Homologous Series
Definition and meaning of Homologous Series in chemistry.
A homologous series is a group of organic compounds sharing the same general formula and functional group, in which each successive member differs from the last by a fixed CH2 unit.
In more detail
Because each member is built by adding one CH2 unit to the previous one, physical properties such as boiling point, melting point, and density change gradually and predictably as chain length increases. Chemical properties stay similar across the series since every member has the same functional group and undergoes analogous reactions. This regularity allows chemists to predict the properties of an untested member from trends shown by its neighbors, and it underlies the systematic naming of organic compounds.
Key facts
| Field | Organic Chemistry |
|---|---|
| Alkane general formula | CnH2n+2 |
| Repeating unit | CH2 (mass 14 u) |
| Property trend | Boiling point rises steadily with chain length |
The alkanes methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10) form a homologous series: each member differs from the one before it by a CH2 unit, and all fit the general formula CnH2n+2.
Frequently asked questions
Why does boiling point increase along a homologous series?
Adding CH2 units increases molecular size and surface area, which strengthens the London dispersion forces between molecules, so more energy is needed to separate them.
Do all members of a homologous series react in exactly the same way?
They undergo similar reactions because they share the same functional group, though reaction rate or ease can vary slightly with chain length due to effects like increased steric hindrance.