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Lesson Video: Homologous Series Chemistry

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

16:31

Video Transcript

In this video, we will learn what a homologous series or family of organic chemicals is. We will learn how to write the general formula for a homologous series, and we will also investigate the trends in physical and chemical properties within a series.

What do we mean when we talk about an homologous series? Firstly, in general, an organic compound is a chemical compound that contains carbon and hydrogen covalently bonded together. Many organic compounds also contain other covalently bonded atoms such as oxygen, sulfur, or nitrogen. Every organic compound belongs to a specific family of organic compounds. A family of organic compounds is called an homologous series. Each member or each compound within the same family or same homologous series are identical in terms of structure except that each successive member differs from each other by a simple repeating unit. Member one and two differ from each other only by this simple repeating unit.

In the same way, member two and three differ from each other by the same simple repeating unit. And again, members three and four differ from each other by that same simple repeating unit. We could go on and add more members to the series in the same manner. We can define an homologous series as a family of compounds with the same functional group and each compound in the series differing from the previous by a simple structural unit. We will see that the members of an homologous series or family of compounds have the same general formula in much the same way as members of a family tend to have the same surname and that compounds in the same family tend to have the same or similar properties in much the same way as members of a family tend to look similar to each other.

Let’s now have a look at the most basic homologous series, the alkanes. The alkane homologous series or alkane family has methane as the first and simplest member. Methane’s molecular formula is CH4, and its structural formula is shown here. Methane has one carbon atom and four hydrogen atoms. Ethane, the next alkane in this series, has two carbon atoms and six hydrogen atoms. Ethane differs from methane by one carbon and two hydrogens, which is a simple CH2 unit. The next compound in the alkane homologous series is propane, whose molecular formula is C3H8, with the following structural formula. Propane differs from ethane by one carbon atom and two hydrogen atoms, which is again a simple CH2 structural unit.

All the compounds in the alkane homologous series follow this pattern. Each compound differs from the previous or from the next by the same simple structural unit, a CH2 unit. We call compounds that differ from each other by a repeating unit homologs. Methane, ethane, and propane are homologs in the same homologous series. We can write a general formula for all the alkanes since they differ by the same unit. The general formula for the alkanes is C𝑛H2𝑛+2, where 𝑛 is a natural number such as one, two, three, four, et cetera.

Many homologous series, such as the alkenes, have one or more functional groups. Ethene, propene, and butene all have one carbon-carbon double bond. Ethene is C2H4, propene C3H6, and butene C4H8. As with the alkanes, the homologs, ethene, propene, and butene successively differ from each other by one carbon and two hydrogens or a CH2 unit. As a side note, the two isomers of butene, but-1-ene and but-2-ene, both also differ from propene by a simple CH2 unit, even though the double-double carbon bond is in a different place in each compound. The general formula of alkenes is C𝑛H2𝑛.

Now, there are many homologous series. Let’s look at two more examples briefly, and those are the alcohols and carboxylic acids. The simple straight-chain alcohols all contain the hydroxy or hydroxyl functional group. The homologs, methanol, ethanol, propanol, butanol, and so on all differ from each other by a simple repeating unit and all have the same general formula C𝑛H(2𝑛+1)OH. Even those compounds in the series, such as propanol and butanol, which have more than one isomer, still differ from each other by the same simple repeating unit and have the same general formula.

The simple straight-chain carboxylic acids all have the same functional group, the carboxyl group or carboxylic acid group, COOH. Methanoic acid, ethanoic acid, propanoic acid, and butanoic acid again all differ from each other by the successive addition of a CH2 unit. And they all share the same general formula C𝑛H(2𝑛+1)COOH. Now we know that compounds within an homologous series all have a similar structure except that successive structures differ from the previous or next by a simple repeating unit. We also know that all compounds within the same family have the same functional group and the same general formula. But what about their properties? How do properties compare?

Let’s start with the chemical properties within an homologous series. The alkanes all tend to react in the same manner because they have similar structures and they are all fully saturated with no other functional groups. For example, when ethane reacts with chlorine gas in the presence of UV light, one of the chlorine atoms is substituted for a hydrogen atom to give a monosubstituted product, 1-chloroethane. Other alkanes react in the same way with halogens. In other words, they undergo substitution reactions.

Now, the alkenes tend to react in a similar manner to each other. For example, ethene, which is unsaturated with a carbon-carbon double bond, can react with chlorine gas to give a disubstituted product, 1,2-dichloroethane. This is correctly called an addition product. And alkenes all tend to undergo addition reactions because extra atoms are added on to the double bond. We have seen here how members of the same homologous series tend to react in a similar manner.

