Organic Chemistry: Classification and Nomenclature

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

The Language of Organic Chemistry — Functional Groups:

Organic compounds are classified by their functional groups — specific atoms or groups of atoms that determine the compound’s chemical behaviour. The major functional groups in increasing order of priority for IUPAC naming:

Functional Group	Suffix	Example	Formula
Alkane	-ane	Propane	C₃H₈
Alkene	-ene	Propene	C₃H₆
Alkyne	-yne	Propyne	C₃H₄
Alcohol	-ol	Propanol	C₃H₇OH
Amine	-amine	Propylamine	C₃H₇NH₂
Aldehyde	-al	Propanal	C₃H₇CHO
Ketone	-one	Propanone	C₃H₇COCH₃
Carboxylic acid	-oic acid	Propanoic acid	C₃H₇COOH

Priority rule: When multiple functional groups are present, the highest priority group gets the suffix, and lower priority groups are indicated as prefixes.

IUPAC Nomenclature — Systematic Naming:

Steps for naming:

1. Identify the longest carbon chain (parent chain)
2. Number from the end that gives the lowest locant to the principal functional group
3. Identify and name substituents
4. Assemble the name: substituent names + parent (with suffix) + prefixes

Example: CH₃-CH(CH₃)-CH₂-CH(OH)-CH₃ → 4-methylpentan-2-ol (6-carbon chain, -ol at position 2, methyl substituent at position 4). Note: the suffix -ol takes priority over the methyl prefix, so the chain is numbered to give -ol the lowest possible number.

⚡ ECAT Tip: Cyclic compounds use the prefix cyclo-: cyclohexane (C₆H₁₂), cyclopentanol (5-membered ring with -OH). Aromatic compounds use benzene as parent: toluene (methylbenzene), phenol (hydroxybenzene), aniline (aminobenzene).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Hydrocarbons — Saturation and Unsaturation:

Saturated hydrocarbons (alkanes): Contain only single bonds (C-C and C-H). General formula C_nH₂n₊₂. They are chemically inert (no double or triple bonds to add to). Combustion is their main reaction: CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O.

Unsaturated hydrocarbons:

• Alkenes (C_nH₂n): one C=C double bond. Addition reactions: hydrogenation (H₂/Pt), halogenation (Br₂, visible decolourisation), hydration (H₂O/H⁺), polymerisation.
• Alkynes (C_nH₂n₋₂): one C≡C triple bond. Alkynes undergo the same addition reactions as alkenes but in two steps: first adding one molecule gives an alkene, then a second addition gives an alkane. Terminal alkynes (C≡C-H) are weakly acidic: they react with NaNH₂ or AgNO₃ to form acetylide salts.

Homologous Series — Systematic Patterns:

A homologous series is a family of compounds with the same functional group and similar chemical properties, where each successive member differs by CH₂ (molecular mass increases by 14). Members show gradual changes in physical properties (boiling point increases with molecular mass due to increasing London dispersion forces).

For straight-chain alkanes: methane (-162°C bp), ethane (-88°C), propane (-42°C), butane (-0.5°C), pentane (36°C), hexane (69°C), heptane (98°C), octane (126°C). The boiling point increases roughly 20-30°C per CH₂ added for smaller alkanes.

⚡ ECAT Tip: Isomerism is a key concept: structural isomers (different connectivity: chain, position, functional group) and stereoisomers (same connectivity, different spatial arrangement: geometric E/Z, optical R/S). Butane C₄H₁₀ has two structural isomers: n-butane (CH₃CH₂CH₂CH₃) and isobutane (2-methylpropane). Pentane C₅H₁₂ has three structural isomers.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Functional Group Interconversions — Synthesis Logic:

Organic synthesis requires thinking about how to build complex molecules from simple ones by introducing functional groups in the right order. Key transformations:

Alkane → Halogenoalkane: Free radical halogenation (UV light, Cl₂ or Br₂). Specificity: for propane, bromination is more selective than chlorination for the less substituted position (secondary H is ~80× more reactive than primary H for bromination).

Alkene → Alcohol → Aldehyde/Ketone → Carboxylic acid: This is the oxidation ladder. Mild oxidants (Na₂Cr₂O₇/H₂SO₄ or PCC for alcohol → aldehyde) vs strong oxidants (KMnO₄/H⁺ for alcohol → carboxylic acid). Terminal alkenes (R-CH=CH₂) give aldehydes (R-CHO) when oxidised. Internal alkenes give ketones (R-CO-R’).

Alkyne → Acetylide → Various: Terminal alkyne (R-C≡CH) deprotonated by NaNH₂ to form acetylide anion (R-C≡C:⁻ Na⁺). This nucleophile attacks alkyl halides (SN2) to form longer chain alkynes. This is a key C-C bond forming reaction.

Resonance and Electron-Donating/Withdrawing Effects:

In benzene and aromatic compounds, substituents affect reactivity:

• Electron-donating groups (-OH, -NH₂, -OCH₃, -CH₃): activate the ring toward electrophilic aromatic substitution, direct ortho/para
• Electron-withdrawing groups (-NO₂, -CN, -COOH, -SO₃H): deactivate the ring, direct meta

The nitro group (-NO₂) on benzene makes it much less reactive toward further substitution. This is why nitration of benzene to dinitrobenzene requires more forcing conditions, and trinitrobenzene (TNT) requires yet more severe conditions.

Bridge between Functional Groups and Reaction Mechanisms:

Organic chemistry’s “queen” is mechanism — understanding electron movement in 3D. Curly arrow notation shows how electrons move: from a nucleophile (electron pair donor) to an electrophile (electron pair acceptor). Four main types of organic reactions:

1. Addition: Two reactants combine (alkenes, alkynes, carbonyls)
2. Elimination: One reactant splits (alkanes → alkenes, alcohols → alkenes)
3. Substitution: Two reactants swap parts (halogenoalkanes, aromatic substitution)
4. Rearrangement: One molecule reorganises (isomerisation)

The nature of the functional group determines which mechanism is possible. Carbonyls undergo nucleophilic addition because the C=O is electrophilic. Halogenoalkanes undergo nucleophilic substitution because the carbon attached to the halogen is electrophilic (especially if the halogen is a good leaving group like I⁻ > Br⁻ > Cl⁻ > F⁻).

⚡ ECAT Pattern: ECAT frequently tests: (1) naming organic compounds from structure; (2) identifying functional groups in complex molecules; (3) ranking compounds by reactivity (e.g., acidity: carboxylic acid > phenol > water > alcohol; basicity: amine > amide); (4) isomer counting; and (5) predicting the major product of addition reactions to alkenes (Markovnikov’s rule — H adds to the carbon with more hydrogens already, Cl/Br adds to the carbon with fewer hydrogens). A common ECAT question: “Draw and name all isomers of C₄H₁₀O. Which isomers are alcohols, which are ethers?” Answer: 4 structural isomers — 2 butanols (n-butanol and isobutanol as primary), 2-methyl-2-propanol (tert-butanol, tertiary), and diethyl ether + methyl propyl ether.