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Amines are Organic Compound s 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 Substituent s 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 Amide s rather than amines. Amides and amines have different structures and properties, so the distinction is chemically important. Somewhat confusing is the fact that amines wherein an N-H group has been replaced by an N-M group (M = metal) are also called amides. Thus (CH3)2NLi is lithium dimethylamide.

See the 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.

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


Aromatic amines

Aromatic amines have the nitrogen atom connected to an Aromatic ring as in Aniline s. 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 affect. 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:



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 Phosphine s, but generally lower than the corresponding Alcohol s. 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 Solvent s, especially polar organic solvents. Primary amines react with Ketone s 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.




Chirality

Tertiary amines of the type NHRR' and NRR'R" are 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 Bases and are reasonably strong (see table for examples of Conjugate Acid Ka values). The basicity of amines depends on:
#The availability of lone pair on N.
#The electronic properties of the substituents (alkyl groups enhance the basicity, aryl groups diminish it).
#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.

: +I effect of alkyl groups raises the energy of the lone pair of electrons, thus elevating the basicity.

: +M effect of aromatic ring delocalise the lone pair electron into the ring, resulting in decreased bascitiy.

The degree of protonation of protonated amines:




SYNTHESIS

The following laboratory methods exist for the preparation of amines:


  • Hoffmann elimination of quaternary ammonium salts

    • :::R3N+CH2CH2R' + OH- → R3N + H2C=CHR' + H2O


: Nitrile s are reduced to amines using hydrogen in the presence 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 Compound s to amines can be accomplished with elemental Zinc , Tin or Iron with an Acid .
  • Nucleophilic substitution of haloalkanes. 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. Many Quaternary Ammonium Salt s can be prepared by this route with diverse R groups and many halide and pseudohalide anions.




REACTIONS

Amines react in a variety of ways:


:Because amines are basic, they neutralize Carboxylic Acid s to form the corresponding ammonium carboxylate salts. Upon heating to 200 °C, the primary and secondary amine salts dehydrate to form the corresponding Amide s.

  • By ammonium salt formation. Amines R3N react with strong acids such as Hydroiodic Acid , Hydrobromic Acid and Hydrochloric Acid in neutralization reactions forming Ammonium Salt s R3NH+.

  • By diazonium salt formation. Nitrous Acid with formula HNO2 is unstable, therefore usually a mixture of NaNO2 and dilute Hydrochloric Acid or Sulfuric Acid is used to produce nitrous acid indirectly. Primary aliphatic amines with nitrous acid give very unstable diazonium salts which spontaneously decompose by losing N2 to form carbonium ion. The carbonium ion goes on to produce a mixture of 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 Aniline (phenylamine) forms a more stable Diazonium ion at 0–5 °C. Above 5 °C, it will decompose to give Phenol and N2. Diazonium salt can be isolated in the crystalline form but are usually used in solution and immediately after preparation, due to rapid decomposition on standing even in cold. Solid salt explosive on shock or on mild warming.


: RNH2 + R'2C=O → R'2C=NR + H2O

:Secondary amines react with ketones and aldehydes to form Enamine s
: R2NH + R'(R"CH2)C=O → R"CH=C(NR2)R' + H2O


USE OF AMINES


Dyes

Primary aromatic amines are used as a starting material for the manufacture of Azo Dye s. 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 the helps to relief 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.

  • Amphetamine , Methamphetamine , and Methcathinone are amines that are listed as controlled substances by the DEA .



SEE ALSO