IUPAC Nomenclature of Organic Chemicals
Module 7 of the Year 12 HSC Chemistry syllabus is called ‘Organic Chemistry’. Organic chemicals are chemicals that contain carbon and hydrogen atoms. From Year 11 Module 1 to Year 12 Module 6, you have studied inorganic chemistry. If you recall, Year 11 Module 1 began with learning how to name inorganic covalent and ionic compounds. Similarly, to progress in Module 7, we need to begin by learning how to name organic chemicals.
To name organic chemicals:
Step 1: Identify and name the parent chain, the longest carbon chain containing the main functional group.
The prefix of the name depends on the number of carbons in the chain.
The suffix of the name depends on the main functional group.
Combine the prefix and suffix to form one word.
Example:
There are 6 carbons in a chain so the prefix is ‘hex’. All carbons are single-bonded to each other so the suffix is ‘ane’. The IUPAC name of this structure is hexane.
Step 2: Number each carbon in the parent chain so the carbon with the main functional group has the smallest number. These numbers are called location numbers. Write the number before the name in Step 1 and separate the number and name with a hyphen (-).
Example:
There are 5 carbons in a chain so the prefix is ‘pent’. All carbons are single-bonded to each other so the suffix is ‘ane’. There is a C-OH functional group so remove the ‘e’ from the alkane name and replace with an additional suffix ‘ol’. Numbering from right to left gives the smaller location number 1. The IUPAC name of this structure is 1-pentanol.
A location number is not required to name the functional groups of alkanes, carboxylic acids, aldehydes and amides.
Example:
There are 3 carbons in a chain so the prefix is ‘pent’. All carbons are single-bonded to each other so the suffix is ‘ane’. There is a C=O functional group on a terminal carbon so replace the ‘e’ from the alkane name with the suffix ‘al’. Aldehydes, by definition, always have the C=O bond on the terminal carbon so the location number is redundant. The IUPAC name is propanal.
Step 3: Identify and name any carbon branches or halogen substituents bonded to the parent chain.
For branches, the prefix also depends on the number of carbons and the suffix is ‘yl’.
For halogen substituents, replace the ‘ine’ suffix on the name of the element with the suffix ‘o’. For example, ‘chlorine’ becomes ‘chloro’ and ‘bromine’ becomes ‘bromo’.
Write the location number of the carbon that the branch or halogen substituent is bonded to before its name. Separate the number and name with a hyphen.
If branches are bonded to the nitrogen (N) atom in amines or amides, give it the location letter ‘N’ instead of a number.
Example:
There are 4 carbons in a chain so the prefix is ‘but’. All carbons are single-bonded to each other so the suffix is ‘ane’. The parent chain name is butane. Numbering from right to left, there is an additional carbon atom bonded to carbon number 2 in the chain. The prefix for one carbon is ‘meth’ and the suffix for a branch is ‘yl’. The IUPAC name is 2-methylbutane.
Example:
There are 2 carbons in a chain so the prefix is ‘eth’. All carbons are single-bonded to each other so the suffix is ‘ane’. There is a chlorine substituent on carbon number 1, if numbering from left to right. Replace the ‘ine’ suffix on the halogen with ‘o’. The IUPAC name is 1-chloroethane.
Example:
There are 3 carbons in a chain with the O=C-N functional group so the parent chain name is propanamide. There is an additional carbon bonded to the nitrogen. The IUPAC name is N-methylpropanamide.
Step 4: Write an additional prefix and location number before a name if there are multiple branches or halogen substituents with the same name. For example, if there is one methyl branch on carbon number 2 and another methyl branch on carbon number 3 in the parent chain, the branches would be named ‘2,3-dimethyl’.
These are the same prefixes you used to name covalent compounds in Year 11 Module 1: Properties and Structure of Matter.
Example:
There are 2 carbons in a chain so the prefix is ‘eth’. The carbons are double-bonded to each other so the suffix is ‘ene’. A location number for the functional group is redundant here since there are only 2 carbons and 1 bond between carbons in total. There are two chlorine substituents both on carbon number 1. The IUPAC name is 1,1-dichloroethene.
Step 5: Order the names of branches and halogen substituents alphabetically, ignoring the lettering of any additional prefixes added in Step 4. For example, an ‘ethyl’ branch would be named before ‘dimethyl’ branches before ‘e’ comes before ‘m’. The ‘d’ in the prefix ‘di’ is ignored.
Example:
There are 2 carbons single bonded so the parent chain name is ethane. Numbering from left to right gives the smallest combination of location numbers for all 5 halogen substituents. Bromine is named first, followed by chlorine and lastly, fluorine. The IUPAC name is 2-bromo-2-chloro-1,1,1-trifluoroethane.
Question: Write the IUPAC names of the organic chemicals with the structural formulae below. (3 marks)
(a)
(b)
(c)
Answers:
(a) 2-methyl-2-butene
(b) 5-bromo-2-pentanone
(c) 2-methylpentanoic acid
