Amine
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Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms are replaced by organic substituents such as alkyl and aryl groups. Compounds with the nitrogen atom next to a carbonyl of the structure R-C(=O)NR2 are called the amides and have different chemical properties. Important amines include amino acids, biogenic amines, trimethylamine and aniline; see Category:Amines for a list of amines.
Contents
[edit] Introduction
[edit] 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.
Similarly, an organic compound with multiple amino groups is called a diamine, triamine, tetraamine and so forth.
[edit] Aromatic amines
Aromatic amines have the nitrogen atom connected to an aromatic ring as in anilines. The aromatic ring strongly decreases the alkalinity of the amine, depending on its substituents. Interestingly, the presence of an amine group strongly increases the reactivity of the aromatic ring, due to an electron-donating effect. One organic reaction involving aromatic amines is the Goldberg reaction.
[edit] Naming conventions
Systematic names for some common amines:
methylamine
2-aminopentane
(or sometimes: pent-2-yl-amine or pentane-2-amine)
[edit] Physical properties
[edit] General properties
[edit] Chirality
Tertiary amines of the type NHRR' and NRR'R" are chiral: the nitrogen atom bears four distinct substituents counting the lone pair. The energy barrier for the 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 such as aziridines. Quaternary ammonium salts with four distinct groups on the nitrogen are capable of exhibiting optical activity.
[edit] Properties as bases
Like ammonia, amines act as bases and are reasonably strong (see table for examples of conjugate acid Ka values). The basicity of amines depends on:
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 a two dots above or next to the N. The water solubility of simple amines is largely due to hydrogen bonding between protons on the water molecules and these lone electron pairs.
- +I effect of alkyl groups raises the energy of the lone pair of electrons, thus elevating the basicity. Thus the basicity of an amine may be expected to increase with the number of alkyl groups on the amine. However, there is no strict trend in this regard, as basicity is also governed by other factors mentioned above. Consider the Kb values of the methyl amines given above. The increase in Kb from methylamine to dimethylamine may be attributed to +I effect; however, there is a decrease from dimethylamine to trimethyl amine due to the predominance of steric hindrance offered by the three methyl groups to the approaching Lewis acid.
- -M effect of aromatic ring delocalises the lone pair of electrons on nitrogen into the ring, resulting in decreased basicity. Substituents on the aromatic ring, and their positions relative to the amine group may also considerably alter basicity as seen above.
The degree of solvation of the protonated amine depends on the approachability of solvent molecules. If the molecule is sterically hindered (as in the case of trimethylamine), the protonated form is not well-solvated, thereby reducing basicity. This also explains the order of basicity of the methyl amines (see above). In the case of aprotic polar solvents (like DMSO and DMF), wherein the extent of solvation is not as high as in protic polar solvents (like water and methanol), the basicity of amines is almost solely governed by the electronic factors within the molecule.
[edit] 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 zinc, tin or iron with an acid.
-
For more details on this topic, see Reduction of nitro compounds.
[edit] Reactions
Amines react in a variety of ways:
- Because amines are basic, they neutralize carboxylic acids to form the corresponding ammonium carboxylate salts. Upon heating to 200 °C, the primary and secondary amine salts dehydrate to form the corresponding amides.
- NaNO2 + HCl → HNO2 + NaCl
- Primary aromatic amines, such as aniline (phenylamine) form more stable diazonium ions at 0–5 °C. Above 5 °C, they will decompose to give phenol and N2. Arenediazonium salts can be isolated in the crystalline form but are usually used in solution immediately after preparation, due to rapid decomposition on standing even when cold. The solid arenediazonium salt is explosive upon shock or mild warming. Because of their greater stability, arenediazonium salts are more synthetically useful than their aliphatic counterparts. Since it is not necessary to isolate the diazonium salt, once it is formed another reagent such as cuprous cyanide can simply be added to the mixture, and with gentle heating of the solution, a replacement reaction takes place along with the evolution of nitrogen. In addition, arenediazonium ions can also undergo a coupling reaction with a highly activated aromatic compound such as a phenol to form an azo compound.
- 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
[edit] Biological activity
Amines have strong, characteristic 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.
[edit] Use of amines
[edit] 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:
