Hydrocarbon Nomenclature
This is part of HSC Chemistry course under the topic of Hydrocarbons
HSC Chemistry Syllabus
-
construct models, identify the functional group, and write structural and molecular formulae for homologous series of organic chemical compounds, up to C8 (ACSCH035):
Hydrocarbon Nomenclature
This video will teach you the IUPAC conventions of naming hydrocarbons. The rules are applied for other organic compounds. Nomenclature refers to the rules to be followed for naming organic substances in order to avoid confusion and ambiguity.
This is a very important skill to learn in organic chemistry as names of organic compounds are used often in communication, teaching, and exams.
How do we Name Hydrocarbons?
Nomenclature of organic compounds including hydrocarbons must account for:- Functional group(s) and their positions
- Number of carbon atoms in the longest chain (stem)
- Type and position of substituents
Prefixes for the number of carbon atoms in the longest chain
|
No. of Carbons |
Name |
Molecular formula |
|
1 |
Methane |
CH4 |
|
2 |
Ethane |
C2H6 |
|
3 |
Propane |
C3H8 |
|
4 |
Butane |
C4H10 |
|
5 |
Pentane |
C5H12 |
|
6 |
Hexane |
C6H14 |
|
7 |
Heptane |
C7H16 |
|
8 |
Octane |
C8H18 |
Functional Groups of Hydrocarbons
|
Functional group |
Suffix |
Example |
|
Alkane |
-ane |
Ethane |
|
Alkene |
-ene |
Ethene |
|
Alkyne |
-yne |
Ethyne |
Common Substituents for Hydrocarbons
In addition to the longest carbon chain, organic compounds including hydrocarbons can have ‘branches’ or formally – substituents. The presence of substituents in organic molecules are denoted in the name as a prefix.
It is important to know the prefix name of common substituents shown in the table.
|
Substituent |
Prefix |
Example |
Substituent |
Prefix |
Example |
|
–CH3 |
Methyl- |
2-methylbutane |
–Cl |
Chloro- |
chloroethane |
|
–CH2CH3 |
Ethyl- |
3-ethylpentane |
–Br |
Bromo- |
bromoethane |
|
–F |
Fluoro- |
Fluoroethane |
–I |
Iodo- |
iodoethane |
IUPAC Rules for Naming
The following steps can be followed when the structure of an organic compound is provided.
1. Find the longest continuous carbon chain containing the principal functional group. This will help identify which carbons belong to a substituent group.
2. Number the carbon chain such that the functional group receives the smallest number. After prioritising the functional group, the substituents should also receive the smallest number possible.
3. Name each substituent and indicate the position (separate by a ‘-‘)
Example 1
2-methyl-1-butene
Step 1: there are four consecutive carbons in the structure on the right. The molecule contains a double carbon-carbon bond, so it is an alkene. Therefore, the name will contain 'butene' denoting '4'.
Step 2: the carbon atoms are numbered from right to left such that the C=C receives the bond position of '1' instead of '3'. Therefore, the name will be 1-butene.
Step 3: the remaining carbon atom and its three hydrogen atoms is a 'methyl' substituent. Since the methyl group is attached to the second carbon atom (counting from the right), the name will be 2-methyl-1-butene.
Example 2
3-methyl-1-butene
The structure presented in example 2 is similar to that in example 1. The name of this compound is 3-methyl-1-butene. The longest continuous carbon chain still contains four carbon atoms, and the C=C bond is still in the first position.
The only difference is that the methyl group is now on the third carbon atom (counting from the right).
4. If two or more identical substituent groups are present, specify using the prefix:
|
di |
Two identical groups |
|
tri |
Three identical groups |
|
tetra |
Four identical groups |
Each additional substituent group requires its own number in the name to denote its position.
Example 3
3,3-dimethyl-1-butene
The structure in example 3 has two methyl groups bonded to the third carbon atom (counting from the right). As a result, the name of this compound is 3,3-dimethyl-1-butene.
5. If there are more than one substituent, arrange them in alphabetical order. If the sum of their positions remains the same, the first substituent (after rearrangement) should have the smaller number.
Example 4
4-bromo-3,3-dimethyl-1-butene
The structure in example 4 has a bromine atom bonded to the fourth carbon atom (counting from the right). The name of this compound is 4-bromo-3,3-dimethyl-1-butene.
In this case, the bromine receives the higher number (4) than the methyl groups (3) because the C=C (principal functional group) must receive the lowest number possible (step 1)
Naming Cyclic Organic Compounds
Carbon atoms in an organic molecules can form a ring in which case they are referred to as cyclic compounds.
The names of cyclic compounds are added with a special prefix 'cyclo'.
cyclohexane
For example, when six carbon atoms form a ring-shaped molecule, it is given the name cyclohexane.
The position of functional groups and substituents is denoted by its number on the longest carbon chain. Since there are no 'ends' in a cyclic molecule, the principal functional group is automatically assigned the number '1'.
cyclohexene
The structure above is given the name cyclohexene. The position of the C=C bond is not denoted by '1' because it is assumed in the nomenclature to be assigned '1'.
The direction of counting is either clockwise or anticlockwise starting from the principal functional group. The direction is chosen such that substituent groups (if any) are assigned the lowest number(s) possible.
2-bromocyclohexene
In the structure above, the carbon atoms are counted in an anticlockwise direction. This is so that the bromine atom is assigned '2' instead of '5' if counted clockwise.