Characteristics of a Homologous Series
A family of organic compounds in which successive members differ by a common structural unit (−CH₂−). Members share the same functional group, the same general formula, show a gradual trend in physical properties (e.g. Boiling point), and possess similar chemical properties.
Functional Group: A specific atom or group of atoms within a molecule responsible for its characteristic chemical reactivity.
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 | C₄H₁₀O (cannot be 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 Systematic Nomenclature Rules
- Find the longest continuous carbon chain → stem/root name (meth-, eth-, prop-, but-, pent-, hex-).
- Identify any principal functional group (determines suffix: -ol, -al, -one, -oic acid).
- Number the chain from the end giving lowest position numbers to the principal group, then substituents.
- Identify & name branch substituents (methyl, ethyl, fluoro, chloro, bromo) with location locants in alphabetical order.
- Use prefixes (di-, tri-, tetra-) for multiple identical substituents.
IUPAC Nomenclature in Practice
2-chloro-3-methylpentane
5C chain (pentane), Cl at C2, CH₃ branch at C3.
2,4-dichlorohexane
6C chain (hexane), two Cl at positions 2 and 4 → dichloro.
3-bromopropene
3C chain with C=C. Number from C=C end; Br is at C3.
Butan-2-ol
4C chain (butane), −OH on C2 (suffix "-2-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
Cis-Trans (Geometric) Isomerism
Stereoisomers have the same structural formula but a different 3D spatial arrangement of atoms in space.
Cis-Trans Isomerism
- Occurs due to restricted rotation around a \(\text{C}=\text{C}\) double bond or in cyclic rings.
- Requires each carbon of the double bond to have two different substituent groups attached.
- Cis: Identical/priority groups on the same side of the double bond.
- Trans: Identical/priority groups on opposite sides.
Trends in Boiling Points of Homologous Series
Alkanes have higher boiling points as chain length increases because the larger electron cloud increases the strength of London dispersion forces between molecules, requiring more thermal energy to separate them.
Common IUPAC Naming Pitfalls
- Longest Chain: The longest chain is not always drawn horizontally! Look around bends and branches.
- Numbering Direction: Always number from the end that gives the principal functional group the lowest number.
- Alphabetical Order: Halogen and alkyl prefixes are written alphabetically (e.g. bromo- before chloro- before methyl-); ignore di-/tri- multipliers.
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