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Amines

The organic molecules mainly consist of carbon atoms with some hetero atoms like hydrogen, nitrogen, halogen, nitrogen, oxygen and phosphorus. Hydrocarbons are composed of carbon and hydrogen atoms. They can be classified as saturated and unsaturated on the basis of single and multiple covalent bonds. Hydrocarbons with single covalent bonds are known alkanes. On the contrary, multiple bonds either double or triple bonds between carbon-carbon atoms.

Alkenes have a double covalent bond whereas alkynes have a triple covalent bond between carbon atoms. If there is any other atom or group of atom is bonded to the parent chain, it alters the chemical and physical of hydrocarbons. For example, if there is a -OH group bonded to the carbon chain by the replacement of hydrogen atom, it forms alcohols. Similarly the presence of –COOH group on the carbon chain forms carboxylic acid which shows different chemical and physical properties compare to alkanes.

Let’s discuss some chemical and physical properties of amines which contain –$NH_2$ group directly bonded to the parent carbon chain of the molecule. If there is an amino group directly bonded to the carbon atom of the aromatic ring, it forms aromatic amine which is commonly called as aniline.

 

What are Amines?

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Amines are derivatives of ammonia(NH3). Amines are formed by replacing one or more hydrogen atoms of ammonia (NH3) with an alkyl or an aryl group. These are further classified as primary, secondary and tertiary amines according to one, two or three hydrogen atoms of ammonia being replaced by alkyl or aryl groups.
For example,

1. Primary (amino, NH2)

Primary amines have one alkyl group attached to the nitrogen atom.

Primary Amines

2. Secondary (imino, NH)

Secondary amines have two alkyl groups attached to the nitrogen atom.

 Secondary Amines

3. Tertiary (nitrile, N)

Tertiary amines have three alkyl groups attached to the nitrogen atom.

Tertiary Amines

Naming Amines

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In the common system, amines are named by adding the suffix -amine to the name of the alkyl group or groups. The entire name is written as one word. The names of the alkyl groups are arranged in alphabetic order.

In IUPAC system, primary aliphatic amines are named by replacing the final 'e' of the parent alkane with -amine.(Alkanamine). If necessary, a number is added to indicate the position of the -NH2 group.

CH3CH2NH2 - Ethanamine, CH3CH(NH2)CH3 - 2 - Propanamine (since the NH2 is attached to the second carbon)

The secondary and tertiary amines are named as N- substituted derivatives of the primary amines. The largest of the groups attached to the nitrogen is chosen as the organic group of the primary amine. The remaining alkyl groups are named as substituents by using the prefix N- to indicate that they are attached to nitrogen. The higher alkyl group is considered as the residue of the parent alkane.


Examples of secondary and tertiary amines
Amines IUPAC Name
This indicates the number of C-atoms in the chain: the other groups are considered as the substituent. For diamines, the final -e of the hydrocarbon name is retained.
H2NCH2CH2CH2CH2CH2CH2NH2 : 1, 6 Hexanediamine

When it is necessary to name NH2 as a substituent, it is called the amino group.

Primary Amines

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Primary Amine IUPAC Name

For primary amines containing three or more carbon atoms, there can be more than one isomers of the concerned amine. For example, there are two propylamines.

Two Propylamines


Examples of Amines

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A few examples of aliphatic and aromatic amines are as follows.

Aliphatic amines

Examples of Amines

Aromatic amines

Aromatic Amines



Structure of Amines

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Let us consider methylamine (CH3NH2) for illustrating the orbital make up of amines. In methylamine, both nitrogen and carbon are sp3 hybridized. One of the sp3 orbitals of nitrogen is completely filled and cannot take part in the bond formation. The C-N $\sigma $ bond in methylamine is formed by overlap of an sp3 orbital of carbon and an sp3 orbital of nitrogen.

Each N-H $\sigma $ bond is formed by the overlap of an sp3 orbital of nitrogen and s orbital of hydrogen.

