The key difference between functional group and homologous series is that functional group is a moiety of a chemical compound that is responsible for the reactivity of that chemical compound whereas homologous series is a sequence of chemical compounds having the similar functional group and similar chemical reactivity.
The terms functional group and homologous series are two related terms. This is because a homologous series forms according to the functional groups of chemical compounds.
CONTENTS
1. Overview and Key Difference
2. What is a Functional Group
3. What is a Homologous Series
4. Side by Side Comparison – Functional Group vs Homologous Series in Tabular Form
5. Summary
What is Functional Group?
Functional groups are specific substituents that occur within a molecule and are responsible for the characteristic chemical reactions those molecules undergo. For example, if the functional group is the same for two molecules that have different chemical structures, the two molecules will undergo similar types of reactions, no matter the size of the molecules. Generally, functional groups are very important in different aspects; in identifying unknown molecules, in determining end products of reactions, in chemical synthesis reactions for the designing and synthesis of new compounds, etc.
Figure 01: Functional Groups in Different Organic Compounds
Normally, functional groups attach with a molecule via covalent chemical bonds. In polymers, the functional groups are attached to the nonpolar core of carbon atoms, giving the polymer its specific characteristic features. However, functional groups are sometimes charged chemical species. i.e. carboxylate ion group. This makes the molecule a polyatomic ion. In addition, functional groups that attach to a central metal atom in coordinate complexes are called ligands. Some common examples for functional groups include hydroxyl group, carbonyl group, aldehyde group, ketone group, carboxyl group, etc.
What is a Homologous Series?
A homologous series is a sequence of chemical compounds having the same functional groups and thus, the same chemical reactivity. This term is used mainly in the field of organic chemistry where chemical compounds can be either branched or unbranched structures. A homologous series can form according to the length of a carbon chain or according to the number of monomer units in a homopolymer as well. For example, a homologous series of straight-chain alkanes begin with methane, ethane propane, butane and pentane.
Figure 02: Boiling Points Gradually Change in a Homologous Series
Typically, the members of a homologous series contain a fixed set of functional groups that give these molecules similar chemical and physical properties. E.g. the primary straight chain alcohols have a hydroxyl at the end of the carbon chain. The properties of the homologous series gradually change along with the series, and these changes can often be explained by slight differences in molecular size and mass. However, if we cannot arrange the members of the series according to a linear order depending on a single parameter, the compounds collection is called a chemical family or a class of homologous compounds, not a series.
What is the Difference Between Functional Group and Homologous Series?
The terms functional group and homologous series are two related terms. The key difference between functional group and homologous series is that functional group is a moiety of a chemical compound that is responsible for the reactivity of that chemical compound whereas homologous series is a sequence of chemical compounds having the similar functional group and thus, similar chemical reactivity.
Below infographic presents more details of the difference between functional group and homologous series.
Summary – Functional Group vs Homologous Series
A homologous series forms according to the functional groups of chemical compounds. Therefore, the key difference between functional group and homologous series is that functional group is a moiety of a chemical compound that is responsible for the reactivity of that chemical compound whereas homologous series is a sequence of chemical compounds having the similar functional group and thus, similar chemical reactivity.
Reference:
1. “Functional Group Names, Properties, and Reactions.” Boundless Chemistry – Lumen, Available here.
Image Courtesy:
1. “OSC Microbio 07 01 TBLcomFun” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Alkane boilingpoints (centigrade) from methane to nonadecane” By AgnesLager – Own work (CC BY-SA 4.0) via Commons Wikimedia