Halogenoalkane
A concise revision guide to halogenoalkane functional groups, halogen prefixes, position numbers, alphabetical ordering, and how to name compounds that contain both halogen substituents and C=C double bonds.
What Is a Halogenoalkane?
A halogenoalkane is an organic compound in which at least one hydrogen atom in an alkane has been replaced by a halogen atom.
The halogen may be fluorine, chlorine, bromine or iodine. In organic names, these atoms are treated as substituents attached to the carbon chain.
The functional group is usually represented as C-X, where X is a halogen atom such as F, Cl, Br or I.
Key idea: In naming, the halogen is not usually the main suffix. It is written as a prefix before the parent hydrocarbon name.
| Homologous series | Functional group | Prefix | Example |
|---|---|---|---|
| Halogenoalkanes | F, Cl, Br or I attached to a carbon chain | fluoro-, chloro-, bromo- or iodo- | 1-chloropropane |
Halogen Prefixes
A halogen attached to a carbon chain is named using the appropriate prefix: fluoro-, chloro-, bromo- or iodo-.
A position number is included when needed to show which carbon atom the halogen is attached to. The parent chain is numbered to give the substituent the lowest possible number, unless a higher-priority group controls the numbering.
F attached to the chain
Use the prefix fluoro-.
Cl attached to the chain
Use the prefix chloro-.
Br attached to the chain
Use the prefix bromo-.
I attached to the chain
Use the prefix iodo-.
The chloro- prefix is used because chlorine is attached to the first carbon in the propyl chain.
The position number 2 shows that bromine is attached to the second carbon atom in the butane chain.
More Than One Halogen or Side Chain
When a molecule contains more than one functional group or side chain, the prefixes are listed in alphabetical order.
Prefixes such as di- and tri- are ignored when deciding alphabetical order. For example, dibromo is alphabetised using bromo, not the letter d.
If there are two identical halogen atoms, use di-. If there are three identical halogen atoms, use tri-. Position numbers are still needed to show where each halogen is attached.
Exam focus: Number the chain carefully, then list the substituents alphabetically using the root prefix name, such as bromo, chloro, fluoro or iodo.
The name lists bromo, fluoro and methyl substituents with position numbers, while di- shows that two bromine atoms are present.
Halogenoalkanes with Alkenes
If a molecule contains both a halogen substituent and an alkene group, the alkene has higher priority for numbering than the halogen substituent.
This means the carbon-carbon double bond is given the lowest possible number when numbering the carbon chain.
The halogen is still included in the name as a prefix, with its own position number.
The parent chain is numbered so that the C=C bond starts at carbon 1, while the halogens and methyl group are named as prefixes.
Building the Name Step by Step
For halogenoalkanes, start by finding the longest carbon chain. Then number the chain to give the substituents the lowest suitable position numbers.
Next, identify each halogen substituent and write the correct prefix. If there is more than one substituent, arrange the prefixes alphabetically, ignoring multiplier prefixes such as di- and tri-.
Finally, combine the position numbers, prefixes and parent alkane or alkene name into one IUPAC name.
| Step | What to check | Example application |
|---|---|---|
| 1 | Find the parent carbon chain. | propane, butane, pentane or pentene depending on the structure. |
| 2 | Number the chain to show positions clearly. | 2-bromobutane shows bromine on carbon 2. |
| 3 | Name halogens as prefixes. | Cl becomes chloro-, Br becomes bromo-, F becomes fluoro- and I becomes iodo-. |
| 4 | Use alphabetical order when several prefixes are present. | Ignore di- or tri- when deciding the order. |
Check Your Understanding
Use these short activities to check halogenoalkane prefixes, position numbers, alphabetical ordering and priority when alkenes are also present.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.