Similarly, each C-H $\sigma $ bond is formed by the overlap of an sp3 orbital of carbon and s orbital of hydrogen. All bond angles are approximately tetrahedral.

Methylamine

Aromatic Amines

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Aromatic amines are the derivatives of aromatic hydrocarbons in which a hydrogen of the benzene ring has been replaced by an amino group, -NH2.
Aromatic Amines
All such compounds in which an amino or substituted amino group is bonded directly to an aromatic ring are termed Aromatic amines. Like their aliphatic counterparts, aromatic amines could be designated as primary, secondary or tertiary amines.

C6H5-NH2 - Aniline (1o amine)

C6H5-NH-C6H5 - Diphenylamine (2oamine)

C6H5-N(C6H5 )2 - Dimethyl aniline (3oamine)


The derivatives of arenes in which NH2 is bonded to a side chain are to be regarded as aryl substituted aliphatic amines.

C6H5-CH2-NH2 - Benzylamine

Nomenclature


Simple aromatic amines are named as derivatives of aniline. An amino toluene is given special name of Toludine. Substituents on the nitrogen atom are named as prefixes by N- or N, N-.

C6H5-NH(CH3) - Methyl aniline.

Basicity of Amines

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The basicity of a compound is a measure of a compound's ability to accept a proton(H+).

Amines are basic in nature. This is because they possess an unshared pair(lone pair) of electrons on nitrogen. This lone pair of electrons is available for the formation of a new bond with a proton or Lewis acids. Thus, amines react with acids to form salts.
$RNH_{2} + HCl \rightarrow RNH_{3}^{+} \ Cl^{-}\overset{OH^{-}}{\rightarrow}RNH_{2} + H_{2}O$
Amine Alkyl ammonium chloride

Basicity constants


Strong bases like NaOH, KOH, etc completely ionize in aqueous solution. Amines are weak bases. They are only partially ionized in aqueous solution and an equilibrium exists between the ionized and unionized forms.
$RNH_{2} + H_{2}O \rightleftharpoons RNH_{3}^{+} + OH^{-}$

The extent of ionization by an equilibrium constant Kb, which is known as Basicity constant. It is defined as the concentration of the products of ionization per liter by the concentration of the unionized base.
$K_{b}$ = $\frac{[RNH_{3}^{+}][OH^{-}]}{[RNH_{2}]}$

The basicity constant describes the relative strength of a weak base. Stronger bases will have higher numerical values of basicity constants.

Relative basicities


Aliphatic amines are stronger bases than ammonia. This is because the alkyl groups are electron-releasing. They increase the electron density around the nitrogen, thereby increasing the availability of the lone pair of electrons.

" The greater the number of electron-releasing alkyl groups, the greater the availability of nitrogen's lone pair and stronger the base."


Compound
kb value
Ammonia (NH3) 1.8 x 10-5
Methyl amine (CH3NH2) 37 X 10-5
Di methyl amine (CH3-NH-CH3) 54 X 10-5
Tri methyl amine (CH3-N(CH3)2 6.7 X 10-5

The dimethylamine is a stronger base than methylamine. However, trimethylamine is a weaker base than both dimethylamine and methylamine. Although in trimethylamine the electron- density is further increased, the steric crowding of three methyl groups makes the approach and bonding by a proton relatively difficult. The electrons are there but the path is blocked.

Basicity of aliphatic amines in increasing order


Basicity of Aliphatic Amines

In an aliphatic amine, the non bonding electron pair on N atom is localized and is fully available for coordination with a proton. On the other hand, in aromatic amines, the non-bonding electron pair is delocalized into the benzene ring by resonance.

Delocalization in Aromatic Amines

The non-bonding electron pair in the hybrid being dispersed over the benzene ring as also the N atom, is less available for protonation. Hence, aromatic amines are less basic than aliphatic amines due to resonance.

More topics in Amines
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Alkaloids
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