Amine

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The general structure of an amine

Amines are organic compounds and a type of functional group that contain nitrogen as the key atom. Structurally amines resemble ammonia, wherein one or more hydrogen atoms are replaced by organic substituents such as alkyl and aryl groups. An important exception to this rule is that compounds of the type RC(O)NR2, where the C(O) refers to a carbonyl group, are called amides rather than amines. Amides and amines have different structures and properties, so the distinction is 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 Somewhat confusing is the fact that amines in which an N-H group has been replaced by an N-M group (M = metal) are also called amides.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 life processes known, as the core part of amino acids.

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

Contents

  • 1 Introduction
    • 1.1 Aliphatic Amines
    • 1.2 Aromatic amines
  • 2 Naming conventions
  • 3 Physical properties
    • 3.1 General properties
    • 3.2 Chirality
    • 3.3 Properties as bases
  • 4 Synthesis
  • 5 Reactions
  • 6 Biological activity
  • 7 Use of amines
    • 7.1 Dyes
    • 7.2 Drugs
    • 7.3 Gas Treatment
  • 8 See also

[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. Note: the subscripts on the R groups are simply used to differentiate the organic substituents . However, the number subscripts on the H atoms show how many H atoms there are in that group. 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.

[edit] Aromatic amines

Main article: Aromatic amines

Aromatic amines have the nitrogen atom connected to an aromatic ring as in anilines. The aromatic ring strongly decreases the basicity 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:

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)

[edit] Physical properties

[edit] General properties

  1. Hydrogen bonding significantly influences the properties of primary and secondary amines as well as the protonated derivatives of all amines. Thus the boiling point of amines is higher than those for the corresponding phosphines, but generally lower than the corresponding alcohols. Alcohols, or alkanols, resemble amines but feature an -OH group in place of NR2. Since oxygen is more electronegative than nitrogen, RO-H is typically more acidic than the related R2N-H compound.
  2. Methyl-, dimethyl-, trimethyl-, and ethylamine are gases under standard conditions, while diethylamine 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 solvents, especially polar organic solvents. Primary amines react with ketones such as acetone, and most amines are incompatible with chloroform and carbon tetrachloride.
  6. The 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. 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.

[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.

[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:

  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 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.

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

[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 alliphatic 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, disagreeable odors, and are toxic. The smells of ammonia, 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:

[edit] Drugs

[edit] Gas Treatment