Homologous Series Chemistry Tutorial

Key Concepts

• In organic chemistry (carbon chemistry), a homologous series refers to a family of compounds that:

  (i) have the same functional group

  (ii) differ by a constant unit as you proceed from one member to another

• A homologous series can be described by a general formula.

  Some Names of Homologous Series and the Corresponding General Formula
  Homologous series	alkanes	alkenes	alkynes	alkanols	alkanals	alkanones	alkanoic acid
  General formula	CnH2n+2	CnH2n	CnH2n−2	CnH2n+2O	CnH2nO	CnH2nO	CnH2nO2

• Each compound in the homologous series (a member) is referred to as a homologue (sometimes spelt homolog).
• The homologues in a homologous series undergo similar chemical reactions because they have the same functional group.
• There is a gradation in the physical properties of the homologues of the series due to the increasing length of the carbon chain.

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Alkane Homologous Series

The alkane homologous series contains compounds that:

• are composed of ONLY carbon (C) and hydrogen (H) atoms
• have single covalent bonds between all the carbon atoms (C−C)
• contain no functional group

Since one carbon will make a total of 4 covalent bonds, the first member (homologue) of the alkane homologous series must be 1 carbon atom bonded to 4 hydrogen atoms as shown below:

which has the molecular formula CH₄.

The next homologue in the alkane homologous series will contain 2 carbon atoms with a single covalent bond between them, each carbon atom will make up its quota of 4 covalent bonds by bonding to 3 hydrogen atoms as shown below:

and this molecule has the molecular formula C₂H₆.

The next homologue in the series will have 3 carbon atoms and 8 hydrogen atoms (C₃H₈), followed by 4 carbon atoms and 10 hydrogen atoms (C₄H₁₀), and so on and so forth...

Notice a pattern? For each successive homologue we add CH₂ to the chain.

Start with CH₄ (the first homologue)

then add CH₂ to make C₂H₆ (the next homologue in the series)

then add CH₂ to make C₃H₈ (the next homologue in the series)

then add CH₂ to make C₄H₁₀ (the next homologue in the series)

then add CH₂ to make C₅H₁₂ (the next homologue in the series)

Which means the difference in the molar mass between adjacent homologues in the series is 14 g mol^-1:

C has a molar mass of 12 g mol^-1
H has a molar mass of 1 g mol^-1
Therefore CH₂ has a molar mass of 12 + (2 × 1) = 12 + 2 = 14 g mol^-1

So, each member of the alkane series, each homologue, contains a chain that is n lots of carbon atoms long, C_n.

Each homologue will also contain 2 × n hydrogen atoms (from each CH₂ unit), plus there is an extra hydrogen atom at each end of the chain that is, an extra 2 H atoms, so the total number of hydrogen atoms in the chain is 2n+2, H_2n+2.

Putting it all together allows us to write a general formula for the homologous series of alkanes: C_nH_2n+2

For the first member of the alkane series (the first homologue), n=1 so the molecular formula will be C₁H_2×1+2 which is CH₄

For the second member of the alkane series (the second homologue), n=2 so the molecular formula will be C₂H_2×2+2 which is C₂H₆

Click to investigate the properties of alkanes.

Alkene Homologous Series

Alkene homologous series contains compounds that:

• are composed of ONLY carbon (C) and hydrogen (H) atoms
• have a double bond between two of the carbon atoms (C=C)
• contain no other functional group

The first member, the first homologue, of the alkene homologous series must contain 2 carbon atoms which are joined by a double bond, and, because each carbon atom must make 4 bonds in total, each of the carbon atoms must be bonded to 2 hydrogen atoms as shown below:

For the next homologue we add 1 more C atom, and it will need to make 4 bonds in total as shown in the structure below:

For the next homologue we add 1 more C atom, and it will need to make 4 bonds in total as shown in the structure below:

For the next homologue we add 1 more C atom, and it will need to make 4 bonds in total as shown in the structure below:

So we can see that we are adding a CH₂ unit to one homologue to make the next one in the series.
Since each CH₂ unit has a molar mass of 14 g mol^-1, the difference in molar mass between adjacent homologues is 14 g mol^-1.

And we can describe the homologous series of alkenes by a general formula, C_nH_2n.

For the first member of the series (the first homologue), n=2, so the molecular formula for this compound will be C₂H_2×2 which is C₂H₄

For the second member of the series (the second homologue), n=3, so the molecular formula for this compound will be C₃H_2×3 which is C₃H₆

Click to investigate the properties of alkenes.

Alkyne Homologous Series

Alkyne homologous series contains compounds that:

• are composed of ONLY carbon (C) and hydrogen (H) atoms
• have a triple bond between two of the carbon atoms (C≡C)
• contain no other functional group

The first member, the first homologue, of the alkyne homologous series must contain 2 carbon atoms which are joined by a triple bond, and, because each carbon atom must make 4 bonds in total, each of the carbon atoms must be bonded to 1 hydrogen atom as shown below:

The next member in the series, the next homologue, must contain 3 carbon atoms, and each C atom must make 4 bonds in total:

Which is followed by the next homologue with 4 carbon atoms:

So once again we can see that we are adding a CH₂ unit to one homologue to make the next one in the series.
Since each CH₂ unit has a molar mass of 14 g mol^-1, the difference in molar mass between adjacent homologues is 14 g mol^-1.

And we can describe the homologous series of alkynes by a general formula, C_nH_2n−2.

