Amines lecture 9B

Note that the N atoms are trivalent (have three covalent bonds), and have one lone pair of electrons, so they have a full outer octet.

Bronsted Basicity of Amines

The Brønsted basicity of amines refers to their ability to act as proton (H⁺) acceptors. In other words, it measures how readily an amine can accept a proton. The basicity of amines is influenced by several factors, including the structure of the amine and the electron-donating or electron-withdrawing effects of any substituents attached to the nitrogen atom.

Factors Affecting the Brønsted Basicity of Amines:

1. Electron-Demand on Nitrogen:

• Amines are basic because the nitrogen atom has a lone pair of electrons that can be donated to a proton (H⁺). The more electron-rich the nitrogen, the more easily it can accept a proton, thus increasing the basicity.

• Alkyl groups are electron-donating through induction, increasing the electron density on nitrogen, and thus making amines more basic. For example, methylamine (CH₃NH₂) is more basic than ammonia (NH₃) because the methyl group donates electron density to nitrogen.

• Aryl groups (like phenyl groups) are electron-withdrawing due to resonance, which decreases the electron density on nitrogen and reduces the basicity of the amine. For instance, aniline (C₆H₅NH₂) is less basic than alkyl amines.

2. Steric Effects:

• The size and steric hindrance of substituents around the nitrogen atom can also influence basicity. For instance, bulkier groups can make it harder for the nitrogen to approach a proton, decreasing basicity.

• Primary amines (RNH₂) generally have higher basicity than secondary amines (R₂NH), and tertiary amines (R₃N) can sometimes have lower basicity due to steric hindrance.

3. Resonance Effects:

• In aromatic amines like aniline, the lone pair on nitrogen can delocalize into the benzene ring, reducing the electron density on nitrogen and making it less basic. The more resonance stabilization an amine has, the less basic it tends to be.

4. Inductive Effects:

• Electron-withdrawing groups attached to the amine (such as -NO₂, -CF₃, etc.) lower the electron density on nitrogen, making the amine less basic.

• Electron-donating groups (such as -OH, -OCH₃) increase the electron density on nitrogen, making the amine more basic.

General Order of Basicity:

• Alkylamines (RNH₂) > Ammonia (NH₃) > Aryl amines (C₆H₅NH₂).

• Alkyl groups are electron-donating and increase nitrogen’s electron density, making alkyl amines more basic than ammonia or aryl amines.

• Aryl groups, on the other hand, withdraw electron density from the nitrogen via resonance, making aryl amines less basic than alkyl amines.

Conclusion: The Brønsted basicity of amines is primarily influenced by the electron density on the nitrogen atom, which can be affected by the presence of electron-donating or electron-withdrawing groups, as well as steric factors. Alkylamines are generally more basic than ammonia or arylamines due to the electron-donating effects of alkyl groups.