Just like we have the first name that differentiates us from the others but our surnames that groups us with our family, organic compounds too can be classified into families. Organic compounds are divided into several groups depending on the general formula, properties, structures, etc. The series differentiating the organic compounds are known as Homologous series.

Methane is used as a fuel in the form of natural gas. Similarly, Propane is also used as a fuel in cooking gas. These have similar properties, but do they belong to the same family? Let’s dive in to discover this widespread family tree of organic chemistry. Continue reading to learn more!

What is a Homologous Series?

Organic compounds are divided into different groups having the same general formula, properties, and related structures. Such a series of organic compounds are known as homologous series.

A homologous series is a series of organic compounds whose members can be denoted by a single general formula and differ from each other by a \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group. The individual members of a homologous series are called homologues. While transforming a compound into its next homologue, \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group gets added to the chain.

Process of Homologous Series

The name or the nomenclature of an organic compound is written out with the substituents in an alphabetical order followed by the base name (derived from the number of carbon atoms in the parent chain). Commas are used between numbers, and dashes are used between letters and numbers.

There are no spaces in the name. The most frequently used nomenclature worldwide is the one created and developed by the International Union of Pure and Applied Chemistry (IUPAC). The basename or the parent name of an organic compound depends on the total number of carbon atoms. This is shown as below:

Name	Number of C’s in the Parent Chain
Meth-	\(1\)
Eth-	\(2\)
Prop-	\(3\)
But-	\(4\)
Pent-	\(5\)
Hex-	\(6\)
Hept-	\(7\)
Oct-	\(8\)
Non-	\(9\)
Dec-	\(10\)

Alkanes in Homologous Series

The simplest example of homologous series is the first four hydrocarbons: methane, ethane, propane, and butane.

Methane having one carbon atom is surrounded by four hydrogen atoms. Hence, its molecular formula is \({\rm{C}}{{\rm{H}}_4}.\) The chemical formula of ethane differs from that of methane by the \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group. Similarly, the propane formula differs from the ethane by the \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group. This shows that the homologous series occurs in succession.

The general formula of this homologous series Alkanes is \({{\rm{C}}_{\rm{n}}}\) \({{\rm{H}}_{{\rm{2n + 2}}}}\) where \({\rm{n = 1,2,3 \ldots }}\)
The combustion of all the above hydrocarbons produces carbon dioxide and water. This means that the organic compounds in the homologous series have similar chemical properties.

\({\rm{C}}{{\rm{H}}_{\rm{4}}}\left( {\rm{g}} \right){\rm{\; + \;2}}{{\rm{O}}_{\rm{2}}}\left( {\rm{g}} \right){\rm{\;}} \to {\rm{C}}{{\rm{O}}_{\rm{2}}}\left( {\rm{g}} \right){\rm{\; + \;2}}{{\rm{H}}_{\rm{2}}}{\rm{O\;}}\left( {\rm{g}} \right)\)
\({\rm{2C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{3}}}\left( {\rm{g}} \right){\rm{\; + 7}}{{\rm{O}}_{\rm{2}}}\left( {\rm{g}} \right)\) \( \to \) \({\rm{4C}}{{\rm{O}}_{\rm{2}}}\left( {\rm{g}} \right){\rm{\; + \;6}}{{\rm{H}}_{\rm{2}}}{\rm{O\;}}\left( {\rm{g}} \right)\)

The table above shows that with the addition of the \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group, there is an increase in the boiling point of the respective hydrocarbon.

Alkenes in Homologous Series

The homologous series of alkenes consists of ethene, propene, butene, pentene and so on. Ethene has two carbon atoms bonded to each other through a double bond and is surrounded by four hydrogen atoms. Hence, its molecular formula is \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{4}}}{\rm{.}}\) The chemical formula of propene differs from that of ethene by the \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group. Similarly, the butene formula differs from the propene by the \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group.

