Chapter 12.6 - Nomenclature When Functional Groups are Present

In the previous section, we saw how the cyclic compounds are named. In this section, we will see the nomenclature when functional groups are present.

Nomenclature of organic compounds having functional groups

Some basics about functional groups can be written in 5 steps:
1. We know that, functional group is an atom or a group of atoms. It gives special characteristic properties to the organic compound to which it is attached.
• We have seen some basics about this topic in our earlier chemistry classes (Details here).
2. Consider a molecule of an organic compound containing a functional group.
• That molecule will be more reactive at the region where the functional group is attached.
3. Different organic compounds may be having the same functional group.
• For example, the three compounds in fig.12.39 below, have the same functional group ㅡOH

Fig.12.39

4. The above three compounds have similar chemical properties.
• For instance, they all liberate hydrogen on reaction with sodium metal.
• That means, different compounds become similar due to the presence of common functional groups.
5. So compounds carrying common  functional groups can be put together into various classes. Let us see some examples:
• All organic compounds carrying the functional group ㅡOH come under the class: Alcohols.
• All organic compounds carrying the functional group ㅡCHO come under the class: Aldehydes.
• All organic compounds carrying the functional group >C=O come under the class: Ketones.
(What does the '>' of the keto group indicate? Answer can be seen here)
• We will see more such classes as we continue our discussion.

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We have seen the basics of naming functional groups in our earlier chemistry classes. (Details here) Now we will see some advanced details. They can be written in 5 steps:
1. Identify the functional group present in the given molecule.
• Based on the functional group, the class of the molecule can be decided.
• Based on the class, the appropriate suffix can be decided.
• For example, if the molecule falls in the class of alcohols, the suffix is ‘ol’
2. Identify the parent chain (the chain with the largest number of C atoms)
• Based on the number of C atoms in the parent chain, the root name can be fixed.
• For example, if the parent chain has three C atoms, and if the hydrocarbon is an alkane, the root name will be propane.
3. Number the C atoms in the parent chain in such a way that, the C atom carrying the functional group gets the lowest possible number.
4. Now remove ‘e’ from the root name and put the following format in it’s place: -[no space]number of C atom[no space]-[no space]suffix denoting the functional group.
• For example, the IUPAC name of the molecule in fig.12.40(a) below is: Butan-2-ol
• Note that, 'Butan' is obtained by removing 'e' from Butane.

Fig.12.40

5. In some cases, in addition to the functional group, the parent chain may contain branches also. In such cases, the names of the branches should be written as prefixes.
• Let us see an example. It can be written in steps:
(i) Consider the molecule in fig.12.40(b) above.
• Imagine that, there is no methyl branch. Then the name of the molecule would be: octan-2-ol
(ii) Now, we must give appropriate name for the methyl branch.
• We know that, it’s name is: 6-methyl.
• We must use this as the prefix to the result in (i).
(iii) So the IUPAC name will be: 6-Methyloctan-2-ol

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• We have seen that in IUPAC system, the suffix is based on the functional group.
• For example, in 6-Methyloctan-2-ol, the suffix is octan-2-ol , which is based on the ㅡOH functional group.
• If there are different functional groups, which one shall we choose?
• We can write the answer in 7 steps:
1. If more than one functional group is present in a molecule, we must select one of them as the principal functional group. The remaining functional groups in the molecule will be called subordinate functional groups.
2. How do we know which one is the principal functional group?
• For that, first we make a list of all the functional groups in our molecule.
• Then we compare the list with the table 1 given below:

Table 1: Order of priority

ㅡCOOH, ㅡSO3H, ㅡCOOR, ㅡCOCl, ㅡCONH2, ㅡC☰N, ㅡHC=O, >C=O, ㅡOH, ㅡNH2, >C=C<, ㅡC☰Cㅡ • R represents an alkyl group like CH2ㅡ, CH3CH2ㅡ etc.,

• When compared with the above table 1, the functional group which comes in the top most position in our list will be the principal functional group.
(Note that, the above table-1 does not contain all the groups specified by the IUPAC. It contains only those groups which we encounter frequently in our present discussions)
3. There are some groups which are always considered as subordinate groups. They are given in table 2 below:

Table 2: Groups which are always subordinate

RㅡX, RㅡOㅡR, RㅡSㅡR, NO2 etc., • R represents an alkyl group like CH2ㅡ, CH3CH2ㅡ etc., • X represents a halogen like Cl, Br etc.,

(Note that, the above table-2 does not contain all the groups specified by the IUPAC. It contains only those groups which we encounter frequently in our present discussions)
4. Let us apply the above table 1 to an example.
• Consider the molecule in fig.12.41(a) below.

Fig.12.41

• We want to write the IUPAC name of the molecule. It can be done in 5 steps:
(i) First we make a list of the functional groups in the given molecule. We get: ㅡCOOH and ㅡOH
(Recall how the COOH group is attached to a molecule. We have seen it in our earlier chemistry classes. Details here)
(ii) Next we compare the list with table 1 above. We see that, ㅡCOOH gets top priority. So it is the principal functional group. The ㅡOH is the subordinate functional group.
(iii) Imagine that, the ㅡOH is not present. Then the IUPAC name of the molecule will be based on the ㅡCOOH group.
    ♦ When the component obtained from ㅡCOOH is the suffix, we use ‘-oic acid’  
    ♦ When the component obtained from ㅡCOOH is the prefix, we use ‘carboxy’
• Here, suffix is to be used because, ㅡCOOH has priority and the final name is going to be based on it.
• So the main component of the final name will be: hexanoic acid
(Main component will be always the suffix. Recall that, in methane, the main component is 'ane', which is the suffix)
(iv) Now we have to account for the ㅡOH group.
• The component of the name obtained from the ㅡOH will be the prefix.
    ♦ When the component obtained from ㅡOH is the suffix, we use ‘-ol’  
    ♦ When the component obtained from ㅡOH is the prefix, we use ‘hydroxy-’
(v) Now we give the position number. The ㅡOH is at position 4. So the component will be 4-hydroxy
• Thus the IUPAC name of the molecule will be: 4-Hydroxyhexanoic acid.
5. Let us apply the above table 1 to another example.
• Consider the molecule in fig.12.41(b) above. We want to write the IUPAC name of the molecule. It can be done in 5 steps:
(i) First we make a list of the functional groups in the given molecule. We get: ㅡC☰N and ㅡOH
(ii) Next we compare the list with table 1 above. We see that, ㅡC☰N gets top priority. So it is the principal functional group. The ㅡOH is the subordinate functional group.
(iii) Imagine that, the ㅡOH is not present. Then the IUPAC name of the molecule will be based on the ㅡC☰N group.
    ♦ When the component obtained from ㅡC☰N is the suffix, we use ‘nitrile’  
    ♦ When the component obtained from ㅡC☰N is the prefix, we use ‘cyano’
• Here, suffix is to be used because, ㅡC☰N has priority and the final name is going to be based on it.
• So the main component of the final name will be: hexanenitrile
(Main component will be always the suffix. Recall that, in methane, the main component is 'ane', which is the suffix)
(iv) Now we have to account for the ㅡOH group.
• The component of the name obtained from the ㅡOH will be the prefix.
    ♦ When the component obtained from ㅡOH is the suffix, we use ‘-ol’  
    ♦ When the component obtained from ㅡOH is the prefix, we use ‘hydroxy-’
(v) Now we give the position number. The ㅡOH is at position 4. So the component will be 4-hydroxy
• Thus the IUPAC name of the molecule will be: 4-Hydroxyhexanenitrile
6. Let us apply the above table 1 to yet another example.
• Consider the molecule in fig.12.42(a) below.

