What is a Homologous Series?
A homologous series is a collection of molecules with comparable chemical characteristics and functional groups that differ by CH2 from one another. In organic molecules with the same general formula, carbon chains of various lengths have been discovered. A series of organic compounds are formed by organic compounds that differ from one another by a repeating unit and have the same general formula.
This article will study homologous compounds, homologous series of alcohol, and homologous series of alkanes.
Homologous Series of Carbon Compounds
The functional groups are the same for all members of this series. They have identical physical qualities that increase in mass in a fixed gradation. The characteristics of CH3OH, C2H5OH, and C3H7OH are similar and vary gradually as the molecular mass of subsequent members of the series increases. This is due to the fact that as the molecular mass of the compounds grows, so does the number of bonds. As a result, properties such as melting and boiling points, solubility, and other properties that are dependent on the mass and the total number of bonds in a compound alter gradually as the molecular masses of the compounds increase.
Homologous Series of Hydrocarbons
Hydrocarbons can be classified into four groups, namely, Alkanes, Alkenes, Alkynes, and Arenes.
Examples of Hydrocarbons
Homologous Series of Alkanes
Physical Properties of Alkanes:
Solubility: Alkanes are often non-polar compounds due to the small difference in electronegativity between carbon and hydrogen and the covalent character of the C-C or C-H bond.
Polar molecules are soluble in polar solvents, while non-polar molecules are soluble in non-polar solvents, as we know. As a result, alkanes are hydrophobic, meaning they are water-insoluble.
Boiling Point: We see an increase in intermolecular Van Der Waals forces as the molecular size or surface area of the molecule grows.
The boiling point of alkanes rises as their molecular weight rises.
In comparison to their structural isomers, straight-chain alkanes appear to have a greater boiling point.
Melting Point: The melting point of alkanes follows the same pattern as their boiling point, i.e., it rises as molecular weight rises.
This is due to the fact that higher alkanes are solids, making intermolecular forces of attraction difficult to overcome.
Homologous Series of Alkanes are Shown Below in the Table:
Homologous Series of Alkenes
The alkenes form a homologous series.
A Hydrocarbon Homologous Series is a Series of Hydrocarbons Which:
Have the same general formula
In molecular formulas, they differ from neighbouring molecules by CH2.
Demonstrate a steady change in physical attributes, such as boiling and melting points.
chemical qualities that are similar
The homologous series of alkenes have the general formula CnH2n, where n is the number of carbon atoms. The alkene homologous series begins with ethene C2H4 because alkenes are hydrocarbons with at least one carbon-carbon double bond.
The compounds are termed similarly to alkanes but with the suffix "ene" instead of "ane" at the end.
Homologous Series of Alkene is Shown Below:
Homologous Series of Alkynes
The general formula of alkynes is CnH2n-2.
The compounds are termed similarly to alkanes but with the suffix "yne" instead of "ane" at the end.
Alkyne Homologous Series Is Shown in the Table Below:
Functional Groups and Homologous Series
Homologous Series of Alcohol
Alcohol is an organic molecule having an aliphatic carbon atom bearing the hydroxyl (OH) functional group. We often represent alcohols by the general formula ROH, where R is an alkyl group because OH is the functional group of all alcohols. Alcohol is found in abundance in nature. The active element in alcoholic beverages is ethyl alcohol (ethanol), although it is merely one of a group of chemical molecules known as alcohols. In addition to cholesterol and carbs, the family includes other well-known compounds.
Alcohols are given the suffix -ol by the IUPAC system of nomenclature.
An Example of Homologous Series of Alcohol is Shown Below:
Homologous Series of Aldehyde
Aldehyde is a compound containing CHO group
The functional group aldehydes are thought to be the most important. They're commonly referred to as the formyl group. The dehydration of alcohols gives rise to aldehydes. The carbonyl group is linked to at least one hydrogen atom in aldehydes.
Aldehydes are given the suffix -al by the IUPAC system of nomenclature.
Properties
Compounds in a homologous series bearing the same functional group generally display similar chemical properties. The size of the compounds increases in a homologous series with an increase in the number of carbon atoms, thereby raising the molecular weight of the compounds. Increased weight and size sometimes influence the chemistry of the compound such as reactivity and rate of reaction.
Progression of size and weight in a homologous series increases the boiling points of the compounds. For example, ethane has a higher boiling point than methane; the molecules with increasing length have more surface area and experience greater Van der Waals forces with neighbouring molecules.
Uses
Methane gas is found in natural gas, it is used for cooking and heating.
Propane gas is used in cylinders.
Octane is present in gasoline (petrol) and serves as fuel.
Cycloalkanes are found in natural gas, kerosene, diesel and serve as fuels, it is used in motor fuels and oils.
Alkenes can be used as fuels and solvents.
Alkenes due to their reactivity serves as starting material for the synthesis and manufacture of alcohols, plastics, liquors, detergents, and fuels.
Acetylene (C2H2) is used for welding and cutting metals in oxy-acetylene torches.
