In the previous section, we saw the classification and nomenclature of hydrocarbons. In this section we will learn about Cyclic hydrocarbons.
The hydrocarbons that we saw so far were 'chains'. For example:
• In propane, the three carbon atoms bond together to form a chain.
• In pentene, the five carbon atoms bond together to form a chain.
Now we consider a different case.
• If the last carbon atom in the chain is connected back to the first carbon atom, it becomes a 'closed chain'. Then it will resemble a ring. So they are called cyclic or ring hydrocarbons.
• According to the IUPAC rules, the prefix 'cyclo' is used in naming them.
The fig.18.16 below shows some cyclic hydrocarbons.
Let us see each of them in detail:
1. In fig.18.16(a) we have a ring with 3 carbon atoms. So the root name is 'prop'
• All the carbon-carbon bonds are single bonds. So the suffix is 'ane'
• Thus we get: prop + ane → propane
• But since it is a cyclic hydrocarbon, the suffix 'cyclo' must be given.
• So the final name is cyclo + propane → cyclopropane
■ Let us consider the number of hydrogen atoms:
• In cyclopropane, each carbon atom is bonded to 2 hydrogen atoms. The molecular formula is C3H6
• We can see that all hydrogen and carbon atoms have attained octet
• Compare cyclopropane with open chain propane:
♦ Open chain propane has a molecular formula C3H8.
♦ So when the 'open chain' become 'cyclic', two hydrogen atoms are not required. Why is this so?
Ans: We have seen the structural formula of open chain propane. (Details here).
• There is a hydrogen on the extreme left, and another hydrogen on the extreme right.
• These two are not required when the 'open chain' becomes 'cyclic'.
• This is because, the last carbon atom cycles back to the first carbon atom. The two of them share a pair to form a single bond. Thus two hydrogen atoms are no longer required
2. In fig.b we have a ring with 4 carbon atoms. So the root name is 'but'
• All the carbon-carbon bonds are single bonds. So the suffix is 'ane'
• Thus we get: but + ane → butane
• But since it is a cyclic hydrocarbon, the suffix 'cyclo' must be given.
• So the final name is cyclo + butane → cyclobutane
■ Let us consider the number of hydrogen atoms.
• In cyclobutane, each carbon atom is bonded to 2 hydrogen atoms. The molecular formula is C4H8
• We can see that all hydrogen and carbon atoms have attained octet
• Compare cyclobutane with open chain butane:
♦ Open chain butane has a molecular formula C4H10.
♦ So when the 'open chain' become 'cyclic', two hydrogen atoms are not required. Why is this so?
• The answer is same as the one we saw above in the case of cyclopropane. The last carbon atom cycles back to the first carbon atom. The two of them share a pair to form a single bond. Thus two hydrogen atoms are no longer required.
■ So we have seen the details about cyclopropane (fig.a) and cyclobutane (fig.b). Based on them the details about cyclopentane (fig.c) and cyclohexane (fig.d) can be written. The reader may write them in his/her own notebooks.
3. Next we will see fig.e. We have a ring with 4 carbon atoms. So the root name is 'but'
• There is one carbon-carbon double bond. So the suffix is 'ene'
• Thus we get: but + ene → butene
• But since it is a cyclic hydrocarbon, the suffix 'cyclo' must be given.
• So the final name is cyclo + butene → cyclobutene
■ Let us consider the number of hydrogen atoms:
• In cyclobutene, 2 carbon atoms are bonded to 2 hydrogen atoms each. The other 2 carbon atoms are bonded to one hydrogen atom each. The molecular formula is C4H6
• We can see that all hydrogen and carbon atoms have attained octet
• Compare cyclobutene with open chain butane:
♦ Open chain butane has a molecular formula C4H10.
♦ So when the 'open chain' become 'cyclic', and also with atleast one double bond, four hydrogen atoms are not required. Why is this so?
Ans: We have seen the structural formula of butane (Details here)
• There are 3 hydrogen atoms on the extreme left, and another 3 hydrogen atoms on the extreme right.
• Out of them two are not required from either ends, when the 'open chain' becomes 'cyclic'.
• This is because, the last carbon atom cycles back to the first carbon atom. The two of them form a double bond, sharing two pairs of electrons.
• Thus, a total of four hydrogen atoms are no longer required
4. Next we will see fig.f. We have a ring with 5 carbon atoms. So the root name is 'pent'
• There is one carbon-carbon double bond. So the suffix is 'ene'
• Thus we get: pent + ene → pentene
• But since it is a cyclic hydrocarbon, the suffix 'cyclo' must be given.
