Amines and amides are nitrogen-containing functional groups that appear in medicines, proteins, dyes, polymers, and many natural molecules. They may look similar because both contain nitrogen bonded to carbon, but their behavior is very different. Amines often act as bases, while amides are usually much less basic because the nitrogen lone pair is shared with a carbonyl group.
Learning to tell them apart helps students predict reactivity, solubility, bonding, and biological function.
In an amine, nitrogen has a lone pair that can accept H+ to form an ammonium ion, so many amines raise pH in water. In an amide, the nitrogen is attached directly to C=O, and resonance spreads electron density across the C, O, and N atoms. This resonance makes the amide bond partially double-bonded, planar, and unusually stable.
The same amide linkage forms peptide bonds, which connect amino acids into proteins.
Key Facts
- Amine general forms: RNH2, R2NH, and R3N for primary, secondary, and tertiary amines.
- Amide general forms: RCONH2, RCONHR, and RCONR2 for primary, secondary, and tertiary amides.
- Amine basicity reaction: RNH2 + H2O ⇌ RNH3+ + OH-.
- Amide resonance: O=C-N ↔ -O-C=N+ helps explain the stability and planarity of the amide bond.
- Henderson-Hasselbalch for amines: pH = pKa + log([base]/[conjugate acid]).
- The peptide bond in proteins is an amide bond formed between the carboxyl group of one amino acid and the amino group of another.
Vocabulary
- Amine
- An amine is an organic compound with nitrogen bonded to one or more carbon groups and usually able to accept a proton.
- Amide
- An amide is a compound with nitrogen bonded directly to a carbonyl carbon in the functional group C(=O)N.
- Carbonyl
- A carbonyl is a C=O group that strongly affects molecular polarity, reactivity, and resonance.
- Resonance
- Resonance is the sharing of electrons over multiple atoms when one Lewis structure cannot fully describe the bonding.
- Peptide bond
- A peptide bond is an amide linkage that joins amino acids together in proteins.
Common Mistakes to Avoid
- Calling every nitrogen-containing group an amine is wrong because amides have nitrogen attached directly to a carbonyl carbon and behave very differently.
- Assuming amides are strongly basic is wrong because the nitrogen lone pair is delocalized by resonance into the carbonyl group.
- Ignoring primary, secondary, and tertiary labels is wrong because these labels describe how many carbon groups are attached to nitrogen and affect hydrogen bonding and structure.
- Drawing the amide bond as freely rotating is wrong because resonance gives the C-N bond partial double-bond character and makes the group nearly planar.
Practice Questions
- 1 A solution contains methylamine, CH3NH2, with pKa of its conjugate acid CH3NH3+ equal to 10.6. If [CH3NH2] = 0.20 M and [CH3NH3+] = 0.050 M, calculate the pH using pH = pKa + log([base]/[acid]).
- 2 Classify each compound as an amine or an amide, then state whether it is primary, secondary, or tertiary: CH3NH2, CH3CONH2, (CH3)2NH, and CH3CON(CH3)2.
- 3 Explain why an amine nitrogen usually accepts H+ more readily than an amide nitrogen, using the idea of the lone pair and resonance.