In the previous section, we saw how the compounds with functional groups are named. In this section, we will see the nomenclature of substituted benzene compounds.
Let us first see some basics about benzene. It can be written in 3 steps:
1. Fig,12.52(a) below shows our familiar hexagonal ring.
 |
| Fig.12.52 |
• We know that, it is the bond-line formula of cyclohexane. We know how the C atoms are arranged in that ring. It is shown again in figs (b) and (c)
• Fig.b shows the condensed formula and fig.c shows the complete structural formula.
• From the complete structural formula, we can see how the valencies of C atoms are satisfied.
2. We know that, all bonds in the cyclohexane above are single bonds.
• Now, if alternate bonds are double bonds, there will be lesser number of H atoms. This is shown in fig.12.53 below:
 |
| Fig.12.53 |
• Fig.12.53(a) shows the bond line formula. Fig.b shows the condensed formula.
• Fig.c shows the complete structural formula. From the complete structural formula, we can see how the valencies of all the C atoms are satisfied.
◼ The compound in fig.12.53 is known as benzene.
3. Benzene has exactly half the number of H atoms that are present in cyclohexane.
♦ The molecular formula of cyclohexane is C6H12
♦ The molecular formula of benzene is C6H6
A large number of organic compounds can be derived from benzene. So we must know how to name them systematically based on the IUPAC rules. We can learn those rules by analyzing some examples.
Example 1
This can be written in 4 steps:
1. Fig.12.54(a) below shows a compound in which, one H atom of benzene is replaced by a methyl group.
 |
| Fig.12.54 |
2. In such cases, the benzene is given importance and so it becomes the suffix in the name.
• We can write the name as: Methylbenzene.
3. Note that, the position number of the methyl group is not necessary. The methyl group can be anywhere along the ring. Whatever be the position, the compound will not change.
• This can be explained using the animation in fig.12.55 below.
 |
| Fig.12.55 |
• It can be written in 5 steps:
(i) Positions of the double bond:
• Consider the left side molecule in the animation. The top double bond is on the left side of the branch.
• Consider the middle molecule in the animation. The top double bond is on the right side of the branch.
(ii) So the two molecules appear to be different.
(iii) Now consider the right side molecule. Initially, it appears to be same as the left side molecule. But when it is rotated through 180o, it becomes same as the middle molecule.
(iv) So we can write:
When there is only one branch, the position of the branch is not important.
4. Methylbenzene in fig.12.54(a) has many industrial applications. It’s common name is Toleune. This name is widely accepted.
Example 2
This can be written in 5 steps:
1. Fig.12.54(b) above shows a compound in which, one H atom of
benzene is replaced by a methoxy group.
2. As before, the benzene is
given importance and so it becomes the suffix in the name.
• We can write
the name as: Methoxybenzene.
• Note that, the position number of the methoxy
group is not necessary. The group can be anywhere along the
ring. Whatever be the position, the compound will not change. We saw this in the animation in fig.12.55 above.
3. Note how the valency of the O atom is satisfied. The two bonds around the O indicates that, it has obtained the required two new electrons.
4. The name 'methyl' can be abbreviated.
• IUPAC allows the following abbreviations:
♦ Methyl can be abbreviated as Me
♦ Ethyl can be abbreviated as Et
♦ Propyl can be abbreviated as Pr
♦ Butyl can be abbreviated as Bu
• So our present Methoxybenzene can be shown as in fig.12.54(c) also
5. Methoxybenzene has many industrial applications. It’s common name is Anisole. This name is widely accepted.
Example 3
This can be written in 4 steps:
1. Fig.12.56(a) below shows a compound in which, one H atom of benzene is replaced by a ㅡNH2 group.
 |
| Fig.12.56 |
2. We have seen that, the benzene is to be given importance and so it becomes the suffix in the name.
• We can write the name as: Aminobenzene.
• Recall that:
♦ When ㅡNH2 group is the suffix, we use 'amine'
♦ When ㅡNH2 group is the prefix, we use 'amino'
3.
Here also, the position number of the ㅡNH2 group is not necessary.
4. Aminobenzene has many industrial applications. It’s common name is Aniline. This name is widely accepted.
Example 4
This can be written in 4 steps:
1. Fig.12.56(b) above shows a compound in which, one H atom of benzene is replaced by a ㅡOH group.
2. We have seen that, the benzene is to be given importance and so it becomes the suffix in the name.
• We can write the name as: Hydroxybenzene.
• Recall that:
♦ When ㅡOH group is the suffix, we use 'ol'
♦ When ㅡOH group is the prefix, we use 'hydroxy'
3.
Here also, the position number of the ㅡOH group is not necessary.
