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Amine-2D-general

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 Somewhat confusing is the fact that amines in which an N-H group has been replaced by an N-M group (M = metal) are also called amides.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.

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. 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
Primary-amine-2D-general
Secondary-amine-2D-general
Amine-2D-general

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 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.

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
methylamine

Higher amines have the prefix amino as a functional group.

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

Physical properties

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.
Inversion of Amine

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.

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.

Drugs

See also

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