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Amines

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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 Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substitute such as an alkyl or aryl group.

Amines are derived from the simple nitrogen containing compound ammonia NH3. Replacement of a hydrogen atom by an alkyl group forms an amine. Amines occur widely in living organisms. Many of these naturally occurring compounds are very active physiologically. In addition numerous drugs used for the treatment of metal illness.

Definition

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"Amines are organic compounds that contain nitrogen and are basic. The general formula for an amine is R-NH2". One or two additional alkyl groups could be attached to the nitrogen atom of the amino group in the place of one or more hydrogen atoms.

Amines are classified as primary, secondary or tertiary depending on the number of carbon atoms bonded to nitrogen. Amines are polar compounds and primary and secondary amines associate by intermolecular hydrogen bonding. All classes of amines form hydrogen bonds with water and are more soluble in water than are hydrocarbons of comparable molecular weight.

Primary Amine

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A primary amine is an amine in which the nitrogen atom is bonded to one hydrocarbon group and two hydrogen atoms. The generalized formula for a primary amine is RNH2. The unique structural feature of a primary amine is the presence of an NH2 group. This moiety can stretch and bend giving rise to unique infrared absorbancies. The three most important vibrations of the primary amine functional group are shown below.

Primary amines are obtained by reduction of some nitrogen compounds such as nitroparaffins, nitriles, amides, oximes etc. yield primary amines by reduction.

For example
,


$CH_3NO_2 + 6[H]$ $\xrightarrow[or LiAIH_4]{Sn+HCl}$ $CH_3NH_2 + 2H_2O$

$C_2H_5NO_2 +6[H]$ $\overset{LiAlH_4}{\rightarrow}$ $C_2H_5NH_2 + 2H_2O$

$CH_3CN + 4[H]$ $\overset{Na+C_2H_5OH}{\rightarrow}$ $CH_3CH_2NH_2$
(Medium Method)

Naming 

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To name amines using IUPAC nomenclature follow these steps.

Step 1: Determine the parent compound.
Step 2:
  • Drop the final -e of the alkane name of the parent hydrocarbon and add the suffix -amine.
  • A second system for naming amines uses the suffix -ylamine for naming amines with similar hydrocarbon substituents. For example, CH3NH2 is called methylamine.
Step 3: Name the alkyl groups of secondary and tertiary amines as substituents to t
e parent compound name by adding the letter N to show that the group is bonded to the nitrogen atom.
  1. For secondary amines, add one N.
  2. For tertiary amines, add two Ns.
Step 4: If the molecule contains a second nitrogen, denote any substituents on that nitrogen with the letter N.
Step 5: Use the prefix amino or alkylamino, when the amine is a substituent on a parent molecule with another functional group. For example, when an amine and an alcohol are in the same molecule, the alcohol group determines the parent name.
Step 6: For some amines particularly the cyclic amines chemists sometimes use the prefix aza-, which is similar to the oxa- prefix of ethers. The name for an amine group attached to a carbon atom of a benzene ring is aniline.

Functional Group

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The amino group NH2 is found in primary amines. The amines may be aliphatic, aromatic or mixed depending on the nature of the functional groups and are classified as
  • Primary: For example, methylene (primary aliphatic), aniline (primary aromatic)
  • Secondary: For example, dimethyl amine (secondary aliphatic), diphenalamine (secondary aromatic).
  • Tertiary: For example, trimethyl amine (tertiary aliphatic).
Amines are present in amino acids and alkaloids. It is the derivative of ammonia (NH3), where one or more organic groups replaces hydrogen atoms. In a primary amine an organic group replaces one hydrogen atom. In secondary an tertiary amines, two or three organic groups respectively replace two or three hydrogen atoms.

Classification of Amines

Structure

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Amines are derivatives of NH3, aliphatic amines have a pyramidal shape with N sp3 hybridised and one lone pair of electron present. Amine of the type R-NH-R' or R-N(R'')-R' contains chiral nitrogen atoms and exist in two forms.

Structure of Amines

But these amines cannot be resolved into enantiomer to its mirror image. In aromatic amines the C-N bond is slightly stronger due to partial double bond character which arises as a result of delocalisation of the lone pair of N with the benzene ring.

Reactions 

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Amines are widely used as organic bases in organic chemistry. In aqueous media, amines exist in equilibria with their conjugated acids, the ammonium ions and hydroxide ion. In addition to serving as bases, common reactions of amines include serving as the nucleophile to react with various organic electrophiles. Because the nitrogen atom in amine is softer than the oxygen atom in water, such nucleophile reactions with softer electrophiles can occur readily in water.

Nucleopilic substitution reactions and nucleophilic conjugate addition reactions occur readily in aqueous media. Organic amines are also excellent ligands for various transition metals in water because of the softness of both amines and transition metals. In addition the electron lone pair on the nitrogen of amines redenrs them prone to oxidation reactions.

1. Alkylation

Because the SN2 nucleophilic substitution of uncharged amines with uncharged aliphatic organic halides involves a transition state that is more polar than that of the starting materials, such substitution reaction occurs readily in aqueous media. For example, morpholine reacts with aqueous HCHO and RhCl3.3H2O as a catalyst to give 90% N-methylmorpholine.

Alkylation of Amines
2. Diazotization or Nitrozation

Primary aromatic amines can be readily converted to diazonium salts by treatment with nitrous acid in aqueous media. The reaction also occurs with aliphatic primary amines. However aliphatic diazonium salts are extremely unstable and decompose to give a complicated mixture of substitution, elimination and rearrangement products.

EtO2C-CH2-NH2 $\overset{aq.HONO}{\rightarrow}$ EtO2C-CH=N(+)=N(-)

3. Oxidation

The oxidation of amines can be carried out by a variety of oxidizing reagents. The one electron oxidation of aromatic amines and diamines by hydroxyl radicals in water initially gives radical adducts that decay by first order kinetics and have lifetimes of approximately 5-50 $\mu$sec.

Oxidation of Amines

Basicity 

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Basicity of amines is best explained in terms of the pKa of their conjugate acids than as basicity constants Kb.
  • The more basic the amine, the weaker its conjugate acid.
  • The more basic the amine, the larger the pKa of its conjugate acid.
Citing amine basicity according to the pKa of the conjugate acid makes it possible to analyze acid-base reactions of amines according to the usual Bronstead relationships. For example, we see that amines are converted to ammonium ions by acids even as weak as acetic acid.

Basicity of Amines

The basicity provides a means by which amines may be separated from neutral organic compounds.

Uses 

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Aliphatic and aromatic amines are widely used as key intermediates or products in industrial organic chemistry.
  1. The manufacture of pharmaceutical and agrochemical products is an example that frequently includes the selective hydrogenation of a nitro group.
  2. Other applications are the synthesis of intermediates for azo dyes, pigments and photographic chemicals.
  3. Aromatis amines represent the most important class of chemical compounds for the polymer industry and for the rubber industry.
  4. The use of amines as antioxidants, corrosion inhibitors and vulcanization accelerators are of minor importance. All these products require selective and efficient synthetic routes.
  5. In industry, the majority of aliphatic and aromatic amines are manufactured by the catalytic hydrogenation of the corresponding nitro compound using precious metal or activated base metal catalysts.