GCSE OCR Gateway Organic Chemistry C6.2a-b Homologous Series

GCSE OCR Gateway Organic Chemistry C6.2a-b Homologous Series

C6.2a To be able to recognize functional groups and identify members of the same homologous series to include alkanes, alkenes, alcohols and carboxylic acids

C6.2b To be able to name and draw the structural formulae, using fully displayed formulae, of the first four members of the straight chain alkanes, alkenes, alcohols and carboxylic acids

What is a homologous series?

A homologous series is a series of organic compounds where one organic compound differs from the member next to it by a “CH₂” group. 

Each member of the series is a homologue of the other members that means that the series can be described by a general formula.

So for the alkanes that general formula is C_nH_2n+2 , for the alkenes C_nH_2n for the alcohols C_nH_2n+1OH and for the carboxylic acids C_nH_2n+1.COOH where n represents the number of carbon atoms in the particular molecule.

However, you have to ask to what extent these terms are useful today given the fact that they are of limited help in identifying any particular molecule because a general formula C_nH_2n can also describe a cyclic alkane and a large alkene can have the double bond in several different positions. 

The general formula tells us nothing about where that double bond might be situated on the carbon chain.  For example an alkene

C_nH_2n C₄H₈ could be CH₃CH₂CH=CH₂  or CH₃CH=CHCH₃ so the general formula or molecular formula tells you nothing about the structural differences. 

It is better to examine individual hydrocarbons and their derivative’s names to find out the structural information that is very useful.  Take but-2-ene :

First but- tells us that the molecule contains four carbon atoms and that due to the –ene suffix it has 8 hydrogen atoms since it is an alkene. i.e. C₄H₈.

So what does the -2- tell us? The -2- in but-2-ene tells us that the double bond starts on the second carbon atom of the chain, that is, it is the CH₃CH=CHCH₃ above.  But but-1-ene would tell us that the double bond starts on the first carbon atom in the chain that is CH₃CH₂CH=CH₂ above.  But-1-ene and but-2-ene are structural isomers: same molecular formulae but different arrangement of the atoms in the molecule.

The tables below show the names, molecular formulae, structural formulae and displayed formulae of the first four members of each of these homologous series: alkanes, alkenes, alcohols and carboxylic acids.

1. Alkane Homologous Series

Let’s now look at the alkanes up to alkane number four with four carbon atoms butane:

2. Alkene Homologous Series

The following table shows the first four alkenes and includes the butene isomers:

There is no methene because for an alkene to possess a double bond the molecule requires a minimum of two carbon atoms.

3. Alcohol Homologous Series

This table shows the first four straight chain alcohols in the homologous series,

4. Carboxylic acid Homologous Series 

Lastly let’s look at the carboxylic acid homologous series:

Make a note that in constructing the name of the acid from its displayed formula the carbon of the acid group is counted together with the carbon atoms in the carbon chain.

My next post will examine some of the reactions of these compounds.