Heterocyclic amines basicity

Table 24.1 lists pXa values of some ammonium ions and indicates that there is a substantial range of amine basicities. Most simple alkylainines are similar in their base strength, with p/q/s for their ammonium ions in the narrow range 10 to 11. Aiylctmines, however, are considerably less basic than alkylamines, as are the heterocyclic amines pyridine and pyurole. 

Other common five-membered heterocyclic amines include imidazole and thiazole. Imidazole, a constituent of the amino acid histidine, has two nitrogens, only one of which is basic. Thiazole, the five-membered ring system on which the structure of thiamin (vitamin Bt) is based, also contains a basic nitrogen that is alkylated in thiamin to form a quaternary ammonium ion. 

The use of heterocyclic diazo components is particularly important in the preparation of blue disperse dyes. It is often difficult to prepare, diazotise and/or couple the highly electronegatively substituted diazo components of the benzene series needed to achieve blue hues, and feebly basic aminoheterocycles offer a convenient alternative. The small molecular size of these heterocyclic amines, compared with that of heavily substituted anilines, is another factor favouring their use. 

For weakly basic amines, i.e. anilines with poly nitro groups, e.g. 2, 4,-dintro-6-bromoaniline, and also most heterocyclic amines, an alternative method must be adopted using nitrosylsulfuric acid. This diazotising medium can be made by dissolving sodium nitrite in concentrated sulfuric acid, but is also available preformed from chemical suppliers. The weakly basic amine is added gradually to the nitrosyl-sulfuric acid with cooling, and after reaction is complete the mixture is poured into ice and water to give a solution of the diazonium salt. 

Not only alkylations, but also Michael-type reactions, with weakly basic heterocyclic amines, can be accomplished by PTC. Yamada and Ohki158 have recently reported such a reaction with pyridazinones (99). 

Tertiary amines catalyze the homopolymerization of epoxy resins in the presence of hydroxyl groups, a condition which generally exists since most commercial resins contain varying amounts of hydroxyl functionality (B-68MI11501). The efficiency of the catalyst depends on its basicity and steric requirements (B-67MI11501) in the way already discussed for amine-catalyzed isocyanate reactions. A number of heterocyclic amines have been used as catalytic curatives pyridine, pyrazine, iV,A-dimethylpiperazine, (V-methylmorpholine and DABCO. Mild heat is usually required to achieve optimum performance which, however, is limited due to the low molecular weight polymers obtained by this type of cure. 

Although tryptophan contains a heterocyclic amine, it is considered a neutral amino acid. Explain why the indole nitrogen of tryptophan is more weakly basic than one of the imidazole nitrogens of histidine. 

The a chloroketone b simply nude by chlorination, and substitution b rapid and efficient even with the weekly basic (Chapter 0) heterocyclic amine. 

Predict the basicity of alkylamines, arylamines and heterocyclic amines. 

Most low-molecular-weight aliphatic amines are somewhat stronger bases than ammonia. Table 28-2 shows that aliphatic amines are much stronger bases than aromatic and heterocyclic amines. The basicities of amines often decrease roughly in the order tertiary > secondary > primary. Other structural factors and solvation effects, however, may outweigh this tendency, especially with tertiary amines. 

BU4NF, THF, rt-reflux, 75-98% yield. This method is specific for electron-deficient amines such as heterocyclic amines and electron poor anilines. Because TBAF contains about 4% water and is considered basic, some amides are also cleaved. 

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