What is a Homologous Series?
A family of organic compounds in which successive members differ by a common structural unit, typically −CH₂−. Members share the same functional group and can be described by a general formula. They show a gradual trend in physical properties (e.g. Boiling point) and similar chemical properties.
Functional group: A specific group of atoms within a molecule responsible for its characteristic chemical reactions and physical properties.
Key Homologous Series
| Series | General Formula | Functional Group | Suffix |
|---|---|---|---|
| Alkanes | CnH2n+2 | C−C (single bonds only) | -ane |
| Alkenes | CnH2n | C=C (double bond) | -ene |
| Alkynes | CnH2n−2 | C≡C (triple bond) | -yne |
| Alcohols | CnH2n+1OH | −OH (hydroxyl) | -ol |
| Halogenoalkanes | - | −X (F, Cl, Br, I) | fluoro-/chloro-/bromo- |
| Aldehydes | CnH2nO | −CHO (terminal C=O) | -al |
| Ketones | CnH2nO | C=O (internal carbonyl) | -one |
| Carboxylic acids | CnH2n+1COOH | −COOH (carboxyl) | -oic acid |
| Esters | - | −COO− (ester linkage) | -oate |
| Amines | CnH2n+1NH₂ | −NH₂ (amino) | -amine |
| Amides | - | −CONH₂ (amido) | -amide |
Types of Formulas
| Formula Type | What It Shows | Example (Butanol) |
|---|---|---|
| Molecular | Actual number of each atom | C₄H₁₀O |
| Empirical | Simplest ratio of atoms | C₂H₅O (if simplified) |
| Displayed (full) | Every atom and every bond drawn out | Shows all C−H, C−C, C−O, O−H bonds |
| Condensed | Groups atoms without drawing bonds | CH₃CH₂CH₂CH₂OH |
| Skeletal | Carbons at vertices/ends; H on C omitted | Zig-zag line ending in OH |
IUPAC Naming Steps
- Find the longest continuous carbon chain → root name (meth-, eth-, prop-, but-, pent-, hex-)
- Identify any substituent groups (alkyl branches or halogens)
- Number the chain from the end giving the lowest numbers to the substituents/functional group
- Name branches with position numbers; use di-, tri- for multiples; alphabetical order
- Add the suffix for the principal functional group (-ol, -al, -one, -oic acid, etc.)
Worked Examples
2-chloro-3-methylpentane
5C chain (pentane), Cl at C2, CH₃ branch at C3. Number from the end nearest the substituents.
2,4-dichlorohexane
6C chain (hexane), two identical Cl at positions 2 and 4 → dichloro.
3-bromopropene
3C chain with C=C. Number to give the double bond the lowest number; Br ends up at C3.
Butan-2-ol
4C chain (butane), −OH on C2. The "2" goes before the suffix "-ol".
Primary, Secondary & Tertiary Classification
Alcohols, halogenoalkanes and amines are classified by the number of alkyl groups attached to the carbon bearing the functional group:
Primary (1°)
Carbon bonded to one alkyl group
e.g. Propan-1-ol, 1-bromopropane
Secondary (2°)
Carbon bonded to two alkyl groups
e.g. Propan-2-ol, 2-bromopropane
Tertiary (3°)
Carbon bonded to three alkyl groups
e.g. 2-methylpropan-2-ol
🔬 HL. Stereoisomerism
Stereoisomers have the same structural formula but a different spatial arrangement of atoms.
Cis-Trans Isomerism
- Occurs due to restricted rotation around a C=C double bond
- Requires each C of the double bond to have two different groups attached
- Cis = same groups on the same side; Trans = same groups on opposite sides
- Also occurs in C₃ and C₄ cycloalkanes
- E/Z nomenclature will NOT be assessed on the IB exam
Optical Isomerism (Chirality)
- A chiral carbon is bonded to four different groups
- Produces enantiomers. Non-superimposable mirror images
- Drawn using wedge-dash notation to show 3D arrangement
- Enantiomers have identical physical/chemical properties except in chiral environments
- A 50:50 mixture of enantiomers is called a racemic mixture
Think About It
Why do boiling points increase down a homologous series?
As chain length increases, the number of electrons increases, which strengthens London dispersion forces. Stronger intermolecular forces require more energy to overcome → higher boiling point.
⚠️ Common Exam Mistakes
- Confusing displayed and condensed formulas. Displayed shows every bond
- Numbering the chain from the wrong end. Always give the lowest numbers to substituents/functional groups
- Forgetting that aldehydes and ketones share the same general formula (CnH2nO). They're functional group isomers!
- Thinking all C=C compounds show cis-trans isomerism. Both C atoms must have two different groups