MCAT Organic Chemistry · Lesson 10
Nitrogen & Phosphorus Containing Compounds
2 min read3 sectionsUpdated
3 sections
10.1 Amino Acids and Peptides
Amino acids (AA) are the building blocks of proteins, containing amino and carboxyl groups attached to an alpha carbon. Most are chiral and optically active, forming zwitterions in solution and peptide bonds through condensation reactions.
- Structure & Chirality
- Glycine: achiral
- All other AAs: L-isomers, amino group on left
- S configuration for all except cysteine (R configuration due to priority rules)
- Properties
- Amphoteric: act as acids and bases
- Zwitterions in solution
- Form peptide bonds → polypeptides
- Resonance in CN bond of amide → partial double bond → planar geometry
- Limited rotation due to partial double bond character
10.2 Synthesis of 𝛼-Amino Acids
Alpha-amino acids can be synthesized via Strecker synthesis and Gabriel synthesis, both producing racemic mixtures of L and D amino acids.
- Strecker Synthesis
- Step 1: Aldehyde + NH₄Cl + KCN
- Carbonyl O protonated → ↑ electrophilicity
- Ammonia attacks → forms imine
- CN⁻ attacks → forms aminonitrile
- Step 2: Nitrile N protonated → electrophilicity ↑
- Water attacks → forms intermediate
- Imine attacked by another water → carbonyl formed → COOH
- Product: racemic mixture of AA (both L and D)
- Step 1: Aldehyde + NH₄Cl + KCN
- Gabriel Synthesis
- React potassium phthalimide with diethyl bromo malonate → SN2 reaction → phthalimidomalonic ester
- Alkylation at deprotonated C → attacks bromoalkane
- Hydrolysis & decarboxylation → amino acid
- No dehydration reaction
- Racemic mixture of L and D AA
10.3 Phosphorus-Containing Compounds
Phosphorus compounds, including inorganic phosphate (Pi) and nucleotides, are essential for energy transfer and DNA structure. Phosphate groups act as buffers and store/release energy upon bond cleavage.
- Inorganic & Organic Phosphates
- Pi = HPO₄²⁻, H₂PO₄⁻
- Pyrophosphate (PPi, P₂O₇⁴⁻) released during DNA polymerization → provides energy
- Organic phosphate: Pi attached to carbon (e.g., DNA backbone)
- Properties
- Phosphoric acid (H₃PO₄) has 3 acidic hydrogens
- H₂PO₄⁻ in mildly acidic conditions
- HPO₄²⁻ in slightly basic
- PO₄³⁻ in strongly basic conditions
- H₂PO₄⁻ / HPO₄²⁻ system acts as buffer
- Adjacent phosphates in nucleoside triphosphates → strong charge repulsion → high energy release upon cleavage
- Phosphoric acid (H₃PO₄) has 3 acidic hydrogens
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