Organic Families, Formulae and Naming
ChemistryΒ· 11.1, 11.2Β· 15 min read
1. 1. Homologous Series and Functional Groups (Core)β βββββ± 4 min
Homologous Series
A family of organic compounds that share the same functional group, same general formula, similar chemical properties, and show a gradual trend in physical properties (e.g. boiling point increases as carbon chain length increases).
Example:
Alkanes are a homologous series with general formula CβHββββ
All compounds in the same homologous series have the same functional group: the atom or group of atoms that gives the compound its characteristic chemical properties. For example, all alkenes have a C=C double bond functional group, so they all undergo addition reactions.
State two features shared by all members of the alcohol homologous series.
- 1
- Recall the definition of a homologous series: shared features include same functional group, same general formula, similar chemical properties, gradual change in physical properties.
- 2
- Select two valid features for alcohols: All alcohols have the -OH functional group, and all follow the general formula CβHββββOH.
Exam tip:
You will often be asked to list 2-3 features of homologous series in 2-3 mark questions; memorize the 4 key features to score full marks.
2. 2. Organic Formula Types (Core)β β ββββ± 5 min
Displayed Formula
A formula that shows every atom and every covalent bond in an organic molecule, drawn as a full structural diagram.
Example:
Displayed formula of ethane shows two C atoms single bonded to each other, each bonded to three H atoms.
You need to know four formula types for 0620: 1. General formula: applies to all members of a homologous series, uses n for number of carbon atoms. 2. Molecular formula: shows the actual number of each atom in one molecule of a compound. 3. Displayed formula: shows all atoms and bonds. 4. Structural formula: shortened version of displayed formula, shows groups of atoms without drawing every C-H bond, e.g. CHβCHβ for ethane.
Write the general, molecular, displayed and structural formula for propane (3 carbon atoms, alkane series, general formula CβHββββ).
- 1
- General formula for alkanes: CβHββββ (applies to all alkanes, including propane)
- 2
- Molecular formula for propane (n=3): CβHβ
- 3
- Structural formula: CHβCHβCHβ
- 4
- Displayed formula: Draw 3 C atoms in a single-bonded chain. Each end C has 3 H atoms attached, the middle C has 2 H atoms attached, with all single bonds shown explicitly.
Exam tip:
When drawing displayed formula, you must show every bond, including all C-H and C-O bonds, to get full marks. Missing bonds will cost you marks.
3. 3. Naming Unbranched Organic Compounds (Core)β β β βββ± 5 min
IUPAC naming rules for unbranched organic compounds use two parts: 1. Prefix: tells you the number of carbon atoms. 2. Suffix: tells you the homologous series the compound belongs to.
1 C: meth-
2 C: eth-
3 C: prop-
4 C: but-
-ane: alkane (only single C-C bonds)
-ene: alkene (contains one C=C double bond)
-ol: alcohol (contains -OH functional group)
-oic acid: carboxylic acid (contains -COOH functional group)
Name the unbranched compound with 3 carbon atoms in the alkene homologous series.
- 1
- Find the prefix for 3 carbon atoms: prop-
- 2
- Find the suffix for alkene series: -ene
- 3
- Combine prefix and suffix: propene
Exam tip:
For unbranched alkenes with 3 or 4 carbons, the double bond is always on the first carbon, so you do not need to number the position of the double bond at IGCSE level.
4. 4. Esters: Naming and Formulae (Extended only)β β β β βExtended onlyβ± 4 min
Ester
An organic compound formed by the reaction between an alcohol and a carboxylic acid, with the functional group -COO-
Example:
Ethyl ethanoate is formed from ethanol and ethanoic acid
Ester naming follows two parts, reversed from the reactants: 1. First part: name of the alkyl group from the alcohol (e.g. ethyl from ethanol). 2. Second part: name of the carboxylate group from the carboxylic acid (e.g. ethanoate from ethanoic acid). The general formula for esters is CβHββOβ.
Name the ester formed from methanol and propanoic acid, and write its structural formula.
- 1
- Alcohol is methanol (1 C): alkyl group = methyl-
- 2
- Carboxylic acid is propanoic acid (3 C): carboxylate group = propanoate
- 3
- Combine to get name: methyl propanoate
- 4
- Structural formula: CHβCHβCOOCHβ
Exam tip:
Always remember ester names are alcohol part first, acid part second: it is a common mistake to reverse the order.
5. Common Pitfalls
Wrong move:
Writing the general formula for alkanes as CβHββ instead of CβHββββ
Why:
Confusing alkanes with alkenes, which have the general formula CβHββ
Correct move:
Memorize the four core general formulae explicitly: Alkanes CβHββββ, Alkenes CβHββ, Alcohols CβHββββOH, Carboxylic acids CβHββOβ
Wrong move:
Missing bonds when drawing displayed formula, e.g. not showing the O-H bond in alcohol functional groups
Why:
Rushing to draw the structure, forgetting that all covalent bonds must be shown
Correct move:
Double check every atom has the correct number of bonds (C=4, O=2, H=1) and each bond is explicitly drawn for displayed formula questions
Wrong move:
Naming esters with the carboxylic acid part first, e.g. calling ethyl ethanoate 'ethanoate ethyl'
Why:
Mixing up the order of naming parts for esters
Correct move:
Use the mnemonic: 'Alcohol first, acid last, swap acid's -oic acid for -oate'
Wrong move:
Using prefixes longer than but- (e.g. pent-) for naming compounds
Why:
Assuming longer carbon chains are assessed, but the syllabus only requires up to 4 carbon atoms
Correct move:
Only use meth-, eth-, prop-, but- for all naming questions in 0620 exams
Wrong move:
Classifying compounds with the same molecular formula as the same homologous series
Why:
Forgetting that functional group determines the series, not molecular formula
Correct move:
Always identify the functional group first to classify a compound, e.g. CβHβOβ can be ethanoic acid (COOH group) or methyl methanoate (ester group, Extended only)
6. Quick Reference Cheatsheet
Homologous Series | General Formula | Functional Group | Suffix | 1C Example Name | 1C Example Formula |
|---|---|---|---|---|---|
Alkane | CβHββββ | Only single C-C bonds | -ane | Methane | CHβ |
Alkene | CβHββ | C=C double bond | -ene | N/A (min 2C) | N/A |
Alcohol | CβHββββOH | -OH | -ol | Methanol | CHβOH |
Carboxylic Acid | CβHββOβ | -COOH | -oic acid | Methanoic acid | HCOOH |
Ester (Extended) | CβHββOβ | -COO- | -yl -oate | Methyl methanoate | HCOOCHβ |
7. Frequently Asked
Do I need to name branched organic compounds for 0620?
No, you only need to name unbranched compounds with up to 4 carbon atoms for CIE IGCSE Chemistry 0620. Branched compounds are not assessed at this level.
What formula types do I need to learn for organic compounds?
You need to be able to write general formulae for each homologous series, molecular formulae for individual compounds, displayed formulae showing all atoms and bonds, and shortened structural formulae for unbranched compounds up to 4C.
Going deeper
What's Next
Now that you can classify, name and write formulae for organic compounds, you are ready to learn the specific reactions and properties of each homologous series, which make up a large portion of the organic chemistry section in your CIE IGCSE Chemistry 0620 exam. You will first explore alkanes, including their combustion and substitution reactions, before moving on to reactive alkenes and their addition reactions. Extended learners will also study esterification and polymerisation reactions that build on the naming and formula skills you have mastered here. Make sure you practice drawing displayed formulae regularly, as these are frequently tested in both structured and multiple-choice papers.