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In chemistry, amines (, ) are 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 hetero cyclic compounds. Aniline is the simplest aromatic amine, consisting of a benzene ring bonded to an amino group.
Amines are classified into three types: primary (1°), secondary (2°), and tertiary (3°) amines. Primary amines (1°) contain one alkyl or aryl substituent and have the general formula
The functional group present in primary amines is called the amino group. (2025). 9780073375625, McGraw-Hill. ISBN 9780073375625
A fourth subcategory is determined by the connectivity of the substituents attached to the nitrogen:
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.
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 (R3NH+). Small aliphatic amines display significant solubility in many , 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.
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 (quaternary ammonium salts are resolvable).
| Methylamine (MeNH2) | 10.62 | |
| Dimethylamine (Me2NH) | 10.64 | |
| Trimethylamine (Me3N) | 9.76 | |
| Ethylamine (EtNH2) | 10.63 | |
| Aniline (PhNH2) | 4.62 | |
| p-Anisidine (4-MeOC6H4NH2) | 5.36 | |
| Dimethylaniline (PhNMe2) | 5.07 | |
| 3-Nitroaniline (3-NO2-C6H4NH2) | 2.46 | |
| 4-Nitroaniline (4-NO2-C6H4NH2) | 1.00 | |
| 4-Trifluoromethylaniline (CF3C6H4NH2) | 2.75 |
The basicity of amines depends on:
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.
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.
Hydroamination of alkenes is also widely practiced. The reaction is catalyzed by zeolite-based .
Many amines are produced from aldehydes and ketones via reductive amination, which can either proceed catalytically or stoichiometrically.
Aniline () 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.
Because amines are basic, they neutralize to form the corresponding . When formed from carboxylic acids and primary and secondary amines, these salts thermally dehydrate to form the corresponding .
Amines undergo sulfamation upon treatment with sulfur trioxide or sources thereof:
Anilines and naphthylamines form more stable diazonium salts, which can be isolated in the crystalline form. Diazonium salts undergo a variety of useful transformations involving replacement of the group with anions. For example, cuprous cyanide gives the corresponding nitriles:
Aryldiazoniums couple with electron-rich aromatic compounds such as a phenol to form . Such reactions are widely applied to the production of dyes.
Similarly, secondary amines react with ketones and aldehydes to form :
Mercuric ions reversibly oxidize with an locant hydrogen to iminium ions:
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 , organic azide, including , amides, and Nitro compound. In the case of nitriles, reactions are sensitive to acidic or alkaline conditions, which can cause hydrolysis of the group. is more commonly employed for the reduction of these same groups on the laboratory scale.
Specialized methods
Many methods exist for the preparation of amines, many of these methods being rather specialized.
Staudinger reduction Organic azide This reaction also takes place with a reducing agent such as lithium aluminium hydride. Schmidt reaction Carboxylic acid Aza-Baylis–Hillman reaction Imine Synthesis of allylic amines Birch reduction Imine Useful for reactions that trap unstable imine intermediates, such as Grignard reactions with . Hofmann degradation Amide This reaction is valid for preparation of primary amines only. Gives good yields of primary amines uncontaminated with other amines. Hofmann elimination Quaternary ammonium salt Upon treatment with strong base Leuckart reaction and Reductive amination with formic acid and ammonia via an imine intermediate Hofmann–Löffler reaction Haloamine Eschweiler–Clarke reaction Amine Reductive amination with formic acid and formaldehyde via an imine intermediate
Reactions
Alkylation, acylation, and sulfonation, etc.
Aside from their basicity, the dominant reactivity of amines is their nucleophilicity. Most primary amines are good for metal ions to give coordination complexes. Amines are alkylated by alkyl halides. and acid anhydrides react with primary and secondary amines to form (the "Schotten–Baumann reaction").
Similarly, with sulfonyl chlorides, one obtains . This transformation, known as the Hinsberg reaction, is a chemical test for the presence of amines.
