Amine




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Amines are chemically important. Somewhat confusing is the fact that amines in which an N-H group has been replaced by an N-M group (M = metal) are also called amides. Thus (CH3)2NLi is lithium dimethylamide.

Amines are central in organic chemistry. All known life processes depend on amino acids, each of which contains an amine group.

See Category:Amines for a list of types of amine and some real examples of this class of chemical.

Introduction

Aliphatic Amines

As displayed in the images below, primary amines arise when one of three hydrogen atoms in ammonia is replaced by an organic substituent. Secondary amines have two organic substituents bound to N together with one H. In tertiary amines all three hydrogen atoms are replaced by organic substituents. It is also possible to have four alkyl substituents on the nitrogen. These compounds have a charged nitrogen center, and necessarily come with a negative counterion, so they are called quaternary ammonium salts.

Primary amine Secondary amine Tertiary amine

Similarly, an organic compound with multiple amino groups is called a diamine, triamine, tetraamine and so forth.

Aromatic amines

Main article: Aromatic amines

Aromatic amines have the nitrogen atom connected to an Goldberg reaction.

Naming conventions

  • the prefix "N-" shows substitution on the nitrogen atom
  • as prefix: "amino-"
  • as suffix: "-amine"
  • remember that chemical compounds are not proper nouns, so lower case is indicated throughout.

Systematic names for some common amines:

Lower amines are named with the suffix -amine.


methylamine

Higher amines have the prefix amino as a functional group.


2-aminopentane
(or sometimes: pent-2-yl-amine or pentane-2-amine)

Physical properties

General properties

  1. electronegative than nitrogen, RO-H is typically more acidic than the related R2N-H compound.
  2. Methyl-, dimethyl-, trimethyl-, and triethylamine are liquids. Most other common alkyl amines are liquids; high-molecular-weight amines are, of course, solids.
  3. Gaseous amines possess a characteristic ammonia smell, liquid amines have a distinctive "fishy" smell.
  4. Most aliphatic amines display some solubility in water, reflecting their ability to form hydrogen bonds. Solubility decreases with the increase in the number of carbon atoms, especially when the carbon atom number is greater than 6.
  5. Aliphatic amines display significant solubility in organic carbon tetrachloride.
  6. The aromatic amines, such as conjugated into the benzene ring, thus their tendency to engage in hydrogen bonding is diminished. Otherwise they display the following properties:
    • Their boiling points are usually still high due to their larger size.
    • Diminished solubility in water, although they retain their solubility in suitable organic solvents only.
    • They are toxic and are easily absorbed through the skin: thus hazardous.

Chirality

Tertiary amines of the type NHRR' and NRR'R" are inversion of the stereocenter is relatively low, e.g., ~7 kcal/mol for a trialkylamine. The interconversion of the stereoisomers has been compared to the inversion of an open umbrella in to a strong wind. Because of this low barrier, amines such as NHRR' cannot be resolved optically and NRR'R" can only be resolved when the R, R', and R" groups are constrained in cyclic structures.

Properties as bases

Like ammonia, amines act as conjugate acid Ka values). The basicity of amines depends on:

  1. The availability of the lone pair of electrons on the Nitrogen atom.
  2. The electronic properties of the substituents (alkyl groups enhance the basicity, aryl groups diminish it).
  3. The degree of solvation of the protonated amine.

The nitrogen atom features a hydrogen bonding between protons on the water molecules and these lone electron pairs.

Ions of compound Kb
Ammonia NH3 1.8·10-5 M
Methylamine CH3NH2 4.4·10-4 M
propylamine CH3CH2CH2NH2 4.7·10-4 M
2-propylamine (CH3)2CHNH2 5.3·10-4 M
diethylamine (CH3)2NH 9.6·10-4 M
+I effect of alkyl groups raises the energy of the lone pair of electrons, thus elevating the basicity.
Ions of compound Kb
Ammonia NH3 1.8·10-5 M
Aniline C6H5NH2 3.8·10-10 M
4-methylphenylamine 4-CH3C6H4NH2 1.2·10-9 M
+M effect of aromatic ring delocalise the lone pair electron into the ring, resulting in decreased basicity.

