12.2 Basicity of Amines

Amines are organic compounds derived from ammonia ($N{H}_3$), where one or more hydrogen atoms are replaced by alkyl or aryl groups. Their basic nature is a fundamental property determined by the nitrogen atom's electronic configuration.

1. Structure and Basicity of Amines

• Nitrogen Hybridization: In amines ($RN{H}_2$, $R_{2}NH$, $R_{3}N$), the nitrogen atom is $s{p}^3$-hybridized.
• Molecular Geometry: It possesses a nearly tetrahedral structure, forming three sigma bonds with its substituents (alkyl/aryl groups or hydrogen atoms).
• Lone Pair: The fourth $s{p}^3$-hybrid orbital contains a non-bonding lone pair of electrons.
• Basic Nature: This lone pair of electrons makes amines basic in nature. They can act as Lewis bases (electron pair donors) or Brønsted-Lowry bases (proton acceptors). The behavior of this lone pair is analogous to that in ammonia.

2. Reaction of Amines with Water

When an amine is dissolved in water, the nitrogen atom's lone pair accepts a proton ($H^{+}$) from a water molecule. This proton transfer reaction produces an alkyl ammonium ion (the conjugate acid) and hydroxide ions ($O{H}^{-}$).

${\mathrm{RNH}}_2+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons [{\mathrm{RNH}}_3{]}^{+}+{\mathrm{OH}}^{-}$

The formation of hydroxide ions makes the aqueous solution of amines alkaline. Amines are generally stronger bases than water.

3. Relative Basicities of Aqueous Ammonia, Ethylamine, and Phenylamine

The strength of weak bases like amines is quantified using their $p{K}_b$ values. A lower $p{K}_b$ value indicates a stronger base.

Table: $p{K}_b$ values of selected bases

Based on these values, the order of basic strength is: Ethylamine (strongest) > Ammonia > Phenylamine (weakest)

4. Factors Determining Basic Strength

Two primary factors influence the basic strength of an amine:

1. Ease of Proton Capture: How readily the lone pair of electrons on the nitrogen atom can accept a hydrogen ion ($H^{+}$). This is related to the electron density on the nitrogen.
2. Stability of the Conjugate Acid: The stability of the alkyl ammonium ion ($[{\mathrm{RNH}}_3{]}^{+}$) formed after accepting the $H^{+}$ ion. Greater stability of the conjugate acid corresponds to a stronger base.

4.1 Ethylamine vs. Ammonia

• Structures:
  • Ammonia: $N{H}_3$
  • Ethylamine: $C{H}_{3}C{H}_{2}N{H}_2$ (or $RN{H}_2$ where $R=C_2{H}_5$)
• Electron-Donating Effect: The ethyl group ($C{H}_{3}C{H}_2-$) is an alkyl group, which is an electron-donating group by the positive inductive effect ($+I$ effect).
  • This effect increases the electron density on the nitrogen atom in ethylamine.
  • A higher electron density makes the nitrogen lone pair more available and thus more attractive to hydrogen ions ($H^{+}$).
• Stability of Conjugate Acid: The ethyl ammonium ion ($[C{H}_{3}C{H}_{2}N{H}_3{]}^{+}$), formed when ethylamine accepts a proton, is more stable than the simple ammonium ion ($[N{H}_4{]}^{+}$) formed from ammonia.
  • The electron-donating ethyl group helps to disperse and reduce the positive charge on the nitrogen atom in the ethyl ammonium ion, stabilizing it.
• Conclusion: Both increased electron density on nitrogen and enhanced stability of the conjugate acid contribute to ethylamine being a stronger base than ammonia.

4.2 Phenylamine vs. Ammonia

• Structure: Phenylamine ($C_6{H}_{5}N{H}_2$), also known as aniline, is an aromatic primary amine.
• Resonance Effect: The lone pair of electrons on the nitrogen atom in phenylamine is delocalized into the benzene ring through resonance.
  • This delocalization means the lone pair is not solely localized on the nitrogen atom but is shared across the entire aromatic system.
  • This resonance interaction can be represented by resonance structures where the lone pair moves into the ring, creating partial negative charges on ortho and para carbons, and a partial positive charge on nitrogen.
• Consequence for Basicity: Because the lone pair is involved in resonance, it is less available to combine with a proton ($H^{+}$) compared to the lone pair in ammonia or ethylamine.
  • The effective electron density around the nitrogen atom is significantly reduced.
• Conclusion: The resonance stabilization of the lone pair within the aromatic ring makes phenylamine a much weaker base than ammonia.

4.3 Order of Basic Ability

Considering the effects discussed: Phenylamine < Ammonia < Ethylamine

In the case of substituted amines in aqueous solution, the order is often: Secondary amine > Primary amine > Tertiary amine > Ammonia