{
\underbrace\ce{H-\!\!\overset{\displaystyle R1 \atop |}{\underset
C-OH}_\text{carboxylic acid} ->
}\
\underbrace\ce-H} + R3-COO^-}
\ce{->\text{heat}\text{dehydration}}{
\underbrace\ce{R3-\overset{\displaystyle O \atop \ \underbrace\ce{\overset{\displaystyle R1 \atop |}{\underset
C-R3}_\text{amide} +
\underbrace\ce{H2O}_\text{water}
}
Diazotization
Amines reacts with nitrous acid to give diazonium salts. The alkyl diazonium salts are of little importance because they are too unstable. The most important members are derivatives of aromatic amines such as aniline ("phenylamine") (A = aryl or naphthyl):
(2025). 9783527306732 ISBN 9783527306732
Conversion to imines
Imine formation is an important reaction. Primary amines react with and to form . In the case of formaldehyde (R' H), these products typically exist as cyclic trimers:
Overview
An overview of the reactions of amines is given below:
Amine alkylation Amines Degree of substitution increases Schotten–Baumann reaction Amide Reagents: , Hinsberg reaction Reagents: sulfonyl chlorides Amine–carbonyl condensation Imines Organic oxidation Nitroso compounds Reagent: peroxymonosulfuric acid Organic oxidation Diazonium salt Reagent: nitrous acid Zincke reaction Zincke aldehyde Reagent: pyridinium salts, with primary and secondary amines Emde degradation Tertiary amine Reduction of quaternary ammonium cations Hofmann–Martius rearrangement Aryl-substituted von Braun reaction Organic cyanamide By cleavage (tertiary amines only) with cyanogen bromide Hofmann elimination Alkene Proceeds by β-elimination of less hindered carbon Cope reaction Alkene Similar to Hofmann elimination Carbylamine reaction Isonitrile Primary amines only Hofmann's mustard oil test Isothiocyanate Carbon disulfide and mercury(II) chloride are used. Thiocyanate smells like mustard.
Biological activity
Amines are ubiquitous in biology. The breakdown of releases amines, famously in the case of decaying fish which smell of trimethylamine. Many are amines, including epinephrine, norepinephrine, dopamine, serotonin, and histamine. Protonated (–) are the most common positively charged moieties in , specifically in the amino acid lysine. The anionic polymer DNA is typically bound to various amine-rich proteins. Additionally, the terminal charged primary ammonium on lysine forms salt bridges with carboxylate groups of other amino acids in , which is one of the primary influences on the three-dimensional structures of proteins.
Amine hormones
derived from the modification of amino acids are referred to as amine hormones. Typically, the original structure of the amino acid is modified such that a –COOH, or carboxyl, group is removed, whereas the –, or amine, group remains. Amine hormones are synthesized from the amino acids tryptophan or tyrosine. (2023). 9781947172043, OpenStax CNX. ISBN 9781947172043
Application of amines
Dyes
Primary aromatic amines are used as a starting material for the manufacture of . It reacts with nitrous acid to form diazonium salt, which can undergo coupling reaction to form an azo compound. As azo-compounds are highly coloured, they are widely used in dyeing industries, such as:
Drugs
Most drugs and drug candidates contain amine functional groups:
Gas treatment
Aqueous monoethanolamine (MEA), diglycolamine (DGA), diethanolamine (DEA), diisopropanolamine (DIPA) and methyldiethanolamine (MDEA) are widely used industrially for removing carbon dioxide () and hydrogen sulfide (H2S) from natural gas and refinery process streams. They may also be used to remove from combustion gases and and may have potential for abatement of . Related processes are known as sweetening. (2025). 9783527306732 ISBN 9783527306732
Epoxy resin curing agents
Amines are often used as epoxy resin curing agents. These include dimethylethylamine, cyclohexylamine, and a variety of diamines such as 4,4-diaminodicyclohexylmethane. Multifunctional amines such as tetraethylenepentamine and triethylenetetramine are also widely used in this capacity. The reaction proceeds by the lone pair of electrons on the amine nitrogen attacking the outermost carbon on the oxirane ring of the epoxy resin. This relieves ring strain on the epoxide and is the driving force of the reaction.Howarth G.A "Synthesis of a legislation compliant corrosion protection coating system based on urethane, oxazolidine and waterborne epoxy technology" pages 12, Chapter 1.3.1 Master of Science Thesis April 1997 Imperial College London Molecules with tertiary amine functionality are often used to accelerate the epoxy-amine curing reaction and include substances such as 2,4,6-Tris(dimethylaminomethyl)phenol. It has been stated that this is the most widely used room temperature accelerator for two-component epoxy resin systems.
Safety
Low molecular weight simple amines, such as ethylamine, are toxic with between 100 and 1000 mg/kg. They are skin irritants, especially as some are easily absorbed through the skin. Amines are a broad class of compounds, and more complex members of the class can be extremely bioactive, for example strychnine.
See also
Further reading
External links
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