amino adj : pertaining to or containing any of a group of organic compounds of nitrogen derived from ammonia [syn: aminic] n : the radical -NH2 [syn: amino group]
- Rhymes with: -iːnəʊ
- Relating to an amine.
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 amides and have different chemical properties. Important amines include amino acids, biogenic amines, trimethylamine (fish smell), and aniline; see :Category:Amines for a list of amines.
Aliphatic AminesAs 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.
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.
- 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.
- Primary amines:
- Secondary amines:
- Tertiary amines:
- 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 of the corresponding phosphines, but generally lower than those of 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.
- Methyl-, dimethyl-, trimethyl-, and ethylamine are gases under standard conditions, whereas diethylamine and triethylamine are liquids. Most other common alkyl amines are liquids; high-molecular-weight amines are, of course, solids.
- Gaseous amines possess a characteristic ammonia smell, liquid amines have a distinctive "fishy" smell.
- 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.
- 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.
- The aromatic amines, such as aniline, have their lone pair
into the benzene ring, thus their tendency to engage in hydrogen
bonding is diminished. Otherwise they display the following
- 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.
ChiralityTertiary 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.
Properties as basesLike 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 electronic properties of the substituents (alkyl groups enhance the basicity, aryl groups diminish it).
- Steric hindrance offered by the groups on nitrogen.
- 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.
- Inductive effect of alkyl groups