4. Hydroxybenzene has many industrial applications. It’s common name is Phenol. This name is widely accepted.
◼ Similar to the above four examples, we can explain Nitrobenzene and Bromobenzene also. They are shown in fig.12.56(c) and (d) above.
• When there is more than one substituent, numbering must be done in such a way that, the substituents gets the lowest possible numbers.
• Let us see an example:
♦ In figs.12.57(a) and (b) below, the correct numbering is the one in fig.a.
✰ Based on this numbering, the name will be 1,2-dibromobenzene.
♦ Based on the numbering fig.b, the name will be 1,6-dibromobenzene.
✰ This name is not valid.
 |
| Fig.12.57 |
• Let us see another example:
♦ In figs.12.57(c) and (d) above, the correct numbering is the one in fig.c.
✰ Based on this numbering, the name will be 1,3-dibromobenzene.
♦ Based on the numbering fig.d, the name will be 1,5-dibromobenzene.
✰ This name is not valid.
• When there are two ㅡBr functional groups in the benzene ring, only three arrangements are possible. So it is convenient to give three distinct names. This can be explained in 5 steps:
1. The three possible arrangements are shown in fig.12.58 below:
 |
| Fig.12.58 |
2. In fig.12.58(a), the functional groups are at positions 1,2
• The two functional groups are on adjacent C atoms.
• This arrangement is indicated by the word 'ortho'
• So the trivial name of 1,2-dibromobenzene is: ortho-dibromobenzene
♦ This can be further abbreviated as: o-dibromobenzene.
3. In fig.12.58(b), the functional groups are at positions 1,3
• There is one C atom between the two functional groups.
• This arrangement is indicated by the word 'meta'
• So the trivial name of 1,3-dibromobenzene is: meta-dibromobenzene
♦ This can be further abbreviated as: m-dibromobenzene.
4. In fig.12.58(c), the functional groups are at positions 1,4
• There are two C atoms between the two functional groups.
• This arrangement is indicated by the word 'para'
• So the trivial name of 1,4-dibromobenzene is: para-dibromobenzene
♦ This can be further abbreviated as: p-dibromobenzene.
5. The above trivial names are widely accepted.
When there are more functional groups in the benzene ring, we will have to strictly follow the IUPAC rules. The steps can be explained with the help of some examples:
Example 1:
This can be written in 6 steps:
1. Fig.12.59(a) below shows a compound in which, one H atom of benzene is replaced by a ㅡOMe group.
• We know that, this compound is called anisole.
 |
| Fig.12.59 |
2. But besides OMe, two more groups are present: ㅡCl and ㅡCH3.
• To write the IUPAC name, we have to assign proper locant numbers to these functional groups.
3. We take anisole as the base compound.
• The functional group which converts the benzene into anisole is given the number 1.
• So ㅡOMe gets the number 1
4. Once the number 1 is assigned, we can give numbers to the other functional groups. The direction of numbering is chosen in such a way that, the group closest to ㅡOMe gets the smallest number.
• In our present case, the group closest to ㅡOMe is ㅡCl.
5. So we number the atoms in a clockwise direction. This is shown in fig.a.
• The ㅡCl gets the number 2
• If the numbering is in the anti-clockwise direction, Cl will get number 6. This is not acceptable.
6. So we have the required locant numbers:
♦ ㅡOMe at 1
♦ ㅡCl at 2
♦ ㅡCH3 at 4
• Number 1 for OMe need not be written because, it is the group which converts benzene into anisole. It is understood that, OMe will be at position 1 in anisole.
• We have to write the other two groups and it's numbers. They must be written in alphabetical order. So 'chloro' comes before 'methyl'
• Thus the name of the compound is: 2-chloro-4-methylanisole.
Example 2:
This can be written in 6 steps:
1. Fig.12.59(b) above shows a compound in which, one H atom of benzene is replaced by a ㅡNH2 group.
• We know that, this compound is called aniline.
2. But besides ㅡNH2, two more groups are present: CH3 and C2H5.
• To write the IUPAC name, we have to assign proper locant numbers to these functional groups.
3. We take aniline as the base compound.
• The functional group which changes the benzene into aniline is given the number 1.
• So ㅡNH2 gets the number 1
4.
Once the number 1 is assigned, we can give numbers to the other
functional groups. The direction of numbering is chosen in such a way
that, the group closest to NH2 gets the smallest number.
• In our present case, the group closest to NH2 is CH3.
5. So we number the atoms in a clockwise direction. This is shown in fig.b. The CH3 gets the number 2
• If the numbering is in the anti-clockwise direction, CH3 will get number 6. This is not acceptable.
6. So we have the required locant numbers:
♦ ㅡNH2 at 1
♦ ㅡCH3 at 2
♦ ㅡC2H5 at 4
•
Number 1 for NH2 need not be written because, it is the group which
converts benzene into aniline. It is understood that, NH2 will be at
position 1 in aniline.