Now, let’s investigate physical properties within an homologous series. The physical properties within a series are also similar between compounds except that they tend to follow a recognizable trend. This rough graph shows the boiling points of some straight-chain alkanes. The short molecules, which have few carbon atoms, have weak Van der Waals attractive forces between them. And so their boiling points are low as little energy is required to overcome the intermolecular attraction during boiling. However, the longer the carbon chain becomes, the stronger the Van der Waals attractive forces between the molecules are, and so the higher the boiling points are because more energy is needed to overcome these attractive forces to separate the molecules when boiling. We can see this general trend. An increasing number of carbon atoms corresponds with an increasing boiling point.

Viscosity, which is the resistance to flow, is another physical property with a gradually changing recognizable trend within a homologous series. The graph shows the viscosity of some straight-chain alkanes with five carbons or more. Shorter carbon chains have low viscosity and flow easily. But the longer the chains are or the more carbon atoms they are, the higher the viscosity. We say these substances are more viscous or sticky and they flow very slowly. This is because of increased or stronger Van der Waals forces of attraction between these larger molecules. We can see this general increasing trend. An increasing number of carbon atoms corresponds with an increase in viscosity.

Let’s look at the physical property flammability. A flammable substance catches fire immediately when it is exposed to a flame. Flammability in the alkanes tends to follow an opposite trend to boiling point and viscosity. That is, with an increasing number of carbon atoms in the compound chain, there is a decreasing flammability. In other words, the larger the molecule, the less flammable or the less easily it burns. Large alkane molecules tend to undergo incomplete combustion, giving off a smoky flame.

Lastly, let’s look at the density of the straight-chain alkanes and their trends. The table shows the density of some alkanes at 20 degrees Celsius and one atmosphere. Notice that as the number of carbon atoms increases, so the density increases. In general, with an increasing number of carbon atoms, in other words, an increasing carbon chain length or increasing molecular mass, there is a corresponding increase in density. It’s interesting to note there is a sudden big increase in density going from butane to pentane. That is because the compounds up to butane are all gaseous at 20 degrees Celsius. But pentane and those following in the table are liquid and thus are much more dense.

Now, it’s time to practice.

Which of the following is the general formula of haloalkanes that contain one halogen atom? (A) C𝑛H2𝑛+1𝑋, (B) C𝑛H2𝑛+2𝑋, (C) C𝑛H2𝑛-1𝑋, (D) C𝑛H2𝑛𝑋, or (E) C𝑛H2𝑛-2𝑋.

The question asks for the general formula of alkane compounds containing one halogen atom. Alkanes belong to the alkane homologous series, in other words, the alkane family, whereas the haloalkanes belong to the haloalkane homologous series or the haloalkane family. A homologous series is a family of compounds with the same functional group with each compound in the series differing from the previous or next compound by a simple structural unit. Because each compound in the series differs from the next or previous by the same unit, every compound within the same homologous series has the same general formula. The alkanes all have the same general formula C𝑛H2𝑛+2, where 𝑛 is a natural number such as one, two, three, four, et cetera.

A haloalkane is an alkane where one of the hydrogen atoms has been removed and replaced with a halogen. Let’s call the halogen atom 𝑋. So if we remove one hydrogen atom from an alkane, the formula becomes C𝑛H2𝑛+1. But we need to add in a halogen atom. So we can put 𝑋 into the formula in place of the missing hydrogen atom. This formula corresponds with answer option (A). Therefore, the general formula for a haloalkane that contains one halogen atom is C𝑛H2𝑛+1𝑋.

Before we end this question, let’s have a look at two quick examples of haloalkanes. If 𝑛 is, say, two and 𝑋 is, say, chlorine, we can write the general formula as C2H2(2)+1Cl, which is C2H5Cl or chloroethane. Here is another example. If 𝑛 is, say, four and 𝑋 is, say, bromine, then the general formula is C4H2 multiplied by four plus one Br, which is C4H9Br, which is 1-bromobutane or any other isomer of bromobutane.

Let’s summarize what we have learnt. We learnt that an homologous series is a family of compounds that have the same functional group, where each compound in the series differs from the previous compound by a simple repeating structural unit. Compounds in an homologous series have the same general formula, similar chemical properties, and physical properties that gradually change in a recognizable trend within the series.

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