Explain the physical properties of amine compounds. Explain the basic character (nature) of amine compounds.

(N/A) The lower aliphatic amines are gases with a fishy odour. Primary amines with three or more carbon atoms are liquid,and still higher ones are solid.
Aniline and other arylamines are usually colourless but become coloured on storage due to atmospheric oxidation.
Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules.
However,solubility decreases with an increase in the molar mass of amines due to the increase in the size of the hydrophobic alkyl part. Higher amines are essentially insoluble in water.
Amines are soluble in organic solvents like alcohol,ether,and benzene.
Primary and secondary amines are engaged in intermolecular association due to hydrogen bonding between the nitrogen of one molecule and the hydrogen of another molecule.
This intermolecular association is more pronounced in primary amines than in secondary amines as there are two hydrogen atoms available for hydrogen bond formation in primary amines.
Tertiary amines do not have intermolecular association due to the absence of hydrogen atoms available for hydrogen bond formation. Therefore,the order of boiling points of isomeric amines is as follows: Primary > Secondary > Tertiary.
Boiling points of amines,alcohols,and alkanes of almost the same molar mass:

Compound	Molar mass $(g/mol)$ and Boiling points $(K)$
$n-C_4H_9NH_2$	$73, 350.8$
$(C_2H_5)_2NH$	$73, 329.3$
$C_2H_5N(CH_3)_2$	$73, 310.8$
$C_2H_5CH(CH_3)_2$	$72, 300.3$
$n-C_4H_9OH$	$74, 390.3$

Amines,being basic in nature,react with acids to form salts.
Amine salts on treatment with a base like $NaOH$,regenerate the parent amine.
$RNH_3^+X^- + OH^- \longrightarrow RNH_2 + H_2O + X^-$
Amine salts are soluble in water but insoluble in organic solvents like ether.
This reaction is the basis for the separation of amines from non-basic organic compounds that are insoluble in water.
Amines have an unshared pair of electrons on the nitrogen atom,due to which they behave as a Lewis base.
$RNH_2 + H_2O \rightleftharpoons RNH_3^+ + OH^-$
$K = \frac{[RNH_3^+][OH^-]}{[RNH_2][H_2O]}$
$K[H_2O] = \frac{[RNH_3^+][OH^-]}{[RNH_2]}$
$K_b = \frac{[RNH_3^+][OH^-]}{[RNH_2]}$
$pK_b = -\log K_b$
Larger the value of $K_b$ or smaller the value of $pK_b$,stronger is the base.
The $pK_b$ value of ammonia is $4.75$. Aliphatic amines are stronger bases than ammonia due to the $+I$ effect of alkyl groups,leading to high electron density on the nitrogen atom.
Their $pK_b$ values lie in the range of $3$ to $4.22$.
On the other hand,aromatic amines are weaker bases than ammonia due to the electron-withdrawing nature of the aryl group.