Acetylene is used in manufacturing for PVC, polyvinyl acetate, polyvinyl ether, orlon and neoprene rubbers.
Interesting Facts
Alkenes are often referred to as olefins.
Alkynes are often referred to as acetylenes.
Key Features
Homologous series is a family of compounds that can be represented by a general formula.
The compounds in a series have similar chemical properties.
Homologous series of hydrocarbons are classified as Alkanes, Alkenes, Alkynes, and Arenes.
Homologous series of Alkanes have general formula: CnH2n+2
Homologous series of Alkenes have general formula: CnH2n
Homologous series of Alkynes have general formula: CnH2n-2
Arenes are cyclic unsaturated compounds with at least one aromatic ring.
Did You Know?
Ethanol, or ethyl alcohol, is the “alcohol” in alcoholic beverages including wine, beer, and spirits. It is the only kind of alcohol that can be consumed without producing substantial bodily harm.
Alcohol is a sedative. This means that it causes the brain's activity to slow down.
According to the National Survey on Drug Use and Health (NSDUH), 86.4 percent of individuals have consumed alcohol at least once in their lives.
According to the NSDUH, 70.1 percent of American adults consumed alcohol in the previous year, and 56.0 percent consumed alcohol in the preceding month.
The effects of alcohol are numerous. It causes the release of dopamine, a neurotransmitter linked to pleasure and contentment, in the brain.
Another benefit of using alcohol is the reduction of stress. An increase in the absorption of another neurotransmitter called GABA causes this.
FAQs on Homologous Series
1. What is a homologous series in chemistry?
A homologous series is a family of organic compounds that share the same functional group and have similar chemical properties. Each successive member of the series differs from the next by a -CH₂ group. This structural similarity leads to a consistent and predictable pattern in their physical and chemical properties.
2. What are the main characteristics of a homologous series?
All compounds within a single homologous series share these key characteristics:
- They can be represented by a single general formula, such as CₙH₂ₙ₊₂ for alkanes.
- They all contain the same functional group (e.g., the -OH group in alcohols), which dictates their chemical behaviour.
- Each consecutive member differs by a -CH₂ group, which corresponds to a mass difference of 14 atomic mass units.
- They show a gradual and predictable change in physical properties like boiling point and density as the molecular mass increases.
- They exhibit very similar chemical properties due to their common functional group.
3. What are the general formulas for the homologous series of alkanes, alkenes, and alkynes?
The general formulas for these fundamental hydrocarbon series, as per the CBSE syllabus for the 2025-26 session, are:
- Alkanes: CₙH₂ₙ₊₂ (where n ≥ 1)
- Alkenes: CₙH₂ₙ (where n ≥ 2)
- Alkynes: CₙH₂ₙ₋₂ (where n ≥ 2)
In these formulas, 'n' represents the number of carbon atoms in a molecule of the compound.
4. Can you provide examples for the homologous series of alcohols?
Yes, the alcohol homologous series is characterized by the hydroxyl (-OH) functional group. The first few members, which share similar properties like undergoing oxidation, are:
- Methanol (CH₃OH)
- Ethanol (C₂H₅OH)
- Propanol (C₃H₇OH)
- Butanol (C₄H₉OH)
Their names all end with the suffix '-ol', indicating they belong to the alcohol series.
5. How do physical properties like boiling point change within a homologous series?
As you move down a homologous series, the length of the carbon chain and the molecular mass increase. This leads to stronger intermolecular forces of attraction (van der Waals forces) between molecules. Consequently, more energy is required to separate the molecules, causing a gradual increase in both boiling and melting points. For instance, the boiling point of ethane is higher than that of methane.
6. What is the key difference between a functional group and a homologous series?
A functional group is a specific part of a molecule—an atom or a group of atoms—that determines the compound's characteristic chemical reactions. For example, the carboxyl group (-COOH) makes a compound an acid. A homologous series, on the other hand, is the entire family of compounds that contains that specific functional group. In short, the functional group is the feature, while the homologous series is the family defined by that feature.
7. Why do all members of a homologous series show similar chemical properties?
The chemical reactivity of an organic compound is primarily dictated by its functional group. Since all members of a homologous series possess the exact same functional group, they tend to undergo the same types of chemical reactions. The rest of the molecule, the hydrocarbon chain (alkyl part), is generally less reactive and mainly influences the physical properties (like boiling point) rather than the chemical behaviour.
8. How can you identify if two compounds, for example C₂H₅OH and C₄H₉OH, belong to the same homologous series?
To identify if C₂H₅OH and C₄H₉OH are homologues, you check two conditions:
- Same Functional Group: Both compounds contain the -OH (hydroxyl) group, which is the functional group for alcohols.
- Differ by -CH₂ units: Their molecular formulas are C₂H₆O and C₄H₁₀O. The difference between them is C₂H₄, which is equivalent to two -CH₂ units.
Since they share the same functional group and have a structure that fits the same general formula for alcohols (CₙH₂ₙ₊₁OH), they are confirmed to be members of the same homologous series.
