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.
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 CnH2n+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 CnH2n+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
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. CnH2n+1OH is their general formula. Methanol (CH3OH), ethanol (C2H5OH), and propanol (C3H7OH) are a few examples.
Examples
Aldehyde Homologous Series
Aldehyde have functional group -CHO attached to the parent carbon chain. The general formula for Aldehyde homologous series is CnH2n+1CHO where n starts with zero. The members of Aldehyde Homologous series are mentioned below:
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 CnH2n+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 |
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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 CnH2n+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