Amine

In chemistry, amines are organic compounds that contain carbon–nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of electrons. Amines can also exist as heterocyclic compounds. Aniline is the simplest aromatic amine, consisting of a benzene ring bonded to an amino group.
Amines are classified into three types: primary, secondary, and tertiary amines. Primary amines contain one alkyl or aryl substituent and have the general formula RNH2. Secondary amines have two alkyl or aryl groups attached to the nitrogen atom, with the general formula R2NH. Tertiary amines contain three substituent groups bonded to the nitrogen atom, and are represented by the formula R3N.
The functional group present in primary amines is called the amino group.

Classification of amines

Amines can be classified according to the nature and number of substituents on nitrogen. Aliphatic amines contain only H and alkyl substituents. Aromatic amines have the nitrogen atom connected to an aromatic ring.

Primary amine	Secondary amine	Tertiary amine

Amines, alkyl and aryl alike, are organized into three subcategories based on the number of carbon atoms adjacent to the nitrogen :

Primary amines—Primary amines arise when one of three hydrogen atoms in ammonia is replaced by an alkyl or aromatic group. Important primary alkyl amines include methylamine, most amino acids, and the buffering agent tris, while primary aromatic amines include aniline.
Secondary amines—Secondary amines have two organic substituents bound to the nitrogen together with one hydrogen. Important representatives include dimethylamine, while an example of an aromatic amine would be diphenylamine.
Tertiary amines—In tertiary amines, nitrogen has three organic substituents. Examples include trimethylamine, which has a distinctively fishy smell, and EDTA.

A fourth subcategory is determined by the connectivity of the substituents attached to the nitrogen:

Cyclic amines—Cyclic amines are either secondary or tertiary amines. Examples of cyclic amines include the 3-membered ring aziridine and the six-membered ring piperidine. N-methylpiperidine and N-phenylpiperidine are examples of cyclic tertiary amines.
Related compounds

Compounds where the nitrogen atom is attached to a carbonyl group, thus having the structure, are called amides.
Compounds containing four organic substituents on the nitrogen, thus having the structure, are quaternary ammonium salts. Many kinds of anions are found in such compounds.

Naming conventions

Amines are named in several ways. Typically, the compound is given the prefix "amino-" or the suffix "-amine". The prefix "N-" shows substitution on the nitrogen atom. An organic compound with multiple amino groups is called a diamine, triamine, tetraamine and so forth.
Lower amines are named with the suffix -amine.
Higher amines have the prefix amino as a functional group. IUPAC however does not recommend this convention, but prefers the alkanamine form, e.g. butan-2-amine.

Physical properties

significantly influences the properties of primary and secondary amines. For example, methyl and ethyl amines are gases under standard conditions, whereas the corresponding methyl and ethyl alcohols are liquids. Amines possess a characteristic ammonia smell, liquid amines have a distinctive "fishy" and foul smell.
The nitrogen atom features a lone electron pair that can bind H⁺ to form an ammonium ion R₃NH⁺. The lone electron pair is represented in this article by two dots above or next to the N. The water solubility of simple amines is enhanced by hydrogen bonding involving these lone electron pairs. Typically salts of ammonium compounds exhibit the following order of solubility in water: primary ammonium > secondary ammonium > tertiary ammonium. Small aliphatic amines display significant solubility in many solvents, whereas those with large substituents are lipophilic. Aromatic amines, such as aniline, have their lone pair electrons conjugated into the benzene ring, thus their tendency to engage in hydrogen bonding is diminished. Their boiling points are high and their solubility in water is low.

Spectroscopic identification

Typically the presence of an amine functional group is deduced by a combination of techniques, including mass spectrometry as well as NMR and IR spectroscopies. ¹H NMR signals for amines disappear upon treatment of the sample with D₂O. In their infrared spectrum primary amines exhibit two N–H bands, whereas secondary amines exhibit only one. In their IR spectra, primary and secondary amines exhibit distinctive N–H stretching bands near 3300 cm⁻¹. Somewhat less distinctive are the bands appearing below 1600 cm⁻¹, which are weaker and overlap with C–C and C–H modes. For the case of propyl amine, the H–N–H scissor mode appears near 1600 cm⁻¹, the C–N stretch near 1000 cm⁻¹, and the R₂N–H bend near 810 cm⁻¹.

