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What Is a Homologous Series?

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  • Written By: Liz Thomas
  • Edited By: C. Wilborn
  • Last Modified Date: 30 November 2016
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A homologous series refer to organic molecules or compounds that have a similar molecular formula, which causes the compounds to have similar chemical properties. As the molecular size of the compound increases within the homologous series, the physical properties also exhibit a gradual change. The main difference between compounds in such series is the addition of an extra carbon and dihydrogen group, CH2.

Many different types of compounds have homologous series. The most common series include alkanes, ethers, and alcohols; other homologous series include alkenes or olefins and alkynes, and carboxylic acids. Alkanes are compounds that only contain carbon and hydrogen linked by single bonds. Ethers are compounds that have an ether functional group, consisting of an oxygen bound to either two alkane or to two aryl compounds, depicted as R-O-R'. R and R' refer to the different alkane or aryl compounds.

Alcohols contain a functional group made up of an oxygen bound to a hydrogen (-OH), and alkenes have a functional group of a carbon double bonded to another carbon (C=C). Alkynes are similar to alkenes, though the functional group consists of a carbon molecule with a triple bond to another carbon molecule. Carboxylic acids are acids that contain at least one carboxyl group (-COOH).

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The compounds in the series will vary by a CH2 unit and certain molecular mass. The alkane series begins with methane, with a molecular formula of CH4 and mass of 16.04. The next compound is ethane, with a formula of C2H6 and mass of 30.07. This is followed by butane, formula C4H10 and mass 58.12, and pentane, formula C5H12 and mass 72.15. As can be seen, ethane contains one more carbon and two more hydrogen than methane and has a mass of 14 more than methane. The same occurs between ethane and butane, and butane and pentane.

The physical properties of these compounds also gradually change as the molecules get larger. Typically, as the length of the carbon chain increases, the compound's ability to dissolve in water decreases, though this is also dependent on the chemical nature of the functional group, as some functional groups are more soluble in water than others. As the carbon chains within the homologous series become longer, the boiling point of the compound changes. While it increases in many cases, if the carbon chain begins the form branches, the boiling point will begin to decrease. This is just one of the exceptions found within the physical changes.

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