Amines & Biomolecules – SRMJEEE Chemistry Notes
(Prepared for quick revision & exam‑style practice)
1. Simple Explanation
| Concept | What It Means |
|---|---|
| Amines | Organic compounds that contain a nitrogen atom bonded to alkyl or aryl groups. They are derived from ammonia (NH₃). |
| Biomolecules | Large, complex molecules that are essential for life. They are built from smaller “monomer” units (amino acids, nucleotides, etc.). |
Why they matter in exams
- Amines are common in drug design, agrochemicals, and polymer synthesis.
- Biomolecules (proteins, DNA, RNA, carbohydrates) form the basis of biochemistry and molecular biology – many questions involve their structure, synthesis, or properties.
2. Key Points
2.1 Amines
| Category | Representative Formula | Common Examples |
|---|---|---|
| Alkyl Amines | R–NH₂ (primary), R₂NH (secondary), R₃N (tertiary) | Methylamine, Ethylamine, Trimethylamine |
| Aromatic Amines | Ph–NH₂, Ph–NR₂ | Aniline, N,N‑Dimethylaniline |
| Protonated Amines (Aminium ions) | R₃N⁺–H | [Et₃NH]⁺ |
| Amides | R–C(=O)–NR₂ | Acetamide, Urea |
Properties
| Property | Details |
|---|---|
| Basicity | N has a lone pair → acts as a Lewis base. pKb decreases (basicity increases) from primary → secondary → tertiary. |
| Solubility | Primary & secondary amines are miscible with water (due to H‑bonding). Tertiary amines are less soluble. |
| Boiling Points | Higher than alkanes of similar size because of H‑bonding & dipole‑dipole interactions. |
| Reactivity | Nucleophilic substitution (SN2), elimination (E2), acylation (forming amides), alkylation (quaternary salts). |
Important Reactions
-
Formation of Primary Amine (Fischer–Speier)
R–Cl + NH₃ → R–NH₂ + HCl (often with acid/base catalysis). -
Formation of Secondary & Tertiary Amines (Reductive Amination)
R–CHO + R'NH₂ + H₂ (or NaBH₄) → R–CH₂–NR'₂. -
Acylation → Amide
R–COCl + R′NH₂ → R–CO–NR′₂ + HCl. -
Mannich Reaction (C–C bond formation)
R₁CH₂–COOH + NH₃ + CH₂=CH₂ → R₁CH₂–CH(NH₂)–COOH. -
Hydrolysis of Amides → Carboxylic Acids
R–CO–NR₂ + H₂O → R–COOH + R₂NH (acidic or basic).
2.2 Biomolecules
| Class | Monomer | Key Functional Group | Example |
|---|---|---|---|
| Amino Acids | α‑Amino acid | –NH₂, –COOH, side‑chain R | Glycine, Phenylalanine |
| Peptides/Proteins | Polypeptide chains | Peptide bond (–CONH–) | Enzymes, hemoglobin |
| Nucleotides | Nucleic acids | Phosphate, sugar (ribose/deoxyribose), base | ATP, dATP |
| DNA | Double‑helix | Base pairs (A–T, G–C) | Genetic material |
| RNA | Single‑stranded | Uracil replaces thymine | mRNA, tRNA |
| Carbohydrates | Monosaccharides | Aldehyde or ketone | Glucose, Fructose |
Important Concepts
| Topic | Key Points |
|---|---|
| Amino Acid Properties | 20 standard aa, each has a unique side‑chain (R). Some are polar, non‑polar, acidic, or basic. |
| Peptide Bond Formation | Condensation reaction between amino and carboxyl groups → loss of H₂O. |
| Protein Structure | Primary → Secondary (α‑helix, β‑sheet) → Tertiary (3D folding) → Quaternary (multi‑subunit). |
| DNA Structure | Double helix, antiparallel strands, complementary base pairing (A–T, G–C). |
| RNA Types | mRNA (messenger), tRNA (transfer), rRNA (ribosomal). |
| Enzyme Kinetics (Michaelis–Menten) | V₀ = (Vmax [S]) / (Km + [S]). Key parameters: Vmax, Km, kcat. |
3. Worked Example
Example 1 – Basicity of Amines
Problem
Determine the order of basicity among the following amines:
- Methylamine (CH₃NH₂)
- Dimethylamine ((CH₃)₂NH)
- Trimethylamine ((CH₃)₃N)
Solution
| Amine | pKb (approx.) | Basicity |
|---|---|---|
| CH₃NH₂ | 9.4 | Least basic |
| (CH₃)₂NH | 8.4 | More basic |
| (CH₃)₃N | 7.0 | Most basic |
Reasoning
- The lone pair on N becomes more delocalised with increasing alkyl substitution → electron donation increases → stronger base.
- Steric hindrance is minimal in these small amines, so inductive effect dominates.
Answer
Trimethylamine > Dimethylamine > Methylamine.
Example 2 – Peptide Bond Formation
Problem
Sketch the reaction mechanism for the condensation of glycine