Homologous Series

Homologous Series is a family of organic compounds where two consecutive members in the family differ from each other by -CH₂ repeating unit. The mass difference between two such consecutive members is 14 units. They have same functional groups and show similar trends in physical and chemical properties.

This article deals with definitions, formulas, and examples of homologous series. A table discussing all the possible homologous series has been also added to the article. We have to study Homologous Series in Class 10 in the chapter on Carbon and its compound.

What is Homologous Series

Homologous Series is a collection of structurally and chemically identical organic compounds, where each member is distinguished from the others by a repeating unit. This functional group is called a repeating unit. At larger molecular sizes, the physical and chemical characteristics of these compounds often follow similar patterns and have the same basic formula. Carboxylic acids, alcohols, alkanes, alkenes, and alkynes are a few homologous series examples.

Homologous Series

Formula for Homologous Series

A homologous series is a collection of organic compounds that have a general formula but are distinguished from one another by recurrent structural units. The following may be expressed as an equation:

General Formula: R – X

• R is Remainder of Molecule, or Main Carbon Chain
• X is Functional Group that establishes Series

In the alkane series, for instance, the usual formula is C_nH_2n+2. Every series member varies from the others by one CH₂ unit.

Homologous Series Definition

A homologous series consists of a of organic substances with comparable characteristics and structures and differ by a set of repeating group

Example of Homologous Series

Numerous organic molecules fall under the homologous series category; they are all distinguished by unique structural and chemical characteristics. Common homologous series types include the following:

Alkane Homologous Series

Alkane has general formula C_nH_2n+2, these saturated hydrocarbons are made up of carbon-carbon single bonds. Ethane (C₂H₆), propane (C₃H₈), and methane (CH₄) are a few examples. In this, each compound differ by -CH₂ from its preceding compound

Examples

• Methane (CH₄)
• Ethane (C₂H₆)
• Propane (C₃H₈)

Alkenes Homologous Series

Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond. They are sometimes referred to as olefins. CnH₂n is their standard formula. The chemicals propene (propylene, C₃H₆) and ethene (ethylene, C₂H₄) are examples. In this, each compound differ by -CH₂ from its preceding compound

Examples

• Ethene (C₂H₄)
• Propene (C₃H₆)
• Butene (C₄H₈)

Alkynes Homologous Series

The unsaturated hydrocarbons known as alkynes have at least one triple bond between carbon atoms. CnH₂n-2 is their standard formula. Two such compounds are propyne (methylacetylene, C₃H₄) and ethyne (acetylene, C₂H₂). In this, each compound differ by -CH₂ from its preceding compound

Examples

• Ethyne (Acetylene) (C₂H₂)
• Propyne (C₃H₄)
• Butyne (C₄H₆)

Alcohols Homologous Series

These chemical substances have a carbon atom bound to a hydroxyl (-OH) functional group. C_nH_2n+1OH is their general formula. Methanol (CH₃OH), ethanol (C₂H₅OH), and propanol (C₃H₇OH) are a few examples.

Examples

• Methanol (CH₃OH)
• Ethanol (C₂H₅OH)
• Propanol (C₃H₇OH)

Aldehyde Homologous Series

Aldehyde have functional group -CHO attached to the parent carbon chain. The general formula for Aldehyde homologous series is C_nH_2n+1CHO where n starts with zero. The members of Aldehyde Homologous series are mentioned below:

• Methanal(HCHO)
• Ethanal(CH₃CHO)
• Propanal (CH₃CH₂O)

Ketone Homologous Series

Ketone has a general formula of -C-CO-C where, O is bonded to the middle carbon with double bond. Deviating from the other carbon compounds, ketone group starts with three carbon chain. Hence, the first member of Ketone Homologous series is Propanone. The first three members of Ketone Homologous Series are mentioned below:

• Propanone(CH₃COCH₃)
• Butanone(CH₃CH₂COCH₃)
• Pentanone(CH₃CH₂CH₂COCH₃)

Learn, Aldehyde and Ketone

Carboxylic Acids Homologous Series

Carboxylic Acids substances have a carboxyl (-COOH) functional group made composed of a hydroxyl (-OH) group on one carbon atom and a carbonyl (C=O) group on the other. They often have the formula C_nH_2n+1COOH. Acetic acid (CH₃COOH), propionic acid (C₂H₅COOH), and formic acid (HCOOH) are a few examples.

