituents, such as —Cl, -NO2, and -CN, which decrease ring reactivity toward electrophilic substitution, also decrease arylamine basicity. Table 24.2 considers only -substituted anilines, but similar trends are observed for ortho and meta derivatives. 

Para-Substituted anilines are oxidized in low to moderate yields to azobenzenes in basic solution (Table 5, number 4a). By elimination of one / ara-substituent (Cl or OMe) during the reaction, they can also couple to 4-substituted 4-aminodiphenyla-mines [76]. Carbon-carbon coupling can occur with 4-substituted arylamines as substrates. The / ara-substituent is eliminated and a / ara-benzidine is obtained (Table 5, number 6), ortho carbon-carbon coupling occurs (number 7), or a benzyl radical is formed, which subsequently couples (number 8). The ortho- and mera-substituted anilines yield at graphite cloth electrodes in 6iV H2SO4 substituted benzidines and 4-aminodiphenylamines. The latter products are converted by further oxidation and hydrolysis to 2-substituted / (7ra-benzoquinones, sometimes in substantial yields (e.g., 35%) [81],... 

The chemistry of amines is dominated by the lone-pair electrons on nitrogen, which makes amines both basic and nucleophilic. The base strength of arylamines is generally lower than that of aliphatic amines because the nitrogen lone-pair electrons are delocalized by interaction with the aromatic tt system. Electron-withdrawing substituents on the aromatic ring further weaken the basicity of a substituted aniline, while electron-donating substituents increase basicity. 

The propensity for C-N vs. N-H activation correlates well with substituent Hammet parameters groups that increase the basicity of aniline increase the relative rate of N-H activation, suggesting that nucleophilic attack by the amine at an empty d /dy orbital of Ta(silox)3 preceeds oxidative addition. On the other hand, electron-withdrawing substituents decrease the rate of N-H activation and increase the rate of C-N activation, similarly to the effects observed on electrophilic aromatic substitution. Nucleophilic attack by the filled d a orbital of Ta(silox)3 is expected to occur at the arylamine ipso carbon preceding C-N oxidative addition. The carbon-heteroatom cleavages can be accomodated by mechanisms using both electrophilic and nucleophilic sites on the metal center. 

Multiple substitution by strongly electron-withdrawing groups diminishes the basicity of arylamines still more. Aniline is 3800 times as strong a base as p-nitroaniline and 10 times more basic than 2,4-dinitroaniline. 

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