For the first member of the series (the first homologue), n=2, so the molecular formula for this compound will be C₂H_2×2−2 which is C₂H₂

For the second member of the series (the second homologue), n=3, so the molecular formula for this compound will be C₃H_2×3−2 which is C₃H₄

Alkanol Homologous Series

Alkanol homologous series contains compounds that:

• are composed of ONLY carbon (C), hydrogen (H) and oxygen (O) atoms
• have only single bonds between the carbon atoms (C−C)
• contain the hydroxyl functional group (OH)

The first member of the alkanol homologous series contains just 1 carbon atom which needs to make 4 bonds in total, one of these bonds must be to the OH functional group, so the other 3 bonds must be to hydrogen atoms (H), as shown below:

The next homologue contains 2 carbon atoms, each carbon atom must make a total of 4 bonds, one of these bonds is to the OH functional group:

The next member of the alkanol homologous series has 3 carbon atoms, all of which must make 4 bonds, and one of those bonds must be to the OH functional group:

The difference between the molecular formulae of adjacent homologues is -CH₂, which has a molar mass of 14 g mol^-1, so the difference in molar mass of adjacent homologues is 14 g mol^-1.

The general formula for members of the alkanol homologous series is C_nH_2n+2O

The first member of the alkanol homologous series has n=1, so the molecular formula of the first homologue is C₁H_2×1+2O, that is, CH₄O

The second member of the alkanol homologous series has n=2, so the molecular formula of the second homologue is C₂H_2×2+2O, that is, C₂H₆O

Click to investigate the properties of alkanols.

Alkanal Homologous Series

Alkanal homologous series contains compounds that:

• are composed of ONLY carbon (C), hydrogen (H) and oxygen (O) atoms
• have only single bonds between the carbon atoms (C−C)
• contain the carbonyl functional group (C=O) on a terminal carbon atom of the chain (end of the carbon chain, or the first carbon atom of the chain)

The first member (homologue) of the alkanal homologous series contains only 1 carbon atom, which must make 4 bonds in total and 2 of those bonds are to an oxygen atom, as shown below:

The second member (homologue) of the alkanal homologous series contains 2 carbon atoms, each of which must make 4 bonds in total, and the end carbon atom must make 2 bonds to an oxygen atom, as shown below:

The third member (homologue) of the alkanal homologous series contains 3 carbon atoms, each of which must make 4 bonds in total, and the end carbon atom must make 2 bonds to an oxygen atom, as shown below:

The difference between the molecular formulae of adjacent homologues is -CH₂, which has a molar mass of 14 g mol^-1, so the difference in molar mass of adjacent homologues is 14 g mol^-1.

The general formula for members of the alkanal homologous series is C_nH_2nO

The first member of the alkanal homologous series has n=1, so the molecular formula of the first homologue is C₁H_2×1O, that is, CH₂O

The second member of the alkanal homologous series has n=2, so the molecular formula of the second homologue is C₂H_2×2O, that is, C₂H₄O

Click to investigate the properties of alkanals.

Alkanone Homologous Series

Alkanone homologous series contains compounds that:

• are composed of ONLY carbon (C), hydrogen (H) and oxygen (O) atoms
• have only single bonds between the carbon atoms (C−C)
• contain the carbonyl functional group (C=O) on a non-terminal carbon atom of the chain (not at the end of the carbon chain, or not the first carbon atom of the chain)

The first member (homologue) of the alkanone homologous series contains 3 carbon atoms, each of which must make 4 bonds in total, and the carbon atom in the middle of the chain must make 2 bonds to an oxygen atom, as shown below:

The next homologue contains 4 carbon atoms, each of which must make 4 bonds in total, and the carbon atom in the middle of the chain must make 2 bonds to an oxygen atom, as shown below:

The next homologue contains 5 carbon atoms, each of which must make 4 bonds in total, and the carbon atom in the middle of the chain must make 2 bonds to an oxygen atom, as shown below:

The difference between the molecular formulae of adjacent homologues is -CH₂, which has a molar mass of 14 g mol^-1, so the difference in molar mass of adjacent homologues is 14 g mol^-1.

The general formula for members of the alkanone homologous series is C_nH_2nO

The first member of the alkanone homologous series has n=3, so the molecular formula of the first homologue is C₃H_2×3O, that is, C₃H₆O

The second member of the alkanone homologous series has n=4, so the molecular formula of the second homologue is C₄H_2×4O, that is, C₄H₈O

Click to investigate the properties of alkanones.

Alkanoic Acids Homologous Series

Alkanoic acid homologous series contains compounds that:

• are composed of ONLY carbon (C), hydrogen (H) and oxygen (O) atoms
• have only single bonds between the carbon atoms (C−C)
• contain the carboxyl functional group (COOH) on a terminal carbon atom of the chain (at the end of the carbon chain, or the first carbon atom of the chain)

The first member (homologue) of the alkanoic acid homologous series contains only 1 carbon atom, which must make 4 bonds in total and 2 of those bonds are to an oxygen atom, one of those bonds is to an OH group, as shown below:

The second member (homologue) of the alkanoic acid homologous series contains 2 carbon atoms, which must make 4 bonds in total, the first carbon atom must have 2 of those bonds to an oxygen atom and 1 of those bonds is to an OH group, as shown below:

The third member (homologue) of the alkanoic acid homologous series contains 3 carbon atoms, which must make 4 bonds in total, the first carbon atom must have 2 of those bonds to an oxygen atom and 1 of those bonds is to an OH group, as shown below:

The difference between the molecular formulae of adjacent homologues is -CH₂, which has a molar mass of 14 g mol^-1, so the difference in molar mass of adjacent homologues is 14 g mol^-1.

The general formula for members of the alkanoic acid homologous series is C_nH_2nO₂

The first member of the alkanoic acid homologous series has n=1, so the molecular formula of the first homologue is C₁H_2×1O₂, that is, CH₂O₂

The second member of the alkanoic acid homologous series has n=2, so the molecular formula of the second homologue is C₂H_2×2O₂, that is, C₂H₄O₂

Click to investigate the properties of alkanoic acids.