Examples

• Methanoic Acid (Formic Acid) (HCOOH)
• Ethanoic Acid (Acetic Acid) (CH₃COOH)
• Propanoic Acid (Propionic Acid) (C₂H₅COOH)

Amines Homologous Series

These are organic compounds that are created by substituting alkyl or aryl groups for one or more hydrogen atoms in ammonia (NH₃). One nitrogen atom with one lonely pair of electrons is present in them. Methylamine (C₂H₅NH₂), aniline (C₆H₅NH₂), and methylamine (CH₃NH₂) are a few examples.

Examples

• Methylamine (CH₃NH₂)
• Ethylamine (C₂H₅NH₂)
• Propylamine (C₃H₇NH₂)

Characteristics of Homologous Series

The characteristics of homologous series are mentioned below:

Mass and Molecular Formula: Each succeeding member of homologous series differ by -CH₂ in Molecular formula and 14 unit of mass.

Chemical Reactions: Different homologous series members may display varying degrees of reactivity even if they have identical chemical features. While all alkenes, for instance, experience addition reactions, the particular reactions and reactivity may differ slightly based on the molecule’s structure.

Physical Characteristics: In terms of solubility, melting and boiling temperatures, homologous series frequently show consistent patterns.

Functional Groups: The members of every homologous series exhibit comparable chemical behavior due to the presence of a certain functional group. For instance, the -OH functional group in the alcohol series is in charge of all members’ shared processes, such as esterification and oxidation.

Isomerism: There are several forms of isomerism that members of a homologous series might display, such as geometric and structural isomerism. One of the main reasons that a single homologous series may yield such a diverse range of compounds is the variation in isomerism.

Synthesis and Preparation: The process of synthesizing and preparing compounds within a homologous sequence typically involves employing similar chemical processes or synthetic pathways.

Significance of Homologous Series

Significance of Homologous Series are mentioned below:

• Homologus series is methodical arrangement of substances having comparable characteristics.
• It makes Trends in physical attributes, chemical reactivity, and molecular mass that are predictable.
• It makes studying and comprehending chemical molecules easier.
• It supports the synthesis, identification, and categorization of organic compounds.
• It makes it easier to deduce characteristics and actions of unknown substances.

Homologous Series Table

The table summarizes the homologues series of various organic compounds studied above:

Homologous Series	General Formula	Characteristic Functional Group	1st Homologue	2nd Homologue
Alkane	CnH2n+2	-CH3	CH4	C2H6
Alkane	CnH2n	-C=C-	C2H4	C3H6
Alkyne	CnH2n-2	-C[Tex]\equiv [/Tex]C-	C2H2	C3H4
Alcohol	CnH2n+1OH	-OH	CH3OH	C2H5OH
Carboxylic Acid	CnH2n+1COOH	-COOH	HCOOH	CH3COOH
Ester	CnH2n+1COOCmH2m	-COO-	HCOOCH3	HCOOCH2CH3
Amine	CnH2n+1NH2	-NH2	CH3NH2	CH3CH2NH2
Halogenoalkane (Haloalkane)	CnH2n+1X	-X (halogen)	CH3Cl	C2H5Cl

Homologous Series FAQs

What are Homologues?

Homologues are substances that belong to the same homologous series and differ only by a repeating unit and share comparable structures and chemical characteristics.

What are 7 Homologous Series?

Seven homologous common series are,

• Alkanes
• Alkenes
• Alkynes
• Alcohols
• Carboxylic Acids
• Esters
• Amines

What are Properties of a Homologous Series?

Characteristics of a homologous series are,

• Due to the same functional group, they have comparable chemical characteristics.
• Physical characteristics gradually alter as molecule size increases.
• Consistent patterns in solubility, melting, and boiling points.
• Molecular weight steadily increasing with each new component.

What is Difference between Homologous Series and Functional Group?

A functional group is a particular set of atoms that give a molecule its distinctive chemical properties, whereas a homologous series is a collection of compounds with similar structures and characteristics because they share a repeating unit. This is the primary distinction between the two terms.

What is General Formula of Homologous series?

A homologous series’ general formula is the most basic atom-to-atom difference for a molecule in that series. Alkanes, for instance, have the general formula C_nH_2n+2, where n is number of carbon atoms.

Which is First Member of Homologous Series?

A homologous series’ first member usually has the simplest structure and shortest molecular size. Methane (CH₄), for instance, is the first molecule in the alkane series