• So the final name is cyclo + pentene → cyclopentene
■ Let us consider the number of hydrogen atoms.
• In cyclopentene, 2 carbon atoms are bonded to 2 hydrogen atoms each. The other 2 carbon atoms are bonded to one hydrogen atom each. The molecular formula is C5H8
• We can see that all hydrogen and carbon atoms have attained octet
• Compare cyclopentene with open chain pentane:
♦ Open chain pentane has a molecular formula C5H12
♦ So when the 'open chain' become 'cyclic', and also with atleast one double bond, four hydrogen atoms are not required. Why is this so?
• The answer is same as the one we saw above in the case of cyclobutene. The last carbon atom cycles back to the first carbon atom. The two of them share two pairs to form a double bond. Thus four hydrogen atoms are no longer required.
■ From the above discussion, it is clear that, a minimum of two carbon atoms is required to form a cyclic hydrocarbon.
♦ 2 less than the corresponding alkane
• If the cyclic hydrocarbon has one carbon-carbon bonds as double bond:
♦ 4 less than the corresponding alkane
♦ 2 less than the corresponding alkene
Solution:
(i). A series:
1. When the the number of carbon atoms is 3, number of hydrogen atoms is 8
2. When the the number of carbon atoms is 4, number of hydrogen atoms is 10
3. So it is clear that, the general formula for this series is CnH2n+2.
4. The missing compounds are those with number of carbon atoms (n) equal to 2 and 5.
5. The number of hydrogen atoms when n = 2 is ( 2 × 2 + 2) = 6. So the compound is C2H6.
6. The number of hydrogen atoms when n = 5 is ( 2 × 5 + 2) = 12. So the compound is C5H12.
(ii). B series:
1. When the the number of carbon atoms is 3, number of hydrogen atoms is 6
2. When the the number of carbon atoms is 6, number of hydrogen atoms is 12
3. So it is clear that, the general formula for this series is CnH2n.
4. The missing compounds are those with number of carbon atoms (n) equal to 4, 5 and 7.
5. The number of hydrogen atoms when n = 4 is ( 2 × 4) = 8. So the compound is C4H8.
6. The number of hydrogen atoms when n = 5 is ( 2 × 5) = 10. So the compound is C5H10.
7. The number of hydrogen atoms when n = 7 is ( 2 × 7) = 14. So the compound is C7H14.
(iii). C series:
1. When the the number of carbon atoms is 3, number of hydrogen atoms is 4
2. When the the number of carbon atoms is 4, number of hydrogen atoms is 6
3. So it is clear that, the general formula for this series is CnH2n-2.
4. The missing compounds are those with number of carbon atoms (n) equal to 2, 5 and 6.
5. The number of hydrogen atoms when n = 2 is ( 2 × 2 - 2) = 2. So the compound is C2H2.
6. The number of hydrogen atoms when n = 5 is ( 2 × 5 - 2) = 8. So the compound is C5H8.
7. The number of hydrogen atoms when n = 6 is ( 2 × 6 - 2) = 10. So the compound is C6H10.
The completed table is given below:
Note:
• For the A series we found that general formula is CnH2n+2. Also successive compounds differ by CH2. So it is the Alkane series
• For the B series we found that general formula is CnH2n. Also successive compounds differ by CH2. So it is the Alkene series
• For the C series we found that general formula is CnH2n-2. Also successive compounds differ by CH2. So it is the Alkyne series
Solved example 8.4
Given below are the chemical formulae of some hydrocarbons:
C3H6, C2H6, C3H4
(a) Represent their structure
(b) Based on the structure, classify them into alkane, alkene and alkyne.
Solution:
Part (a):
1. C3H6 : We have discussed about the structure of this hydrocarbon in detail here.
2. C2H6 : We have discussed about the structure of this hydrocarbon in detail here.
3. C3H4 : We have discussed about the structure of this hydrocarbon in detail here.
Part (b):
1. There is one double bond in C3H6. So it is an alkene
2. All the carbon-carbon bonds in C2H6 are single bonds. So it is an alkane
3. There is one triple bond in C3H4. So it is an alkyne
In the next section, we will learn about position of atoms in a three dimensional space.
The hydrocarbons that we saw so far were 'chains'. For example:
• In propane, the three carbon atoms bond together to form a chain.
