Factors That Affect the Basicity of Amines

Amines are derived from ammonia. Alkyl or aryl group replaces one or more hydrogen atoms in ammonia. The –NH2 group is known as the amino group. The nitrogen atom has a pair of non-bonded electrons. Amides have chemical properties similar to amines. Amines are further divided into 3 categories: aliphatic amines, aromatic amines and heterocyclic amines. When an alkyl group is attached to nitrogen, it is an aliphatic amine. When an aryl group is attached to the nitrogen, it is an aromatic amine. When the nitrogen of an amine is a part of a cyclic ring, it is a heterocyclic amine. Amines are basic in nature due to the presence of non-bonded electrons on the nitrogen atom, which can easily be protonated.

These notes on the basicity of amines discuss their properties, importance, and the factors affecting the basicity of amines.

Amines and their properties

• Lower aliphatic amines are in a gaseous state; primary amines are liquid at room temperature, while higher amines are in the solid state
• Aliphatic amines with fewer carbon atoms are soluble in water, but this solubility decreases as the size of the carbon chain increases
• Aliphatic amines are more basic than aromatic amines because the non-bonded electron pairs on the nitrogen atom of an aromatic compound are easily delocalised

Amine basicity

• Aliphatic amines show a more basic character compared to aromatic amines as the electron density or the electron charge of the nitrogen atom is utilised in the aromatic ring
• There are various factors that affect the basicity of amines. For example, the presence of an electron-donating group increases the basicity of the amine, whereas the presence of an electron-withdrawing group decreases the basicity of an amine
• Basicity is expressed as Kb. The larger the value of Kb, the stronger the base.
• Basicity can also be expressed as pKb, where pKb= −log Kb. Likewise, the smaller the value of pKb, the stronger the base
• Basicity of amine can also be expressed with the help of the acidity of their conjugate acids. A strong base has small Ka and large pKa hence its conjugate acid is weak
• The position of the electron pair of the nitrogen atom, its hybridisation, and its resonance stabilisation decides the basicity of heterocyclic amines

Factors Affecting Basicity of Amines

The basicity of amines depends on factors that affect the stability of the non-bonded pair of electrons that are present in the nitrogen atom.

• Basicity depends on the -ve charge on the nitrogen atom. With an increase in the -ve charge on the nitrogen atom, the basicity of the amines increases. For example, between ammonia and its conjugate base amide anion, the latter is the stronger base. Amide anion can deprotonate terminal alkynes, which is not possible for ammonia. Likewise, the conjugate base of amide ion, which is amide dianion, is a stronger base than amide ion
• Basicity depends on charge density. Basicity decreases with a decrease in charge density. In the presence of pi bonds, the charge density is decreased as the compound becomes more stabilised. Hence, the basicity also decreases. For example, aniline is a weak base when compared with cyclohexylamine. The addition of another phenyl ring to aniline would decrease its basicity even more. Hence, it is said that the basicity decreases with resonance stabilisation
• Electron withdrawing groups increase the acidity of the compound by removing electron density from its conjugate base. In a similar way, electron-withdrawing groups decrease the basicity
• Resonance and inductive effects decrease the basicity of an amine. When we compare amines with amides, the basicity of amides is lower than amines. The reason for this is that oxygen in amides is more electronegative than nitrogen; hence, it shows an inductive effect
• The presence of nitrogen as a pi-donor decreases the basicity. Let us consider the case of amides. In this case, oxygen works as an electron-withdrawing group that distributes the electron density of the nitrogen atom. But due to the resonance effect, the nitrogen lone pair tends to form a pi bond with the carbon known as a pi donation. Hence, the electrons move from the carbon-oxygen pi bond to the oxygen atom. In this form, where nitrogen forms a pi bond with carbon instead of a pi bond between carbon and oxygen, it restricts nitrogen’s ability to act as a base. Hence, when a pi acceptor, such as oxygen in the case of amides, is present, the basicity of nitrogen decreases or the basicity of amines decreases as nitrogen acts as a pi donor
• It is also seen that hybridisation plays an important role in determining the basicity of amines. Basicity generally increases with a decrease in character. This is because the non-bonded pair of electrons in sp hybridisation is the most stable, whereas, in sp3, it is the least stable. Therefore, as the s character decreases, the basicity increases as the stability of the non-bonded electron pair decrease

Conclusion

Amines are organic compounds with nitrogen atom. Amines are derived from ammonia. Alkyl or aryl group replaces one or more hydrogen atoms in ammonia. The –NH2 group is known as the amino group, and the nitrogen atom has a pair of non-bonded electrons. The stability of this non-bonded pair of electrons determines the basicity of the amines. The basicity of the amines also depends on various other factors like resonance, hybridisation, inductive effect, etc. It is known that aliphatic amines are more basic than aromatic amines. The role of nitrogen acting as a pi-donor or pi-acceptor also plays a very important role in determining the basicity of the amines.

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