What is Organic Chemistry?
Organic chemistry is the study of compounds containing carbon. Carbon is unique: it forms four covalent bonds, can bond to itself in chains or rings, and produces millions of different compounds. Almost all biological molecules — proteins, DNA, fats, sugars — are organic.
Key features of carbon: Tetravalent (4 bonds); forms C–C single bonds, C=C double bonds, C≡C triple bonds; bonds are covalent (not ionic); non-polar C–H bonds give organic compounds hydrophobic character unless a polar functional group is present.
Hydrocarbons
Hydrocarbons contain only carbon and hydrogen. They are classified by the type of C–C bonding:
| Series | General Formula | Bond type | Example |
|---|
| Alkanes | CnH2n+2 | Single bonds only (saturated) | CH4, C2H6, C3H8 |
| Alkenes | CnH2n | At least one C=C double bond | C2H4, C3H6 |
| Alkynes | CnH2n−2 | At least one C≡C triple bond | C2H2 |
IUPAC Naming Rules
IUPAC (International Union of Pure and Applied Chemistry) provides a systematic naming system:
- Step 1 — Parent chain: Find the longest continuous carbon chain containing the principal functional group. The chain length gives the stem: meth(1), eth(2), prop(3), but(4), pent(5), hex(6).
- Step 2 — Suffix: Alkane → -ane; alkene → -ene; alkyne → -yne; alcohol → -ol; aldehyde → -al; ketone → -one; carboxylic acid → -oic acid; ester → -oate.
- Step 3 — Number the chain: Number from the end that gives the functional group or branch the lowest locant (smallest number).
- Step 4 — Name branches: Substituents are named as prefixes: methyl(-CH3), ethyl(-C2H5), chloro-, bromo-, etc. List alphabetically.
- Step 5 — Assemble: locant(s)-prefix(es)-stem-locant-suffix. E.g. 2-methylpropan-1-ol.
but-2-ene → 4-carbon chain, double bond starting at C2
2-methylpropane → propane with a methyl group on C2
3-chlorobutan-1-ol → 4-carbon chain, Cl on C3, OH on C1
Functional Groups
| Functional Group | Structure | Class | Suffix |
|---|
| Hydroxyl | –OH | Alcohols | -ol |
| Carboxyl | –COOH | Carboxylic acids | -oic acid |
| Amino | –NH2 | Amines | -amine |
| Ester linkage | –COO– | Esters | -oate |
| Halogen | –X (F, Cl, Br, I) | Halogenoalkanes | halo- prefix |
| Carbonyl (aldehyde) | –CHO (end) | Aldehydes | -al |
| Carbonyl (ketone) | –C(=O)– (middle) | Ketones | -one |
Structural Formulae
- Expanded (displayed) formula: Shows every bond explicitly — each C–H and C–C bond drawn out. Most detailed; used to show bonding clearly.
- Condensed formula: Atoms written linearly without showing bonds; H atoms implied: CH3CH2OH.
- Skeletal formula: Lines represent C–C bonds; each vertex/end is a carbon; H atoms not shown (assumed to satisfy valency). Most compact; used in complex molecules.
Isomers
Structural isomers have the same molecular formula but different structural arrangements:
- Chain isomers: Different arrangement of the carbon skeleton (e.g. butane vs 2-methylpropane — both C4H10).
- Position isomers: Same functional group in different positions (e.g. propan-1-ol vs propan-2-ol).
- Functional group isomers: Different functional groups with the same formula (e.g. ethanol CH3CH2OH vs methoxymethane CH3OCH3).
Geometric isomers arise from restricted rotation around a C=C double bond. When both carbons of a double bond carry two different groups:
- cis- isomer: same groups on the same side of the double bond.
- trans- isomer: same groups on opposite sides.
Condition for geometric isomerism: Each carbon of the C=C must bear two different substituents. If either carbon has two identical groups, no cis/trans isomers exist.
Physical Properties of Homologous Series
A homologous series is a family of compounds with the same functional group and general formula, differing by –CH2– units. Properties change gradually along the series:
- Boiling point increases with chain length — more carbons mean larger molecules, stronger London (dispersion) forces, more energy needed to separate molecules.
- Branching decreases boiling point — branched molecules are more compact (spherical), reducing surface area for intermolecular contact, weakening dispersion forces.
- Hydroxyl (–OH) groups dramatically raise boiling point via hydrogen bonding (H-bonds are much stronger than dispersion forces). Compare ethanol (bp 78°C) vs propane (bp −42°C) — both have ~3 carbons.
- Solubility in water: short-chain alcohols and carboxylic acids dissolve well (H-bonding with water); long chains are increasingly hydrophobic → insoluble.
| Factor | Effect on BP | Reason |
|---|
| Longer chain | Increases | More surface area, stronger London forces |
| More branching | Decreases | More compact shape, less surface contact |
| –OH group | Greatly increases | Hydrogen bonding (strong intermolecular force) |
| –COOH group | Even higher increase | Dimerisation via 2 H-bonds; also polar C=O |
Worked Naming Examples
CH3CH2CH2CH3 → butane (4-carbon chain, no functional group, alkane suffix -ane)
CH3CH=CHCH3 → but-2-ene (4C chain, double bond at C2)
CH3CH(OH)CH3 → propan-2-ol (3C chain, OH on C2)
CH3COOH → ethanoic acid (2C chain, carboxyl group: suffix -oic acid)
CH3CH(CH3)CH2CH3 → 2-methylbutane (4C parent chain + methyl branch on C2)
⭐ IEB Extension — Stereoisomers & Optical Isomers
Beyond geometric isomers, molecules can exhibit optical isomerism. A carbon atom bonded to four different groups is called a chiral centre (or stereocentre). The two non-superimposable mirror-image structures are called enantiomers.
Enantiomers rotate plane-polarised light in opposite directions: the (+) or R enantiomer and the (−) or S enantiomer. The R/S designation uses the Cahn–Ingold–Prelog (CIP) priority rules:
- Assign priorities 1–4 to the four substituents by atomic number (highest = 1).
- View the molecule with the lowest priority group (4) pointing away from you.
- If 1→2→3 is clockwise → R configuration; anticlockwise → S.
A racemic mixture (50:50 mixture of enantiomers) shows no net optical rotation. Enantiomers have identical physical properties except optical rotation, but can have very different biological activity (e.g. thalidomide, ibuprofen).