• In pentene, the five carbon atoms bond together to form a chain.
Now we consider a different case.
• If the last carbon atom in the chain is connected back to the first carbon atom, it becomes a 'closed chain'. Then it will resemble a ring. So they are called cyclic or ring hydrocarbons.
• According to the IUPAC rules, the prefix 'cyclo' is used in naming them.
The fig.18.16 below shows some cyclic hydrocarbons.
 |
| Fig.8.16 |
1. In fig.18.16(a) we have a ring with 3 carbon atoms. So the root name is 'prop'
• All the carbon-carbon bonds are single bonds. So the suffix is 'ane'
• Thus we get: prop + ane → propane
• But since it is a cyclic hydrocarbon, the suffix 'cyclo' must be given.
• So the final name is cyclo + propane → cyclopropane
■ Let us consider the number of hydrogen atoms:
• In cyclopropane, each carbon atom is bonded to 2 hydrogen atoms. The molecular formula is C3H6
• We can see that all hydrogen and carbon atoms have attained octet
• Compare cyclopropane with open chain propane:
♦ Open chain propane has a molecular formula C3H8.
♦ So when the 'open chain' become 'cyclic', two hydrogen atoms are not required. Why is this so?
Ans: We have seen the structural formula of open chain propane. (Details here).
• There is a hydrogen on the extreme left, and another hydrogen on the extreme right.
• These two are not required when the 'open chain' becomes 'cyclic'.
• This is because, the last carbon atom cycles back to the first carbon atom. The two of them share a pair to form a single bond. Thus two hydrogen atoms are no longer required
2. In fig.b we have a ring with 4 carbon atoms. So the root name is 'but'
• All the carbon-carbon bonds are single bonds. So the suffix is 'ane'
• Thus we get: but + ane → butane
• But since it is a cyclic hydrocarbon, the suffix 'cyclo' must be given.
• So the final name is cyclo + butane → cyclobutane
■ Let us consider the number of hydrogen atoms.
• In cyclobutane, each carbon atom is bonded to 2 hydrogen atoms. The molecular formula is C4H8
• We can see that all hydrogen and carbon atoms have attained octet
• Compare cyclobutane with open chain butane:
♦ Open chain butane has a molecular formula C4H10.
♦ So when the 'open chain' become 'cyclic', two hydrogen atoms are not required. Why is this so?
• The answer is same as the one we saw above in the case of cyclopropane. The last carbon atom cycles back to the first carbon atom. The two of them share a pair to form a single bond. Thus two hydrogen atoms are no longer required.
■ So we have seen the details about cyclopropane (fig.a) and cyclobutane (fig.b). Based on them the details about cyclopentane (fig.c) and cyclohexane (fig.d) can be written. The reader may write them in his/her own notebooks.
3. Next we will see fig.e. We have a ring with 4 carbon atoms. So the root name is 'but'
• There is one carbon-carbon double bond. So the suffix is 'ene'
• Thus we get: but + ene → butene
• But since it is a cyclic hydrocarbon, the suffix 'cyclo' must be given.
• So the final name is cyclo + butene → cyclobutene
■ Let us consider the number of hydrogen atoms:
• In cyclobutene, 2 carbon atoms are bonded to 2 hydrogen atoms each. The other 2 carbon atoms are bonded to one hydrogen atom each. The molecular formula is C4H6
• We can see that all hydrogen and carbon atoms have attained octet
• Compare cyclobutene with open chain butane:
♦ Open chain butane has a molecular formula C4H10.
♦ So when the 'open chain' become 'cyclic', and also with atleast one double bond, four hydrogen atoms are not required. Why is this so?
Ans: We have seen the structural formula of butane (Details here)
• There are 3 hydrogen atoms on the extreme left, and another 3 hydrogen atoms on the extreme right.
• Out of them two are not required from either ends, when the 'open chain' becomes 'cyclic'.
• This is because, the last carbon atom cycles back to the first carbon atom. The two of them form a double bond, sharing two pairs of electrons.
• Thus, a total of four hydrogen atoms are no longer required
4. Next we will see fig.f. We have a ring with 5 carbon atoms. So the root name is 'pent'
• There is one carbon-carbon double bond. So the suffix is 'ene'
• Thus we get: pent + ene → pentene
• But since it is a cyclic hydrocarbon, the suffix 'cyclo' must be given.
