Chemistry: Organic Chemistry

Chemistry: Organic Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Organic chemistry is the study of carbon-containing compounds, with tetravalent carbon (forming four covalent bonds) as the central feature. The topic contributes roughly 4% of the NAT-I (NTS) Subject Knowledge section, and the high-yield items are the four general formulas: alkanes CₙH₂ₙ₊₂, alkenes/cycloalkanes CₙH₂ₙ, alkynes/dienes CₙH₂ₙ₋₂, and the benzene series CₙH₂ₙ₋₆. Master IUPAC nomenclature (longest chain, lowest locants, alphabetical substituent order) and the three core reaction types: substitution (alkanes + Cl₂/uv), electrophilic addition (alkenes + HX, Markovnikov), and electrophilic aromatic substitution (benzene + HNO₃/H₂SO₄). For NAT-I MCQs, always compute the Degree of Unsaturation first: DoU = (2C + 2 + N − H − X)/2; it tells you instantly whether the molecule contains rings, double bonds, or triple bonds.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Hybridization & Bonding

Carbon uses sp³ (tetrahedral, 109.5°, alkanes), sp² (trigonal, 120°, alkenes/benzene), and sp (linear, 180°, alkynes) hybrid orbitals. A double bond = 1 σ + 1 π; a triple bond = 1 σ + 2 π. π-bonds are weaker (~65 kcal/mol vs ~83 for σ) and are the reactive site in alkenes and alkynes.

Hydrocarbon Families

Family	Formula	Character	Example
Alkane	CₙH₂ₙ₊₂	Saturated, σ-only	C₂H₆, ethane
Alkene	CₙH₂ₙ	One C=C	C₃H₆, propene
Alkyne	CₙH₂ₙ₋₂	One C≡C	C₂H₂, ethyne
Aromatic	CₙH₂ₙ₋₆	Benzene ring	C₆H₆, benzene

Functional-Group Priority for IUPAC Naming

Carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene > alkyne > halide. The highest-priority group is the suffix; all others are prefixes (fluoro-, chloro-, hydroxy-, oxo-, amino-). Example: CH₃–CH(OH)–CH₂–CH₃ = butan-2-ol, not 2-hydroxybutane.

Isomerism

• Structural: chain (n-butane vs isobutane), position (propan-1-ol vs propan-2-ol), functional (C₃H₆O: acetone vs propanal).
• Geometric (cis-trans): requires a C=C with two different groups on each carbon, e.g. but-2-ene.
• Optical: chiral carbon (four different substituents) — not usually tested at NAT-I.

Reaction Patterns

• Alkanes + halogen/uv → free-radical substitution (propagation step: Cl• + CH₄ → HCl + •CH₃).
• Alkenes + HX → electrophilic addition, Markovnikov (H adds to the C with more H’s; via the more stable carbocation).
• Benzene + HNO₃/H₂SO₄ → nitrobenzene (electrophilic aromatic substitution, EAS); the ring is activated by –OH, –NH₂ (o/p-directors) and deactivated by –NO₂, –COOH (m-directors).

Common Exam Traps

1. CₙH₂ₙ covers both alkenes AND cycloalkanes — a NAT-I MCQ may list “cyclohexane, C₆H₁₂” as the only CₙH₂ₙ option that is not an alkene.
2. Markovnikov’s rule is reversed for HBr in the presence of peroxides (Kharasch effect, free-radical mechanism) — Br adds to the less substituted carbon.
3. DoU formula: halogens (X) behave like H (subtract), nitrogen adds +1, oxygen is ignored.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Degree of Unsaturation — Worked Example

For C₄H₅NO₂: DoU = (2·4 + 2 + 1 − 5)/2 = (8+2+1−5)/2 = 3. Three degrees of unsaturation in a C₄ skeleton is impossible without an aromatic ring — this is the classic EAS entry-point question.

Mechanism Notes (Qualitative)

• Sₙ2: one-step, backside attack, favoured by 1° substrates, polar aprotic solvents, strong nucleophiles (I⁻, CN⁻). Stereochemistry: inversion (Walden).
• Sₙ1: two-step via carbocation, favoured by 3° substrates, polar protic solvents, weak nucleophiles. Gives racemization at chiral centres.
• E1/E2: elimination (alkene formation); E2 needs a strong base and anti-periplanar β-H.

Connecting Adjacent Topics

• Inorganic Chemistry: combustion analysis (C → CO₂, H → H₂O) gives the empirical formula — a recurring NAT-I bridge.
• Acids & Bases: carboxylic acids (pKa ≈ 5) vs alcohols (pKa ≈ 16) explains why only the former neutralise NaHCO₃.
• Polymers: addition polymers (polyethene from ethene) vs condensation polymers (nylon-6,6 from hexamethylenediamine + adipic acid with loss of H₂O).

Practice Prompts

1. A compound is 52.14% C, 13.03% H, 34.78% O. Find the empirical formula and the molecular formula (M = 46 g/mol). Answer: C₂H₆O, ethanol.
2. Rank by SN2 reactivity in acetone: 1-bromobutane, 2-bromobutane, 2-bromo-2-methylpropane. Answer: 1° > 2° >> 3° (1-bromobutane fastest).

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