Nomenclature
Quick Notes
- Organic compounds can be represented using different formulas:
- Empirical formula – simplest whole number ratio of atoms.
- Molecular formula – actual number of atoms in a molecule.
- General formula – algebraic representation of a homologous series.
- Structural formula – shows how atoms are bonded without drawing structure.
- Displayed formula – shows all atoms and bonds visually.
- Skeletal formula – simplified representation where carbon and hydrogen backbones are shown as diagonal lines.
- Homologous series: A series of compounds with the same functional group, similar chemical properties, and a gradual change in physical properties.
- IUPAC naming rules are used to name organic compounds.
Full Notes
Representing Organic Compounds
Organic molecules can be written using different formula types:
Molecular Formula – Shows the actual number of atoms in a molecule.
ExampleHexane → C6H14
Empirical Formula – Shows the simplest whole number ratio of atoms in a compound.
Example Hexane (C6H14) → C3H7
General Formula – Represents a homologous series using an algebraic expression.
Example Alkanes → CnH2n+2
Structural Formula – Shows how atoms are arranged in a molecule without drawing bonds.
Example Hexane → CH3CH2CH2CH2CH2CH3
Displayed Formula – Shows all bonds and atoms.
Example Hexane
Skeletal Formula – Simplified representation where carbon atoms are at the ends and bends of lines, and hydrogens bonded to carbon atoms aren’t shown.
Example Hexane
Matt’s exam tip - if you find drawing skeletal formulas hard, start by drawing the displayed formula with a diagonal pattern between C-C bonds, then remove all the C-H bonds.
Homologous Series
A homologous series is a family of organic compounds that:
- Have the same general formula.
- Contain the same functional group.
- Have similar chemical properties.
- Show a gradual change in physical properties (e.g., boiling points increase with chain length).
ExamplesCommon Homologous Series:
| Series | General Formula | Functional Group | Example (Name) |
|---|---|---|---|
| Alkanes | CnH2n+2 | None (single C–C) | CH4 (Methane) |
| Alkenes | CnH2n | C=C | C2H4 (Ethene) |
| Alcohols | CnH2n+2O | –OH | C2H5OH (Ethanol) |
| Aldehydes | CnH2nO | –CHO | CH3CHO (Ethanal) |
| Ketones | CnH2nO | >C=O | CH3COCH3 (Propan-2-one) |
| Carboxylic Acids | CnH2nO2 | –COOH | CH3COOH (Ethanoic acid) |
IUPAC Naming Rules
You should be able to use IUPAC (International Union of Pure and Applied Chemistry) naming rules to name:
- Chains and rings with up to six carbon atoms.
- Alkanes, alkenes, alcohols, aldehydes, ketones, carboxylic acids, esters, amines, nitriles, arenes.
Steps for Naming Organic Compounds:
- Find the longest carbon chain
- Meth- (1C), Eth- (2C), Prop- (3C), But- (4C), Pent- (5C), Hex- (6C).
- Identify the main functional group (determines the suffix see table above).
- Number the carbon chain to give the lowest possible numbers to functional groups.
- Identify and name side chains (alkyl groups) as prefixes.
- Use commas between numbers and hyphens between letters and numbers.
Applying IUPAC Rules to Name Organic Compounds
| Class (a–g) | Suffix | Structure / Key Rule | Examples |
|---|---|---|---|
| Alkanes | -ane | Only single bonds between carbon atoms. | CH4 Methane |
| Alkenes | -ene | One or more C=C double bonds. Number the double bond position. | CH3CH=CHCH3 But-2-ene |
| Alkynes | -yne | One or more C≡C triple bonds. | CH≡CCH3 Prop-1-yne |
| Alcohols | -ol | OH (hydroxyl) group attached to a carbon atom. Number the position of the OH group. | CH3CH(OH)CH3 Propan-2-ol |
| Aldehydes | -al | CHO group at the end of a molecule. No need to number the group, it's always at position 1. | CH3CH2CH2CHO Butanal |
| Ketones | -one | C=O (carbonyl) group within the carbon chain. Number to show position of C=O. | CH3CH2COCH3 Butan-2-one |
| Carboxylic Acids | -oic acid | COOH group at the end of the molecule. Always at position 1; no number needed. | CH3CH2CH2COOH Butanoic acid |
Naming Branched and Substituted Compounds
- Identify the longest carbon chain as the base name.
- Name alkyl groups as prefixes (methyl-, ethyl-, propyl-).
- Use numbers to indicate the position of the substituents.
Example 2-Methylpentane (C6H14)
Longest chain = pentane, with a methyl group at carbon 2.
Naming Cyclic Compounds
Cyclic compounds are named with the prefix “cyclo-”.
Example Cyclohexane (C6H12)
Six-membered ring.
Applying IUPAC Rules to Draw Structures from Names
- Identify the longest carbon chain.
- Place functional groups in the correct positions according to numbering.
Example Propan-1-ol
Longest carbon chain = 3 (prop)
Type of functional group: -OH (anol) and Bonding position on carbon 1 (-1-ol)
Giving the final structure of
Summary Table: IUPAC Naming Rules
| Rule | Details | Quick Reference |
|---|---|---|
| Longest Chain | Choose the longest continuous carbon chain. | Meth-, Eth-, Prop-, But-, Pent-, Hex- |
| Functional Group | Determines the suffix (end of name) and carbon number priority. | -ol, -al, -one, -oic acid, -ene, -yne |
| Numbering | Assign lowest possible carbon numbering to the functional group and double/triple bonds. | Use 1-, 2-, etc.; commas between numbers |
| Substituents | Name and number alkyl groups as prefixes; alphabetise if multiple. | 2-Methyl-, 3-Ethyl-, di-/tri- for repeats |
| Punctuation | Commas between numbers; hyphens between numbers and letters. | e.g., But-2-ene; Propan-2-ol |