• So the final name is cyclo + pentene → cyclopentene
■ Let us consider the number of hydrogen atoms.
• In cyclopentene, 2 carbon atoms are bonded to 2 hydrogen atoms each. The other 2 carbon atoms are bonded to one hydrogen atom each. The molecular formula is C5H8
• We can see that all hydrogen and carbon atoms have attained octet
• Compare cyclopentene with open chain pentane:
♦ Open chain pentane has a molecular formula C5H12
♦ So when the 'open chain' become 'cyclic', and also with atleast one double bond, four hydrogen atoms are not required. Why is this so?
• The answer is same as the one we saw above in the case of cyclobutene. The last carbon atom cycles back to the first carbon atom. The two of them share two pairs to form a double bond. Thus four hydrogen atoms are no longer required.
■ In general we can say this about the number of hydrogen atoms in a cyclic hydrocarbon:
• If the cyclic hydrocarbon has all the carbon-carbon bonds as single bonds:♦ 2 less than the corresponding alkane
• If the cyclic hydrocarbon has one carbon-carbon bonds as double bond:
♦ 4 less than the corresponding alkane
♦ 2 less than the corresponding alkene
So we have completed the discussion on cyclic hydrocarbons. We will now see some solved examples based on the discussions that we had so far in this chapter.
Solved example 8.3
Write the chemical formula of the compounds that are missing in the homologous series given below:
(i). A series:
1. When the the number of carbon atoms is 3, number of hydrogen atoms is 8
2. When the the number of carbon atoms is 4, number of hydrogen atoms is 10
3. So it is clear that, the general formula for this series is CnH2n+2.
4. The missing compounds are those with number of carbon atoms (n) equal to 2 and 5.
5. The number of hydrogen atoms when n = 2 is ( 2 × 2 + 2) = 6. So the compound is C2H6.
6. The number of hydrogen atoms when n = 5 is ( 2 × 5 + 2) = 12. So the compound is C5H12.
(ii). B series:
1. When the the number of carbon atoms is 3, number of hydrogen atoms is 6
2. When the the number of carbon atoms is 6, number of hydrogen atoms is 12
3. So it is clear that, the general formula for this series is CnH2n.
4. The missing compounds are those with number of carbon atoms (n) equal to 4, 5 and 7.
5. The number of hydrogen atoms when n = 4 is ( 2 × 4) = 8. So the compound is C4H8.
6. The number of hydrogen atoms when n = 5 is ( 2 × 5) = 10. So the compound is C5H10.
7. The number of hydrogen atoms when n = 7 is ( 2 × 7) = 14. So the compound is C7H14.
(iii). C series:
1. When the the number of carbon atoms is 3, number of hydrogen atoms is 4
2. When the the number of carbon atoms is 4, number of hydrogen atoms is 6
3. So it is clear that, the general formula for this series is CnH2n-2.
4. The missing compounds are those with number of carbon atoms (n) equal to 2, 5 and 6.
5. The number of hydrogen atoms when n = 2 is ( 2 × 2 - 2) = 2. So the compound is C2H2.
6. The number of hydrogen atoms when n = 5 is ( 2 × 5 - 2) = 8. So the compound is C5H8.
7. The number of hydrogen atoms when n = 6 is ( 2 × 6 - 2) = 10. So the compound is C6H10.
The completed table is given below:
• For the A series we found that general formula is CnH2n+2. Also successive compounds differ by CH2. So it is the Alkane series
• For the B series we found that general formula is CnH2n. Also successive compounds differ by CH2. So it is the Alkene series
• For the C series we found that general formula is CnH2n-2. Also successive compounds differ by CH2. So it is the Alkyne series
Solved example 8.4
Given below are the chemical formulae of some hydrocarbons:
C3H6, C2H6, C3H4
(a) Represent their structure
(b) Based on the structure, classify them into alkane, alkene and alkyne.
Solution:
Part (a):
1. C3H6 : We have discussed about the structure of this hydrocarbon in detail here.
2. C2H6 : We have discussed about the structure of this hydrocarbon in detail here.
3. C3H4 : We have discussed about the structure of this hydrocarbon in detail here.
Part (b):
1. There is one double bond in C3H6. So it is an alkene
2. All the carbon-carbon bonds in C2H6 are single bonds. So it is an alkane
3. There is one triple bond in C3H4. So it is an alkyne
In the next section, we will learn about position of atoms in a three dimensional space.
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