• We have to write the other two groups and
it's numbers. They must be written in alphabetical order. So 'ethyl'
comes before 'methyl'
• Thus the name of the compound is: 4-Ethyl-2-methylaniline
Example 3:
This can be written in 6 steps:
1. Fig.12.59(c) above shows a compound in which, one H atom of benzene is replaced by a ㅡOH group.
We know that, this compound is called phenol.
2. But besides ㅡOH, two more groups are present: CH3 and CH3.
• To write the IUPAC name, we have to assign proper locant numbers to these functional groups.
3. We take phenol as the base compound.
The functional group which converts the benzene into phenol is given the number 1.
So ㅡOH gets the number 1
4.
Once the number 1 is assigned, we can give numbers to the other
functional groups. The direction of numbering is chosen in such a way
that, the group closest to OH gets the smallest number.
• In our present case, the group closest to OH is the right side CH3.
5. So we number the atoms in a clockwise direction. This is shown in fig.c. The CH3 gets the number 3
• If the numbering is in the anti-clockwise direction, this CH3 will get number 5. This is not acceptable.
6. So we have the required locant numbers:
♦ ㅡOH at 1
♦ ㅡCH3 at 3
♦ ㅡCH3 at 4
•
Number 1 for OH need not be written because, it is the group which
converts benzene into phenol. It is understood that, OH will be at
position 1 in phenol.
• We have to write the other two groups and
it's numbers.
• Thus the name of the compound is: 3,4-Dimethylphenol.
• In the above discussion, benzene was considered as the parent.
• Groups like , ㅡOH, ㅡCl, ㅡCH3, ㅡCH3CH2 etc., were attached to the benzene ring.
♦ These groups are called substituents.
♦ An example is shown in fig.12.60(a) below.
♦ In this example, a propyl group is the substituent.
 |
| Fig.12.60 |
• In some cases, the benzene ring becomes the substituent.
♦ It will be attached to a parent.
• This can be explained in steps:
1. We know that, benzene is C6H6
• One H atom from the ring is removed so that, it becomes ㅡC6H5
♦ This is called phenyl group.
2. The phenyl group gets attached to another parent chain. An example is shown in fig.12.60(b) above.
• The compound in fig.b appears to be similar to the compound in fig.a.
• Then why is it that,
♦ Benzene ring in fig.a is the parent ?
♦ Benzene ring in fig.b is a substituent ?
3. The answer is:
If the branch attached to benzene has more than six C atoms, then that branch is considered as the parent and benzene is considered as the substituent.
4. There are other occasions also, where benzene becomes the substituent:
(i) If the branch of the benzene contains one or more double bonds.
(ii) If the branch of the benzene contains one or more triple bonds.
(iii) If the branch of the benzene contains one or more functional groups.
• In these three cases, the benzene will become the substituent even if the number of C atoms in the branch is less than 6. Figs (c) and (d) are examples.
Let us see some solved examples:
Solved example 12.12
Write the structural formula of:
(a) o-Ethylanisole (b) p-Nitro-aniline (c) 2,3-Dibromo-1-phenylpentane (d) 4-Ethyl-1-fluoro-2-nitro-benzene.
Solution:
Part (a): o-Ethylanisole
1. 'anisole' indicates that, it is a benzene ring with OMe group as substituent.
2. Ethylanisole indicates that, an ethyl group is also present as a substituent
3. 'o' stands for 'ortho'. So the OMe and ethyl groups are in adjacent positions.
4. Thus we get the structure in fig.12.61(a) below:
 |
| Fig.12.61 |
Part (b): p-Nitro-aniline
1. 'aniline' indicates that, it is a benzene ring with NH2 group as substituent.
2. Nitro-aniline indicates that, a NO2 group is also present as a substituent
3. 'p' stands for 'para'. So the NH2 and NO2 groups have two C atoms in between them.
4. Thus we get the structure in fig.12.61(b) above.
Part (c): 2,3-Dibromo-1-phenylpentane
1. 'pentane' indicates that, a straight chain with five C atoms is the parent.
2. '1-phenylpentane' indicates that, a phenyl group is attached to the first C atom of the parent chain.
3. '2,3-Dibromo' indicates that, there are two Br groups, one at position 2 and the other at position 3
4. Thus we get the structure in fig.12.61(c) above.
Part (d): 4-Ethyl-1-fluoro-2-nitro-benzene
1. 'benzene' indicates that, it is the base.
2. '4-Ethyl-1-fluoro-2-nitro' indicates that, ethyl, fluoro and nitro groups are attached at positions 4, 1 and 2 respectively.
3. Thus we get the structure in fig.12.61(d) above.
In the next section, we
will see isomerism.
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