In chemistry, amines (/əˈmiːn, ˈæmiːn/,^[1][2] UK also /ˈeɪmiːn/^[3]) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (NH₃), wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group^[4] (these may respectively be called alkylamines and arylamines; amines in which both types of substituent are attached to one nitrogen atom may be called alkylarylamines). Important amines include amino acids, biogenic amines, trimethylamine, and aniline. Inorganic derivatives of ammonia are also called amines, such as monochloramine (NClH₂).^[5]

The substituent −NH₂ is called an amino group.^[6]

Compounds with a nitrogen atom attached to a carbonyl group, thus having the structure R−C(=O)−NR′R″, are called amides and have different chemical properties from amines.

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 (1°) amine	Secondary (2°) amine	Tertiary (3°) amine

Amines, alkyl and aryl alike, are organized into three subcategories (see table) based on the number of carbon atoms adjacent to the nitrogen (how many hydrogen atoms of the ammonia molecule are replaced by hydrocarbon groups):^[6][7]

• Primary (1°) 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 (2°) amines—Secondary amines have two organic substituents (alkyl, aryl or both) bound to the nitrogen together with one hydrogen. Important representatives include dimethylamine, while an example of an aromatic amine would be diphenylamine.
• Tertiary (3°) 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.

It is also possible to have four organic substituents on the nitrogen. These species are not amines but are quaternary ammonium cations and have a charged nitrogen center. Quaternary ammonium salts exist with many kinds of anions.

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,^[8] but prefers the alkanamine form, e.g. butan-2-amine.

Physical properties

Hydrogen bonding 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 (RNH⁺
₃) > secondary ammonium (R
₂NH⁺
₂) > tertiary ammonium (R₃NH⁺). 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.^[6] 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.^[9]

Structure

Alkyl amines

Alkyl amines characteristically feature tetrahedral nitrogen centers. C-N-C and C-N-H angles approach the idealized angle of 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 (quaternary ammonium salts are resolvable).

Aromatic amines

In aromatic amines ("anilines"), 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.^[10]

Basicity

Like ammonia, amines are bases.^[11] Compared to alkali metal hydroxides, amines are weaker.

The basicity of amines depends on:

1. The electronic properties of the substituents (alkyl groups enhance the basicity, aryl groups diminish it).
2. 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 (anilines). 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's (basicities in water) 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.^[14]

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:^[5]

${\displaystyle {\ce {ROH + NH3 -> RNH2 + H2O}}}$

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:

${\displaystyle {\ce {RX + 2 R'NH2 -> RR'NH + X}}}$

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.^[5]

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.

From alkenes

Disubstituted alkenes react with HCN in the presence of strong acids to give formamides, which can be decarbonylated. This method, the Ritter reaction, is used industrially to produce tertiary amines such as tert-octylamine.^[5]

Hydroamination of alkenes is also widely practiced. The reaction is catalyzed by zeolite-based solid acids.^[5]

Reductive routes

Via the process of hydrogenation, unsaturated N-containing functional groups are reduced to amines using hydrogen in the presence of a nickel catalyst. Suitable groups include nitriles, azides, imines including oximes, amides, and nitro. In the case of nitriles, reactions are sensitive to acidic or alkaline conditions, which can cause hydrolysis of the −CN group. LiAlH₄ is more commonly employed for the reduction of these same groups on the laboratory scale.

Many amines are produced from aldehydes and ketones via reductive amination, which can either proceed catalytically or stoichiometrically.

Aniline (C₆H₅NH₂) and its derivatives are prepared by reduction of the nitroaromatics. In industry, hydrogen is the preferred reductant, whereas, in the laboratory, tin and iron are often employed.

Specialized methods

Many methods exist for the preparation of amines, many of these methods being rather specialized.

Reactions

Alkylation, acylation, and sulfonation, etc.

Aside from their basicity, the dominant reactivity of amines is their nucleophilicity.^[16] Most primary amines are good ligands for metal ions to give coordination complexes. Amines are alkylated by alkyl halides. Acyl chlorides and acid anhydrides react with primary and secondary amines to form amides (the "Schotten–Baumann reaction").

Similarly, with sulfonyl chlorides, one obtains sulfonamides. This transformation, known as the Hinsberg reaction, is a chemical test for the presence of amines.

Because amines are basic, they neutralize acids to form the corresponding ammonium salts R₃NH⁺. When formed from carboxylic acids and primary and secondary amines, these salts thermally dehydrate to form the corresponding amides.

${\displaystyle \underset{\text{amine}}{\underset{⏟<!--\; ⏟\; -->}{\text{H}-<!--\; -\; -->\stackrel{\genfrac{}{}{0ex}{}{{\displaystyle {\text{R}}_1^{}}}{|}}{\underset{\genfrac{}{}{0ex}{}{|}{{\displaystyle {\text{R}}_2^{}}}}{\text{N}}}:}}+\underset{\text{carboxylic acid}}{\underset{⏟<!--\; ⏟\; -->}{{\text{R}}_3^{}-<!--\; -\; -->\stackrel{\genfrac{}{}{0ex}{}{{\displaystyle \text{O}}}{\Vert <!--\; \Vert \; -->}}{\text{C}}-<!--\; -\; -->\text{OH}}}-<!--\; -\; -->>\underset{}{\underset{}{\text{H}-<!--\; -\; -->\stackrel{\genfrac{}{}{0ex}{}{{\displaystyle {\text{R}}_1^{}}}{|}}{\underset{\genfrac{}{}{0ex}{}{|}{{\displaystyle {\text{R}}_2^{}}}}{{\text{N}}^{+}}}-<!--\; -\; -->\text{H}}}}$Zdroj:https://en.wikipedia.org?pojem=Tertiary_amine
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