Structure

Alkyl amines

Alkyl amines characteristically feature tetrahedral nitrogen centers. C-N-C and C-N-H angles are near 109°. C-N distances are slightly shorter than C-C distances. The energy barrier for the nitrogen inversion of the stereocenter is about 7 kcal/mol for a trialkylamine. The interconversion has been compared to the inversion of an open umbrella into a strong wind.
Amines of the type NHRR' and NRR′R″ are chiral: the nitrogen center bears four substituents counting the lone pair. Because of the low barrier to inversion, amines of the type NHRR' cannot be obtained in optical purity. For chiral tertiary amines, NRR′R″ can only be resolved when the R, R', and R″ groups are constrained in cyclic structures such as N-substituted aziridines.

Aromatic amines

In aromatic amines, nitrogen is often nearly planar owing to conjugation of the lone pair with the aryl substituent. The C-N distance is correspondingly shorter. In aniline, the C–N distance is the same as the C–C distances.

Basicity

Like ammonia, amines are bases. Compared to alkali metal hydroxides, amines are weaker.

Alkylamine or aniline	pK_a of protonated amine	K_b
Methylamine	10.62
Dimethylamine	10.64
Trimethylamine	9.76
Ethylamine	10.63
Aniline	4.62
4-Methoxyaniline	5.36
N,''N''-Dimethylaniline	5.07
3-Nitroaniline	2.46
4-Nitroaniline	1.00
4-Trifluoromethylaniline	2.75

The basicity of amines depends on:

The electronic properties of the substituents.
The degree of solvation of the protonated amine, which includes steric hindrance by the groups on nitrogen.
Electronic effects

Owing to inductive effects, the basicity of an amine might be expected to increase with the number of alkyl groups on the amine. Correlations are complicated owing to the effects of solvation which are opposite the trends for inductive effects. Solvation effects also dominate the basicity of aromatic amines. For anilines, the lone pair of electrons on nitrogen delocalizes into the ring, resulting in decreased basicity. Substituents on the aromatic ring, and their positions relative to the amino group, also affect basicity as seen in the table.

Solvation effects

Solvation significantly affects the basicity of amines. N−H groups strongly interact with water, especially in ammonium ions. Consequently, the basicity of ammonia is enhanced by 10¹¹ by solvation. The intrinsic basicity of amines, i.e. the situation where solvation is unimportant, has been evaluated in the gas phase. In the gas phase, amines exhibit the basicities predicted from the electron-releasing effects of the organic substituents. Thus tertiary amines are more basic than secondary amines, which are more basic than primary amines, and finally ammonia is least basic. The order of pK_b values does not follow this order. Similarly, aniline is more basic than ammonia in the gas phase, but ten thousand times less so in aqueous solution.
In aprotic polar solvents such as DMSO, DMF, and acetonitrile the energy of solvation is not as high as in protic polar solvents like water and methanol. For this reason, the basicity of amines in these aprotic solvents is almost solely governed by the electronic effects.

Synthesis

From alcohols

Industrially significant alkyl amines are prepared from ammonia by alkylation with alcohols:

From alkyl and aryl halides

Unlike the reaction of amines with alcohols the reaction of amines and ammonia with alkyl halides is used for synthesis in the laboratory:
In such reactions, which are more useful for alkyl iodides and bromides, the degree of alkylation is difficult to control such that one obtains mixtures of primary, secondary, and tertiary amines, as well as quaternary ammonium salts.
Selectivity can be improved via the Delépine reaction, although this is rarely employed on an industrial scale. Selectivity is also assured in the Gabriel synthesis, which involves organohalide reacting with potassium phthalimide.
Aryl halides are much less reactive toward amines and for that reason are more controllable. A popular way to prepare aryl amines is the Buchwald-Hartwig reaction.