INTRODUCTION
An amine is a derivative of ammonia. It is composed of one or more alkyl groups which replace the
hydrogen atoms in ammonia (NH3) molecule. Therefore, the alkyl group is directly bonded to the
nitrogen atom. According to the number of alkyl groups that have been attached to the nitrogen atom,
amines are categorized into three broad groups.
Classification of Amine
Primary Amines – One alkyl group is attached to the nitrogen atom.
Secondary Amines – Two alkyl groups are attached to the nitrogen atom.
Tertiary Amines – Three alkyl groups are attached to the nitrogen atom.
According to the type of alkyl group that has been attached, amine are further classified as,
Aliphatic Amines – No ring structures present
Aromatic Amines – Ring structures present
Aliphatic amines have linear or branched alkyl groups attached to the nitrogen atom directly. Aromatic
amines have nitrogen atom directly connected to an aromatic ring structure.
Primary and secondary amines have H atoms bonded directly to the nitrogen atom. Therefore, primary
and secondary amines are capable of having hydrogen bonds. Tertiary amines have no H atoms to make
hydrogen bonds. But all amine types can have intermolecular hydrogen bonds with water (H2O) due to
the presence of a lone electron pair on the nitrogen atom. Therefore, amines with a low molecular
weight can be dissolved in water.
As same as ammonia, amines also act as bases. The reasons for basicity includes the presence of a lone
pair on the nitrogen atom, the presence of alkyl groups (alkyl groups enhance basicity of compounds
Basicity of amines (compare basicity of primary, secondary and tertiary amine)
The basicity of amines depends on:
1. The electronic properties of the substituents (alkyl groups enhance the basicity, aryl groups
diminish it).
2. The degree of solvation of the protonated amine, which includes steric hindrance by the groups
on nitrogen
The order of basicity for primary, secondary and tertiary amines are:
Secondary > Primary > Tertiary > NH3
Amines are basic in nature as they have lone pair of electrons on nitrogen. Therefore, they have a strong
tendency to donate this lone pair of electrons to electron acceptors.
The stability of conjugate acids formed is dependent on the extent of H-bonding,and greater the
number of H-atoms on N more stable is the conjugate acid.
The conjugate acid of primary amine is the most stable with greater number (3) of H-bonds than >
Secondary > tertiary. But primary amines are less basic than secondary as the electron density on the N
atom is less and lone pair of electrons are not readily available for protonation.
The two opposing factors i.e. the stability of conjugate acids formed and ease of availability of electrons
for Protonation, balance each other in case of secondary amines and it is most basic.
1. The electronic properties of the substituents (alkyl groups enhance the basicity, aryl
groups diminish it).
In any base reaction where the mine works as a base, it donates the lone pair to capture a proton. Now,
the stronger the ability of the amine to donate the electron pair, the more basic it is.
For example, ethyl amine is more basic than ammonia because the electron donating effect of the alkyl
group increases the electron density on the nitrogen and thus making it more basic.
And this is a general trend, the more alkyl groups on the nitrogen, the stronger the basicity of the amine.
The steric effects might start influencing this trend.
How do we quantify the base strength of amines?
We know from the acid-base chemistry that, at least in organic chemistry, the use of pKa is the standard
approach when assessing the acid strength. However, because we are looking at the base strength here,
the pKa of the conjugate acid is used to predict the basicity of the amine.
For example, going back to the comparison of ammonia and ethyl amine, we use the pKa values of their
conjugate acids to assess their basicity. The pKa of the conjugate acid of ammonia is 9.3 and that of
ethyl amine is 10.8. These numbers tell us that the conjugate acid of ammonia is a stronger acid than
that of ethyl amine and therefore, it is a weaker base.
Solvation effect in amines
Solvation is the interaction of a solvent with the dissolved solute
In water, the ammonium salts of primary and secondary amines undergo solvation effects (due to
hydrogen bonding) to a much greater degree than ammonium salts of tertiary amines. These solvation
effects increase the electron density on the amine nitrogen to a greater degree than the inductive effect
of alkyl groups.
Reactions of Amines
Due to the unshared electron pair, amines can act as both bases and nucleophiles.
Reaction with acids
When reacted with acids, amines donate electrons to form ammonium salts.
Reaction with acid halides
Acid halides react with amines to form substituted amides.
Aldehydes and ketones react with primary amines to give a reaction product (a carbinolamine) that
dehydrates to yield aldimines and ketimines (Schiff bases).
If you react secondary amines with aldehydes or ketones, enamines form.
Reaction with sulfonyl chlorides
Amines react with sulfonyl chlorides to produce sulfonamides. A typical example is the reaction of
benzene sulfonyl chloride with aniline.