Lesson Explainer: Homologous Series Chemistry

In this explainer, we will learn how to identify a homologous group of organic chemicals.

All organic compounds belong to certain families of compounds. The family that a group of compounds belongs to depends on the structure and properties of those compounds.

Let us investigate the following series of compounds in the table, which are from the family of alkanes, to see what similarities and differences there are.

Methane has one carbon atom and four hydrogen atoms. Its formula is CH4. The next compound in this family series is ethane, which has one more carbon atom and two more hydrogen atoms. Ethane’s formula is CH26. So, if we take methane and add one C atom and two H atoms, we get ethane CH26. Methane and ethane only differ by a CH2 unit.

If we now compare ethane and the next compound in the alkane family, propane, we get the same result. Ethane has two carbon atoms and six hydrogen atoms. Its formula is CH26. The next compound in the alkane family series is propane. Propane has one more carbon atom and two more hydrogen atoms. Propane’s formula is CH38.

So, going from ethane to propane, we have essentially added one C atom and two H atoms.

Notice that each successive compound differs from the previous one by only a CH2 unit. In other words, each compound is the same as the previous compound, except that it has an extra CH2 unit. This is a pattern in this series or group of compounds. We say methane, ethane, and propane are homologs.

Definition: Homologs

Homologs are compounds that differ from each other by a repeating unit.

We call this entire group of straight-chain alkanes the alkane homologous series. A homologous series is a family of compounds that have the same functional group, similar chemical properties, and the same general formula. The specific formula of each compound in the series will differ from the next one by a simple structural unit.

Definition: Homologous Series

A homologous series is a family of compounds that have the same functional group and thus similar chemical properties and the same general formula, with each compound in the series differing from the next by a simple structural unit.

All the compounds in the alkane homologous series have the same general formula CH+, where 𝑛 is 1, 2, 3, and so on.

Let us look at another homologous series. The table below shows information for the alkene homologous series. All alkenes have the same CC functional group.

Ethene has two carbon atoms and four hydrogen atoms. Its formula is CH24. The next compound in the alkene homologous series is propene. Propene has one more carbon atom and two more hydrogen atoms, and its formula is CH36. To get propene, we add a CH2 structural unit to ethene. In the same way, to get but-1-ene, we add a CH2 structural unit to propene. By now you might notice that each compound in the alkene homologous series differs from the previous compound by a simple CH2 unit. But the general formula is not the same as that for alkanes. The general formula for the alkene homologous series is CH.

Example 1: Identifying Molecules Belonging to the Same Homologous Series

Why do the three molecules below all belong to the same homologous series?

CCHHHHCCHCHHHHHCHHCCHHHHCHH
  1. They are all unsaturated.
  2. The ratio of hydrogen atoms to carbon atoms is 21.
  3. They have increasing numbers of carbon atoms.
  4. They are all hydrocarbons.
  5. They all contain the same functional group.

Answer

The three molecules above are ethene, propene, and but-1-ene. They all belong to the alkene homologous series. They are all unsaturated, straight-chain hydrocarbons, with the same functional group (CC). The only difference is the length of the carbon chain. Because they each have a different number of carbon atoms, they also have a different number of hydrogen atoms. Thus, they have a different molecular formula. Ethene is CH24, propene is CH36, and but-1-ene is CH48. Because they belong to the same homologous series, they also all have the same general formula: CH. A homologous series is a family of compounds that have the same functional group and thus similar chemical properties and the same general formula, with each compound in the series differing from the next by a simple structural unit.

All of the answer options apply to these three compounds. All are correct statements that we can use to describe these compounds. However, all of them being unsaturated does not necessarily make these compounds part of the same homologous series. Many compounds are unsaturated but are not necessarily part of the same homologous series. For example, ethyne (CH22) and octene are both unsaturated but are in different homologous series. Ethyne is in the alkyne series, and octene is in the alkene series. So, we can exclude answer option A.

Two or more compounds having a CH ratio of 21, being hydrocarbons, or having an increasing number of carbons also does not necessarily mean the compounds are in the same homologous series.

For example, consider the displayed formula of ethenol:

COHHCHH

And consider that of but-1-ene:

CCHHCHCHHHHH

Both ethenol and but-1-ene have the same ratio of carbon to hydrogen atoms (21) and an increasing number of carbon atoms but are in different series. Thus, we can exclude answer options B and C.