Fig.12.42

• We want to write the IUPAC name of the molecule. It can be done in 5 steps:
(i) First we make a list of the functional groups in the given molecule. We get: ㅡNH₂ and ㅡOH
(Recall how the ㅡNH₂ group is attached to a molecule. We have seen it in our earlier chemistry classes. Details here)
(ii) Next we compare the list with table 1 above. We see that, ㅡOH gets top priority. So it is the principal functional group. The ㅡNH₂ is the subordinate functional group.
(iii) Imagine that, the ㅡNH₂ is not present. Then the IUPAC name of the molecule will be based on the ㅡOH group.
    ♦ When the component obtained from ㅡOH is the suffix, we use ‘-ol’  
    ♦ When the component obtained from ㅡOH is the prefix, we use ‘hydroxy-’
• Here, suffix is to be used because, ㅡOH has priority and the final name is going to be based on it.
• So the main component of the final name will be: hexan-2-ol
(Main component will be always the suffix. Recall that, in methane, the main component is 'ane', which is the suffix)
(Note that, the numbering of C atoms is done in such a way as to give the lowest number to the priority group which is ㅡOH)
(iv) Now we have to account for the ㅡNH₂ group.
• The component of the name obtained from the ㅡNH₂ will be the prefix.
    ♦ When the component obtained from ㅡNH₂ is the suffix, we use ‘-amine’  
    ♦ When the component obtained from ㅡNH₂ is the prefix, we use ‘amino-’
(v) Now we give the position number. The ㅡNH₂ is at position 4. So the component will be 4-amino
• Thus the IUPAC name of the molecule will be: 4-Aminohexan-2-ol
7. Let us apply the above table 1 to one more example.
• Consider the molecule in fig.12.42(b) above. We want to write the IUPAC name of the molecule. It can be done in 5 steps:
(i) First we make a list of the functional groups in the given molecule. We get: C=C and ㅡOH
(ii) Next we compare the list with table 1 above. We see that, ㅡOH gets top priority. So it is the principal functional group. The C=C is the subordinate functional group.
(iii) Imagine that, the C=C is not present. Then the IUPAC name of the molecule will be based on the ㅡOH group.
    ♦ When the component obtained from ㅡOH is the suffix, we use ‘-ol’  
    ♦ When the component obtained from ㅡOH is the prefix, we use ‘hydroxy-’
• Here, suffix is to be used because, ㅡOH has priority and the final name is going to be based on it.
• So the main component of the final name will be: hexan-2-ol
(Main component will be always the suffix. Recall that, in methane, the main component is 'ane', which is the suffix)
(Note that, the numbering of C atoms is done in such a way as to give the lowest number to the priority group which is ㅡOH)
(iv) Now we have to account for the C=C group.
• The component of the name obtained from the C=C will be the prefix.
• But for double bonds and triple bonds, we need not worry about prefix or suffix. • Both prefix and suffix can be written using 'ene' or 'yne'.
• In our present case, hexane becomes hexene.
• Since ㅡOH is present, we remove 'e' from hexene and add 'ol'. Thus we get: hexenol
To indicate the position of the ㅡOH, we write: hexen-2-ol
(v) Now we give the position number of the C=C. It is at position 4.
• Thus the IUPAC name of the molecule will be: 4-Hexen-2-ol

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Now we know how to use the table 1. Let us see a solved example.

Solved example 12.8
Write the IUPAC names of (i) HOCH₂(CH₂)₃CH₂COCH₃ (ii) BrCH₂CH=CH₂
Solution:
Part (i):
1. We are given the condensed formula. Let us draw the complete structural formula. It is shown in fig.12.43(a) below:

Fig.12.43

2. We want to write the IUPAC name of the molecule in fig.12.42(a). It can be done in 5 steps:
(i) First we make a list of the functional groups in the given molecule. We get: >C=O and ㅡOH
(Recall how the >C=O group is attached to a molecule. We have seen it in our earlier chemistry classes. Details here)
(ii) Next we compare the list with table 1 above. We see that, >C=O gets top priority. So it is the principal functional group. The ㅡOH is the subordinate functional group.
(iii) Imagine that, the ㅡOH is not present. Then the IUPAC name of the molecule will be based on the >C=O group.
    ♦ When the component obtained from >C=O is the suffix, we use ‘-one’  
    ♦ When the component obtained from >C=O is the prefix, we use ‘oxo-’
• Here, suffix is to be used because, >C=O has priority and the final name is going to be based on it.
• So the main component of the final name will be: heptan-2-one
(Note that, the numbering is done in such a way as to give the lowest possible number to the principal functional group)
(iv) Now we have to account for the ㅡOH group.
• The component of the name obtained from the ㅡOH will be the prefix.
    ♦ When the component obtained from ㅡOH is the suffix, we use ‘-ol’  
    ♦ When the component obtained from ㅡOH is the prefix, we use ‘hydroxy-’
(v) Now we give the position number. The ㅡOH is at position 7. So the component will be 7-hydroxy
• Thus the IUPAC name of the molecule will be: 7-Hydroxyheptan-2-one.