The degree of protonation of protonated amines:

Ions of compound Maximum number of H-bond
NH4+ 4 Very Soluble in H2O
RNH3+ 3
R2NH2+ 2
R3NH+ 1 Least Soluble in H2O

Synthesis

The following laboratory methods exist for the preparation of amines:

Nitriles are reduced to amines using hydrogen in the presence of a nickel catalyst, although acidic or alkaline conditions should be avoided to avoid hydrolysis of -CN group. LiAlH4 is more commonly employed for the reduction of nitriles on the laboratory scale. Similarly, LiAlH4 reduces amides to amines:
The reduction of nitro compounds to amines can be accomplished with elemental acid.
For more details on this topic, see Reduction of nitro compounds.
  • Nucleophilic substitution of haloalkanes [1]. Primary amines can also be synthesized by alkylaton of ammonia. Haloalkanes react with amines to give a corresponding alkyl-substituted amine, with the release of a halogen acid. Such reactions, which are most useful for alkyl iodides and bromides, are rarely employed because the degree of alkylation is difficult to control. If the reacting amine is tertiary, a quaternary ammonium cation results in the Menshutkin reaction. Many quaternary ammonium salts can be prepared by this route with diverse R groups and many halide and pseudohalide anions.

Reactions

Amines react in a variety of ways:

  • By acylation can still be performed by the use of excess aqeous alkali to facilitate the reaction.
Because amines are basic, they neutralize amides.
  • By ammonium salt formation. Amines R3N react with strong acids such as hydroiodic acid, hydrobromic acid and hydrochloric acid in neutralization reactions forming ammonium salts R3NH+.
  • By diazonium salt formation. Nitrous acid with formula HNO2 is unstable, therefore usually a mixture of NaNO2 and dilute alkenes, alkanols or alkyl halides, with alkanols as the major product. This reaction is of little synthetic importance because the diazonium salt formed is too unstable, even at cold conditions.
NaNO2 + HCl → HNO2 + NaCl
Primary aromatic amines, such as azo compound.
  • By formaldehyde (R' = H), these products are typically cyclic trimers.
RNH2 + R'2C=O → R'2C=NR + H2O
Secondary amines react with ketones and aldehydes to form enamines
R2NH + R'(R"CH2)C=O → R"CH=C(NR2)R' + H2O

Biological activity

Amines have strong, characteristic, disagreeable odors, and are toxic. The smells of ammonia, old fish, urine, rotting flesh, and semen are all mainly composed of amines. Many kinds of biological activity produce amines by breakdown of amino acids.

Use of amines

Dyes

Primary aromatic amines are used as a starting material for the manufacture of azo dyes. It reacts with nitric(III) acid to form diazonium salt, which can undergo coupling reaction to form azo compound. As azo-compounds are highly coloured, they are widely used in dyeing industries, such as:

Drugs

  • Chlorpheniramine is an antihistamine that helps to relieve allergic disorders due to cold, hay fever, itchy skin, insect bites and stings.
  • Chlorpromazine is a tranquillizer that sedates without inducing sleep. It is used to relieve anxiety, excitement, restlessness or even mental disorder.
  • Ephedrine and Phenylephrine, as amine hydrochlorides, are used as decongestants.
  • Methcathinone are amines that are listed as controlled substances by the DEA.
  • Clomipramine are tricylic antidepressants and tertiary amines
  • Amoxapine are tricyclic antidepressants and secondary amines
  • (The tricylics are grouped by the nature of the final amine group on the side chain.)

Gas Treatment

  • Aqueous hydrogen sulphide (H2S) from natural gas streams and refinery process streams. They may also be used to remove CO2 from combustion gases / flue gases and may have potential for abatement of greenhouse gases.

See also

References

  1. ^ For an example see: Org. Synth. 2008, 85, 10-14 Article
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