Organic chemistry is the study of carbon compounds. Carbon is a unique element due to its ability to form a massive variety of stable, complex molecules. In this lesson, we explore how carbon behaves, how we represent organic structures, and the rules of the International Union of Pure and Applied Chemistry (IUPAC) for naming these compounds systematically.
Key Principle
The functional group determines the chemical properties of the molecule, while the longest continuous carbon chain containing the principal functional group defines the parent name of the compound.
Why Carbon is Special
Carbon is situated in Group 14 of the periodic table, meaning it has 4 valence electrons. To achieve a stable octet, it forms 4 covalent bonds. These can be single, double, or triple bonds. Carbon can readily bond to other carbon atoms, forming long chains or rings, a property known as catenation. It also bonds strongly to hydrogen, oxygen, nitrogen, and halogens, facilitating a vast range of diverse chemical structures.
Types of Formulae
We represent organic compounds in different ways depending on how much detail we need to show. The main types of formulas you must know are:
| Type of Formula | Description | Example (Ethanol) | Example (Butanoic Acid) |
|---|---|---|---|
| Empirical Formula | The simplest whole number ratio of atoms of each element in a compound. | C₂H₆O | C₂H₄O |
| Molecular Formula | The actual number of atoms of each element in a molecule. | C₂H₆O | C₄H₈O₂ |
| Structural Formula | The minimal detail showing how the atoms are linked, without drawing bonds. In AQA, this refers to the condensed format. | CH₃CH₂OH | CH₃CH₂CH₂COOH |
| Displayed Formula | Shows every atom and every bond in the molecule. |
H H | | H-C-C-O-H | | H H |
H H H O | | | // H-C-C-C-C | | | \ H H H O-H |
| Skeletal Formula | Shows the carbon carbon skeleton only. Carbon atoms are located at junctions and line ends, with hydrogen atoms on carbon implied. |
When an AQA question asks for the structural formula, write the condensed structure (e.g. CH₃CH₂OH). Do not draw lines representing bonds unless asked for a displayed formula. Drawing single covalent lines in a structural formula question is a common source of lost marks.
IUPAC Naming System
Systematic naming ensures that every unique structure has exactly one name. Follow this step by step process to name a compound:
- Find the longest continuous carbon chain containing the principal functional group. This sets the parent stem:
- 1 Carbon: meth-
- 2 Carbons: eth-
- 3 Carbons: prop-
- 4 Carbons: but-
- 5 Carbons: pent-
- 6 Carbons: hex-
- Identify functional groups and substituents (e.g. chlorine, alkyl chains) to determine suffixes and prefixes.
- Number the carbon chain starting from the end that gives the principal functional group the lowest possible number (locant).
- Alphabetise substituents when assembling the name (e.g., ethyl before methyl), ignoring numerical prefixes like di- and tri-.
- Use hyphens to separate numbers from letters and commas to separate numbers from numbers (e.g., 2,2-dimethylpropane).
Functional Group Table
A functional group is the reactive part of a molecule. Members of a homologous series share the same functional group and general formula, showing a graduation in physical properties and identical chemical properties.
| Homologous Series | Functional Group | General Formula | Prefix / Suffix | Example |
|---|---|---|---|---|
| Alkane | C-C / C-H single bonds | \( \text{C}_n\text{H}_{2n+2} \) | -ane | Ethane (CH₃CH₃) |
| Alkene | C=C double bond | \( \text{C}_n\text{H}_{2n} \) | -ene | Ethene (CH₂=CH₂) |
| Alcohol | -OH hydroxyl group | \( \text{C}_n\text{H}_{2n+1}\text{OH} \) | -ol / hydroxy- | Ethanol (CH₃CH₂OH) |
| Halogenoalkane | -X halogen group | \( \text{C}_n\text{H}_{2n+1}\text{X} \) | chloro- / bromo- / iodo- | Chloroethane (CH₃CH₂Cl) |
| Aldehyde | -CHO carbonyl group (terminal) | \( \text{C}_n\text{H}_{2n}\text{O} \) | -al | Ethanal (CH₃CHO) |
| Ketone | -CO- carbonyl group (internal) | \( \text{C}_n\text{H}_{2n}\text{O} \) | -one | Propanone (CH₃COCH₃) |
| Carboxylic Acid | -COOH carboxyl group | \( \text{C}_n\text{H}_{2n}\text{O}_2 \) | -oic acid | Ethanoic acid (CH₃COOH) |
| Esters | -COOR ester linkage | \( \text{C}_n\text{H}_{2n}\text{O}_2 \) | -yl -oate | Methyl ethanoate (CH₃COOCH₃) |
| Acyl Chloride | -COCl acyl halide | \( \text{C}_n\text{H}_{2n-1}\text{OCl} \) | -oyl chloride | Ethanoyl chloride (CH₃COCl) |
| Nitrile | -C≡N cyano group | \( \text{C}_n\text{H}_{2n+1}\text{N} \) | -nitrile | Ethanenitrile (CH₃CN) |
An atom or group of atoms that determines the chemical reactions of a compound.
A family of organic compounds with the same functional group and general formula, where successive members differ by CH₂.
Worked Examples
Solution:
- The longest carbon chain has 4 carbon atoms, which corresponds to the parent stem butane.
- There is a chlorine substituent, which adds the prefix chloro-.
- Numbering from the left gives the chlorine atom the locant 2 (while numbering from the right would give it the locant 3). We choose the lowest locant.
- Combining these steps, the systematic IUPAC name is 2-chlorobutane.
Solution:
- The parent stem is propane (3 carbons: C₁-C₂-C₃).
- -1-ol tells us the hydroxyl group (-OH) is on Carbon 1.
- 2-methyl tells us a methyl group (-CH₃) is attached to Carbon 2.
- Assembling the structure: Carbon 1 has the -OH and two hydrogens (-CH₂OH); Carbon 2 has one hydrogen and the methyl group (-CH(CH₃)-); Carbon 3 is terminal (-CH₃).
- The structural formula is CH₃CH(CH₃)CH₂OH.
- Counting all atoms gives the molecular formula: C₄H₁₀O.
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