Part (ii):
1. We are given the condensed formula. Let us draw the complete structural formula. It is shown in fig.12.43(b) above.
2. We want to write the IUPAC name of the molecule in fig.12.42(b). It can be done in 5 steps:
(i) First we make a list of the functional groups in the given molecule. We get: C=C and ㅡBr
(ii) Next we compare the list with table 1 above. We see that, C=C gets top priority.
• Also from table 2, we know that X (a halogen) will always be a subordinate group.
• So C=C is the principal functional group. The ㅡBr is the subordinate functional group.
(iii) Imagine that, the ㅡBr is not present. Then the IUPAC name of the molecule will be based on the C=C group.
• The component of the name obtained from the C=C will be the suffix.
• For double bonds and triple bonds, we need not worry about prefix or suffix. Both prefix and suffix can be written using 'ene' or 'yne'.
• In our present case, since there are three C atoms, we get propene.
• To show the position of the double bond, we modify 'propene' as: prop-1-ene
(Note that, the numbering is done in such a way as to give the lowest possible number to the principal functional group)
(iv) Now we have to account for the ㅡBr group.
• The component of the name obtained from the ㅡBr will be the prefix.
    ♦ When the component obtained from ㅡX is the prefix, we use ‘halo’
         ✰ ㅡF becomes fluro
         ✰ ㅡCl becomes chloro
         ✰ ㅡBr becomes bromo
         ✰ ㅡI becomes iodo
(v) Now we give the position number. The ㅡBr is at position 3. So the component will be 3-bromo
• Thus the IUPAC name of the molecule will be: 3-bromoprop-1-ene.

Solved example 12.9
Write the IUPAC names of the compounds shown in figs.12.44 (a) and (b) below:

Fig.12.44

Solution:
Part (a):
• We want to write the IUPAC name of the molecule in fig.12.43(a). It can be done in 4 steps:
(i) First we make a list of the functional groups in the given molecule. We see that there is only one functional group, which is: ㅡOH
• So the name will be based on this group.
(ii) Imagine that, the ㅡCH₃ is not present. Then the IUPAC name of the molecule will be based on the ㅡOH group.
• The component of the name obtained from the ㅡOH will be the suffix.
    ♦ When the component obtained from ㅡOH is the suffix, we use ‘-ol’  
    ♦ When the component obtained from ㅡOH is the prefix, we use ‘hydroxy-’
• Here, suffix is to be used because, ㅡOH has priority and the final name is going to be based on it.
• So the main component of the final name will be: octan-3-ol
(Note that, the numbering is done in such a way as to give the lowest possible number to the principal functional group)
(iii) Now we have to account for the ㅡCH₃ group.
We know that, it is a methyl group.
(iv) Now we give the position number. The ㅡCH₃ is at position 6. So the component will be 6-methyl
• Thus the IUPAC name of the molecule will be: 6-Methyloctan-3-ol.

Part (b):
• We want to write the IUPAC name of the molecule in fig.12.43(b). It can be done in 5 steps:
(i) First we make a list of the functional groups in the given molecule. We get: ㅡCOOH and >C=O
(ii) Next we compare the list with table 1 above. We see that, ㅡCOOH gets top priority. So it is the principal functional group. The >C=O is the subordinate functional group.
(iii) Imagine that, the >C=O is not present. Then the IUPAC name of the molecule will be based on the ㅡCOOH group.
    ♦ When the component obtained from ㅡCOOH is the suffix, we use ‘-oic acid’  
    ♦ When the component obtained from ㅡCOOH is the prefix, we use ‘carboxy’
• Here, suffix is to be used because, ㅡCOOH has priority and the final name is going to be based on it.
• So the main component of the final name will be: hexanoic acid
(Note that, the numbering is done in such a way as to give the lowest possible number to the principal functional group)
(iv) Now we have to account for the >C=O group.
• The component of the name obtained from the >C=O will be the prefix.
    ♦ When the component obtained from >C=O is the suffix, we use ‘-one’  
    ♦ When the component obtained from >C=O is the prefix, we use ‘oxo-’
(v) Now we give the position number. The >C=O is at position 5. So the component will be 5-oxo
• Thus the IUPAC name of the molecule will be: 5-Oxohexanoic acid.

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In the next section, we will see nomenclature when more than one functional group of the same type are present.


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