11.2 Naming Organic Compounds Revision Notes
1. Overview
Organic chemistry is the study of carbon-based compounds. Because carbon can form four covalent bonds and create long chains, millions of different molecules exist; therefore, a systematic naming system (IUPAC) is essential to ensure scientists worldwide can communicate clearly. This topic covers how to name and draw the fundamental building blocks of organic chemistry, including alkanes, alkenes, alcohols, and carboxylic acids.
Key Definitions
- Hydrocarbon: A compound containing only hydrogen and carbon atoms.
- Homologous Series: A family of similar compounds with the same functional group and general formula, where each member differs from the next by a –CH₂– unit.
- Functional Group: An atom or group of atoms that determine the chemical properties of a homologous series (e.g., –OH in alcohols).
- Displayed Formula: A drawing showing all atoms and all the bonds in a molecule.
- Structural Formula: A formula that shows how atoms are grouped together in a molecule without showing every bond (e.g., CH₃CH₂OH).
- Molecular Formula: The total number of atoms of each element in a compound (e.g., C₂H₆O).
Core Content
Naming and Drawing Basics
The name of an organic compound is derived from the number of carbon atoms (the prefix) and the functional group (the suffix).
Prefixes (Number of Carbons):
- 1 Carbon: Meth-
- 2 Carbons: Eth-
Suffixes (Compound Type):
- -ane: Alkane (C–C single bonds)
- -ene: Alkene (C=C double bond)
- -ol: Alcohol (–OH group)
- -oic acid: Carboxylic acid (–COOH group)
Displayed Formulae to Know
| Name | Type | Molecular Formula | Displayed Formula |
|---|---|---|---|
| Methane | Alkane | CH₄ | C atom in center bonded to 4 H atoms |
| Ethane | Alkane | C₂H₆ | 2 C atoms with single bond, each bonded to 3 H atoms |
| Ethene | Alkene | C₂H₄ | 2 C atoms with double bond, each bonded to 2 H atoms |
| Ethanol | Alcohol | C₂H₅OH | 2 C atoms with single bond, first C with 3 H, second C with 2 H and an O-H group |
| Ethanoic Acid | Carboxylic Acid | CH₃COOH | 2 C atoms; first C has 3 H; second C has a double bond to O and a single bond to O-H |
Identifying Compounds
You must be able to identify a compound type from its name or formula:
- C₃H₈: Ends in -ane (Propane) because it fits CₙH₂ₙ₊₂.
- C₂H₄: Ends in -ene (Ethene) because it fits CₙH₂ₙ.
- CH₃OH: Ends in -ol (Methanol) because of the –OH group.
- HCOOH: Ends in -oic acid (Methanoic acid) because of the –COOH group.
Extended Content (Extended Curriculum Only)
Expanded Alkanes, Alkenes, and Alcohols
For the extended curriculum, you must know prefixes for up to 4 carbons:
- 3 Carbons: Prop-
- 4 Carbons: But-
Alkenes (Isomers): The position of the double bond must be indicated by a number.
- But-1-ene: CH₂=CHCH₂CH₃ (Double bond starts at carbon 1)
- But-2-ene: CH₃CH=CHCH₃ (Double bond starts at carbon 2)
Alcohols (Isomers):
- Propan-1-ol: CH₃CH₂CH₂OH (–OH on the end carbon)
- Propan-2-ol: CH₃CH(OH)CH₃ (–OH on the middle carbon)
- Butan-1-ol: CH₃CH₂CH₂CH₂OH
- Butan-2-ol: CH₃CH₂CH(OH)CH₃
Carboxylic Acids (up to C4)
- Methanoic acid: HCOOH
- Ethanoic acid: CH₃COOH
- Propanoic acid: CH₃CH₂COOH
- Butanoic acid: CH₃CH₂CH₂COOH
Esters
Esters are formed from an alcohol and a carboxylic acid.
- Naming: [Alcohol part]-yl [Acid part]-oate.
- Example: Ethyl ethanoate (Made from ethanol and ethanoic acid).
- Structural Formula: CH₃COOCH₂CH₃
- Displayed Formula: CH₃ connected to C=O, then that C connected to O, which is then connected to CH₂CH₃
Key Equations
Organic reactions often involve the production of new named compounds.
1. Combustion of Methane (Complete)
- Word: Methane + Oxygen → Carbon dioxide + Water
- Symbol: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
2. Substitution Reaction (Alkanes + Halogens)
- Word: Ethane + Chlorine → Chloroethane + Hydrogen chloride
- Symbol: C₂H₆(g) + Cl₂(g) → C₂H₅Cl(g) + HCl(g)
3. Addition Reaction (Ethene + Steam to make Ethanol)
- Word: Ethene + Steam → Ethanol
- Symbol: C₂H₄(g) + H₂O(g) → C₂H₅OH(l)
4. Esterification (Ethanoic Acid + Ethanol)
- Word: Ethanoic acid + Ethanol ⇌ Ethyl ethanoate + Water
- Symbol: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l)
Common Mistakes to Avoid
- ❌ Wrong: Drawing carbon with 3 or 5 bonds.
- ✓ Right: Carbon always has 4 bonds. Check every carbon in your drawing.
- ❌ Wrong: Forgetting the "H" in the –OH group of an alcohol.
- ✓ Right: Ensure the bond goes from the Carbon to the Oxygen, then Oxygen to Hydrogen (C–O–H).
- ❌ Wrong: Naming an ester as "Ethyloic ethanyl".
- ✓ Right: Always name the alcohol part first with "-yl" and the acid part second with "-oate" (Ethyl ethanoate).
- ❌ Wrong: Counting the carbon in the –COOH group separately from the chain.
- ✓ Right: If there are 3 carbons total including the one in –COOH, it is propanoic acid.
Exam Tips
- Command Words: "Draw the displayed formula" means show every single bond. If you write "CH₃", you may lose marks; you must draw the three lines for the C–H bonds.
- Identify the Functional Group: Look for the double bond (alkene), the –OH (alcohol), or the –COOH (acid) first before naming the chain.
- Numbering: When numbering carbon chains (e.g., But-1-ene vs But-2-ene), always start numbering from the end that gives the functional group the lowest possible number.
- Esters: In a displayed formula for an ester, the carbon double-bonded to an oxygen (C=O) always comes from the carboxylic acid. The oxygen in the middle of the chain (–O–) connects the acid part to the alcohol part.