Basic properties and structural influence
Amines are organic bases. Like ammonia, they can act as electron-pair donors due to the nitrogen atom. The strength of these properties directly depends on the nature of the attached hydrocarbon radical, which exerts an inductive or mesomeric effect (which is directly opposite to the effect of radicals on the acidity of alcohols and phenols).
In aliphatic (saturated) amines, alkyl groups exhibit a positive inductive effect (+I). They shift electron density toward the nitrogen atom, making its electron pair more accessible for the formation of a donor-acceptor bond. Therefore, saturated amines are stronger bases than ordinary ammonia.
In aromatic amines, such as aniline, a different picture is observed. The lone electron pair of nitrogen enters into conjugation with the $\pi$-electron system of the benzene ring. It is pulled inward into the ring and becomes less accessible for bonding. Consequently, aniline is significantly weaker as a base than ammonia and aliphatic analogues.
The general series of decreasing base strength: Saturated amines > Ammonia > Aromatic amines
If a reaction is carried out between a strong base (ethylamine) and a salt of a weak one (aniline hydrochloride), the equilibrium naturally shifts toward the formation of the weaker base—free aniline.
Interaction with metal ions and nitrous acid
Due to the donor-acceptor mechanism, amines excel as ligands in complexation reactions. They interact with metal ions (predominantly of the $d$-block). For instance, adding methylamine to copper(II) sulfate results in an intensely colored blue solution due to the formation of a complex cation.
Particularly stable complexes are formed by diamines (e.g., ethylenediamine). Each such molecule has two donor nitrogen atoms and binds to a metal (such as a zinc or copper ion) via two bonds simultaneously, realizing the so-called chelate effect.
A specific reaction for primary amines is deamination. It proceeds under the action of nitrous acid (often generated directly in the reaction medium from sodium nitrite and hydrochloric acid). During this process, the amine is converted into the corresponding alcohol, accompanied by the release of nitrogen gas bubbles.
Substitution reactions in the amino group
In primary and secondary amines, the hydrogen atoms attached to nitrogen possess sufficient mobility to participate in substitution reactions:
- Alkylation: upon reaction with halogenated hydrocarbons, the hydrogen atom is replaced by a new radical. This is a classic method for preparing secondary and tertiary amines from primary ones.
- Acylation: reaction with acyl halides (e.g., acetyl chloride). This yields substituted amides in which the hydrocarbon radical is bonded directly to the nitrogen atom.
Properties of tertiary amines and reactivity of aniline
Tertiary amines, lacking mobile hydrogen atoms at the nitrogen, can undergo addition reactions with haloalkanes. This results in the formation of quaternary ammonium salts (e.g., tetraethylammonium chloride). These are crystalline compounds that are highly soluble in water. Unlike ordinary ammonium salts, they are extremely chemically stable: they do not undergo hydrolysis and are not destroyed by the action of alkalis. The most important representative of such cations in the organism is choline — a structural precursor of the neurotransmitter acetylcholine.
In aromatic amines, the amino group exerts a powerful influence on the benzene ring. Exhibiting a positive mesomeric effect (+M), it increases the electron density in the ring, which sharply accelerates electrophilic substitution reactions. This forms the basis of the qualitative test for aniline: upon adding bromine water, the solution decolorizes, and a white precipitate of 2,4,6-tribromoaniline drops out.
Briefly: answers to topical questions
What biogenic amines are formed during the decarboxylation of the amino acids histidine and tyrosine?
During transformations associated with the decarboxylation of amino acids, the following biogenic amines are formed:
- Histamine — formed by the decarboxylation of histidine; it is a mediator of inflammation and allergic reactions.
- Dopamine — formed from tyrosine.
- Norepinephrine — formed from tyrosine via dopamine.
- Epinephrine — formed from tyrosine via norepinephrine.
- Tyramine — a derivative of tyrosine.
Which drug classes contain quaternary ammonium salts?
Quaternary ammonium compounds are part of agents acting on cholinergic synapses.
- Bis-quaternary ammonium compounds — include hexamethonium benzosulfonate, azamethonium bromide, and trimethaphan camsylate.
- Curare-like agents — most representatives of the group, including tubocurarine, have such a structure.
- Myasthenia gravis medications — these include ambenonium chloride, which does not cross the blood-brain barrier.
To remember base strength, imagine a staircase: at the top step stand the strongest—saturated (aliphatic) amines, on the middle step—ammonia, and on the very bottom—weakened aromatic amines.
Class and exam questions
Why is aniline a weaker base than ethylamine?
In aniline, the lone electron pair of nitrogen participates in conjugation with the benzene ring, making it less accessible. In ethylamine, by contrast, the radical donates electron density to the nitrogen due to a positive inductive effect.
What is formed in the reaction of primary aliphatic amines with nitrous acid?
The deamination reaction takes place: a primary amine is converted into an alcohol, while water and free nitrogen gas are released.
How do quaternary ammonium salts differ from regular amine salts?
Quaternary salts are significantly more stable: they do not hydrolyze in aqueous solutions and do not decompose upon the addition of strong alkalis.
What else to study on this topic
- Mechanisms of inductive and mesomeric effects in organic molecules
- Structure and biological functions of choline in the nervous system
- Features of complexation involving diamines (chelate effect)
- Electrophilic substitution reactions in the aromatic ring using aniline as an example
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