Now consider the displayed formula of methane:

CHHHH

And consider that of benzene:

CCCCCCHHHHHH

Methane and benzene are both hydrocarbons, but they are not in the same series. We can thus exclude answer option D.

An increasing number of carbon atoms does not necessarily mean compounds belong to the same homologous series. For example, methane, ethene, and propyne have an increasing number of carbon atoms, but they are not part of the same homologous series. We can therefore exclude answer option C.

The most correct answer is E. They all contain the same functional group.

Example 2: Determining the Formula for the Next Compound in a Homologous Series

What is the formula for the next alkene in the following homologous series: CH24, CH36, CH48?

  1. CH614
  2. CH410
  3. CH510
  4. CH612
  5. CH512

Answer

The three compounds given in the question are CH24 (ethene), CH36 (propene), and CH48 (butene). We are told that they are all in the alkene homologous series. The alkenes all have the same general formula, CH. The compound that will come after butene in this series will have one more carbon atom than butene. It will have five carbon atoms. So, we can determine the formula for the next alkene by substituting 5 for 𝑛 in the general formula CH.

Therefore, substituting 𝑛=5, we get CH52(5).

When we multiply out, we get CH510.

So, the correct answer is C: CH510.

All compounds belong to a specific homologous series. We will not investigate all these families of compounds—there are many. The table below shows some information about the homologous series of alkanes, alkenes, simple alcohols, and simple carboxylic acids.

Homologous SeriesFunctional GroupGeneral FormulaExamples
NameFormula
AlkanesCH+
MethaneCH4
EthaneCH26
PropaneCH38
ButaneCH410
AlkenesCCCH
EtheneCH24
PropeneCH36
ButeneCH48
AlcoholsOHCHOH(+)
MethanolCHOH3
EthanolCHOH25
PropanolCHOH37
ButanolCHOH49
Carboxylic acidsCOOHCHCOOH(+)
Methanoic acidHCOOH
Ethanoic acidCHCOOH3
Propanoic acidCHCOOH25
Butanoic acidCHCOOH37

Example 3: Understanding Why Different Compounds Do Not Belong to the Same Homologous Series

Why is ethane a member of the homologous series of alkanes, but methanol is not?

  1. Ethane has a smaller molecular mass.
  2. Ethane is obtained from crude oil.
  3. Ethane contains the same functional group as other alkanes.
  4. Ethane has two carbon atoms, but methanol only has one.
  5. Ethane is a gas, but methanol is a liquid.

Answer

All alkanes have the general formula CH+. When compounds belong to the same homologous series, they have the same functional group, the same general formula, and the same or very similar chemical properties.

Ethane’s molecular formula is CH26, and its general formula is CH+. So, ethane is an alkane.

Methanol’s formula is CHOH3, and its general formula is CHOH(+). Methanol contains the hydroxyl group OH, but ethane does not.

Ethane and methanol do not have the same general formula because they do not contain the same functional group. So, the correct answer is C. Ethane contains the same functional group as other alkanes.

We can predict the formula of a homologous series related to a homologous series we are familiar with. For example, all alkenes have the same general formula CH. If all the compounds in this series are monosubstituted with chlorine, then they will all have this new general formula CHCl. The number of hydrogen atoms decreases by 1 and so the subscript for hydrogen is no longer 2𝑛; it becomes 2𝑛1. Moreover, Cl is added to the formula.

Example 4: Determining the General Formula for Haloalkanes That Contain One Halogen Atom

Which of the following is the general formula of haloalkanes that contain one halogen atom?

  1. CHX+
  2. CHX+
  3. CHX
  4. CHX
  5. CHX

Answer

Alkanes all have the same general formula: CH+.

If one of the hydrogen atoms in an alkane is replaced with a halogen (X), then the general formula for the compound will no longer be CH+. Instead, the number of hydrogen atoms will decrease by 1 and so the subscript for the number of hydrogen atoms will change from “2𝑛+2” to “2𝑛+1.” The general formula will become CH+.

We must remember to add in the X to represent one halogen atom. So, the general formula for a haloalkane with one halogen atom will be CHX+.

This formula corresponds with answer A: CHX+.

