Homologous Series: Definition & Identification

Homologous Series: Definition & Identification
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  • 0:03 Family Resemblance
  • 0:31 What's a Homologous Series?
  • 1:16 Chemical Properties
  • 2:02 Physical Properties
  • 3:21 Lesson Summary
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Lesson Transcript
Instructor: Sarah Pierce

Sarah has taught high school chemistry and biology, as well as college level chemistry(general, organic, analytical, biochemistry), and has a doctorate in chemistry.

This lesson covers the definition of a homologous series of molecules in organic chemistry. Ways to identify homologous series are discussed as well as the chemical and physical properties of homologous series.

Family Resemblance

Have you ever looked at a family photo at a friend's house and noticed how everyone kind of looked similar, but they didn't look exactly the same? You could definitely tell they were related, but they didn't look like carbon copies of each other. Homologous series in organic chemistry are like family photos. You can tell the molecules are similar or related, but they are a little different. Let's learn a little more about homologous series, homologues, and how to identify them.

What's a Homologous Series?

A homologous series in organic chemistry is a group of organic compounds (compounds that contain C atoms) that differ from each other by one methylene (CH2 ) group. For example, methane, ethane, and propane are part of a homologous series. The only difference among these molecules is that they have different numbers of CH2 groups.

Each member of a homologous series is called a homologue, which can also be spelled ''homolog.'' For example, methane and ethane are homologues and belong to the same homologous series. They differ from each other by one CH2 group. The formula of methane is CH4 and the formula of ethane is C2 H6.

Chemical Properties

Homologues have similar chemical properties because the functional group of the molecule does not change. Remember, the functional group is just the area of the molecule that is reactive or can change in chemical reactions. Because the functional group is the same for each molecule in a homologous series, the homologues have similar reactivities. Some examples of functional groups in homologous series are alcohols (-OH), carboxylic acids (-COOH), aldehydes (COH), ketones (C=O), and amines (N), among many others. So when you are looking to identify homologues, the functional group will be the same. The only thing that will change is the length of the carbon chain.

Physical Properties

So, if the functional group doesn't change, why are homologous series interesting? What can they tell us about the molecules? The reason homologous series are interesting when studying chemistry is because if there is a change in a physical property, it is due to the extra methylene group.

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