Homologous series are organic compounds with similar molecular formulas and chemical properties. As the size of the compound increases, physical properties change due to the addition of a CH2 group. Common series include alkanes, ethers, alcohols, alkenes, alkynes, and carboxylic acids. Physical properties change as the carbon chain lengthens, affecting solubility and boiling point.
A homologous series refers to molecules or organic compounds that have a similar molecular formula, which causes the compounds to have similar chemical properties. As the molecular size of the compound within the homologous series increases, the physical properties also show a gradual change. The main difference between the compounds of these 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 contain only carbon and hydrogen bonded by single bonds. Ethers are compounds that have an ether functional group, consisting of an oxygen bonded to two alkanes or two aryl compounds, described as RO-R’. R and R’ refer to the different alkane or aryl compounds.
Alcohols contain a functional group consisting of an oxygen bonded to a hydrogen (-OH), and alkenes have a functional group consisting of a carbon double bonded to another carbon (C=C). Alkynes are similar to alkenes, although 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).
The compounds in the series will vary by a CH2 unit and a certain molecular mass. The alkane series begins with methane, with the molecular formula CH4 and mass 16.04. The next compound is ethane, with formula C2H6 and mass 30.07. 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 hydrogens than methane and has a mass of 14 more than methane. The same happens 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 ability of the compound to dissolve in water decreases, although this also depends 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 lengthen, the boiling point of the compound changes. While it does go up in many cases, if the carbon chain starts branching, the boiling point will start to go down. This is just one of the exceptions found within physical changes.
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