Besides having the same general formula, all compounds within a homologous series have the same or similar chemical properties. This is because all compounds in a homologous series have the same functional group. For example, all the alkanes are fully saturated, with only carbon and hydrogen atoms and no functional groups. Thus, alkanes tend to react in a similar way. In other words, they have similar chemical properties. Likewise, all alkenes have 1 carbon–carbon double bond, CC, and so alkenes tend to react in the same way as each other. Alcohols in the same homologous series undergo the same reactions because they have the same functional group, namely the hydroxyl, OH, group. Compounds in the carboxylic acid homologous series have similar chemical properties to each other because they all have the COOH group, and so on.

Let us look at two brief instances of this.

Alkanes react with halogens in the presence of UV light to undergo substitution reactions. The equation below shows how the main product of the reaction between ethane and chlorine gas is ethane monosubstituted with an atom of chlorine:

Other alkanes will react in a similar way.

However, alkenes undergo addition reactions with halogens. The following equation shows how alkenes react differently to alkanes. They have a CC bond that opens up to add on two atoms of the halogen:

Alkenes tend to react in the same way because they have the same functional group and belong to the same homologous series.

Physical properties, however, are not the same for compounds of the same homologous series, but they tend to differ according to noticeable trends. For example, straight-chain alkanes have steadily increasing boiling points with an increase in chain length (number of carbon atoms). The graph below shows this trend.

The alkanes are covalent molecules and have weak van der Waals forces between their molecules. For the alkanes with short chain length, little energy is required to overcome the intermolecular attraction, resulting in a very low boiling point. The longer the chain length becomes (the more carbon atoms in the chain), however, the stronger the van der Waals attractive forces become and the higher the boiling point.

Viscosity, which is the resistance to flow, is another example of a physical property that changes gradually within a homologous series. The graph below shows the trend in viscosity (in mPa⋅s) versus the number of carbon atoms in the chain for the alkane homologous series. For the straight-chain alkanes at 25C, the more carbon atoms there are, the more viscous the liquid is. In other words, the longer the carbon chain is, the thicker the alkane liquid is and the slower it flows. Long-chain alkanes are highly viscous. They resist flow. They flow very slowly because they are thick and “sticky.”

Flammability also changes slowly in a homologous series. Flammability tends to decrease with increasing numbers of carbon atoms. In other words, the larger the molecule, the less flammable it is. Also, because there are more carbon atoms, the combustion will be incomplete with larger molecules, giving a smokier flame when the compound burns.

Example 5: Predicting the Trend in Flammability for the Alkane Homologous Series

In a part of the homologous series of alkanes, from methane to hexane, what happens to the flammability as the number of carbon atoms increases?

  1. It decreases then increases.
  2. It decreases.
  3. It stays the same.
  4. It increases.
  5. It increases then decreases.

Answer

We are going from methane (which has one carbon atom) to hexane (which has six carbon atoms) in the alkane homologous series, so the number of carbon atoms increases. As the molecules become larger, the compounds become less flammable and burn with smokier flames. So, from methane to hexane in the alkane series, the flammability decreases as the number of carbon atoms increases. The correct answer is B; it decreases.

We have seen that the number of carbon atoms in the chain, or the size of the molecule, affects the physical properties. Here is one last example. As the molecular mass of an alkane increases, the density increases. The table shows this general trend.

AlkaneDensity (g/mL) at 20C and 1 atmPhysical State of Alkane at 20C and 1 atm
Methane0.000668Gas
Ethane0.00127Gas
Propane0.00187Gas
Butane0.00249Gas
Pentane0.626Liquid
Hexane0.659Liquid
Octane0.703Liquid
Decane0.730Liquid

The longer the length of the carbon chain, the higher the density. There is also a sudden increase in density going from butane to pentane as the state of the alkane changes from gas to liquid.

Key Points

  • Homologs are compounds that differ from each other by a repeating unit.
  • A homologous series is a family of compounds that have the same functional group and thus similar chemical properties and the same general formula, with each compound in the series differing from the next by a simple structural unit.
  • Some common general formulae are
    • CH+ for alkanes,
    • CH for alkenes,
    • CHOH(+) for alcohols,
    • CHCOOH(+) for carboxylic acids.
  • Physical properties are not the same for compounds of the same homologous series, but they tend to differ according to noticeable trends.

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