This shows that the homologous series of alkenes occurs in succession. The general formula of this homologous series alkynes is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n}}}},\) where \({\rm{n}} = 1,2,3 \ldots \)

Alkynes in Homologous Series

The homologous series of alkynes consists of ethyne, propyne, butyne, pentyne and so on. Ethyne has two carbon atoms connected by a triple bond. The two carbon atoms are surrounded by two hydrogen atoms. Hence, its molecular formula is \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{2}}}.\) The chemical formula of propyne differs from that of ethyne by the \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group.

Similarly, the butyne formula differs from the propyne by the \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group. This shows that the homologous series of alkynes occurs in succession.
The general formula of this homologous series alkynes is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n – 2}}}},\) where \({\rm{n}} = 1,2,3 \ldots \)

The chemical formula of propyne differs from that of ethyne by the \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group. Similarly, the butyne formula differs from the propyne by the \({\rm{ – C}}{{\rm{H}}_{\rm{2}}}\) group. This shows that the homologous series of alkynes occurs in succession.

Cycloalkane in Homologous Series

Carbon atoms can join to form rings of carbons. Cycloalkanes are simply cyclic alkanes. The simplest cycloalkane is cyclopropane. There are two hydrogen atoms less in a cycloalkane than its respective straight chain.

This is because the two ends of the molecule are joined together. These are saturated hydrocarbons, and the carbon atom is bonded to the maximum number of hydrogen atoms. As there is an additional bond present, cycloalkanes will have higher boiling points than their corresponding straight-chain partner.

The general formula of this homologous series cycloalkanes is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}}}},\) where \(\mathrm{n}=1,2,3 \ldots\)

Functional Groups in Homologous Series

When the \(-\mathrm{H}\) atom in methane is replaced by some other group such as hydroxyl group \({\rm{ – OH,}}\) then methane is converted into methanol, a type of alcohol with the formula \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{OH}}.\) This hydroxyl group is the functional group that, when attached to an alkyl carbon such as methyl- \({\rm{ – C}}{{\rm{H}}_{\rm{3}}}\) or ethyl \({\rm{ – }}{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}},\) forms a new class of organic compounds called alcohols.

Hence, a functional group is an atom or a group of atoms that replaces hydrogen in an organic compound and is responsible for the characteristic chemical properties of such compounds. These functional groups are the site of chemical reactions in an organic compound.

For example, primary alcohols get oxidised into carboxylic acids when heated in the presence of alkaline \({\rm{KMn}}{{\rm{O}}_{\rm{4}}}{\rm{.}}\) On the other hand, when alcohol is heated with concentrated sulfuric acid, it undergoes dehydration, i.e., removal of water, to produce an alkene:

\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \rightarrow \mathrm{CH}_{3} \mathrm{COOH}\left(\right.\) in the presence of alkaline \(\left.\mathrm{KMnO}_{4}\right)\)
\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \rightarrow \mathrm{C}_{2} \mathrm{H}_{2}+\mathrm{H}_{2} \mathrm{O}\left(\right.\) in the presence of conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) at \(\left.443 \mathrm{~K}\right)\)

The table below lists the general formula and the functional groups for various categories of organic compounds such as halides, alcohols, aldehydes, ketones, and organic carboxylic acid.

It can be observed from the above table that the functional groups for alkyl halides, alcohols, aldehydes, ketones, and carboxylic acids are the halogen atom, hydroxyl group, aldehydic group, ketonic group, and carboxyl group, respectively. Now, how is the naming for these organic compounds done, and what can be their homologous series? Read on to find out.

Alkyl Halides in Homologous Series

The alkyl halides have the general formula of \({\rm{R – X}}\) where \({\rm{R}}\) is some alkyl group, such as methyl \({\rm{ – C}}{{\rm{H}}_{\rm{3}}}\) or ethyl \({\rm{ – }}{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}},\) and \({\rm{X}}\) is a halogen atom fluorine \({\rm{F}},\) chlorine \({\rm{Cl}},\) bromine \({\rm{Br}},\) and iodine \({\rm{I}}.\)
Ethyl chloride \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}{\rm{Cl}}\) and butyl chloride \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{Cl}}\) are examples of alkyl halides.
In alkyl halides, the halogen atom is bonded to an alkyl group \(\left( {\rm{R}} \right)\) Thus, they form a homologous series represented by the general formula \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n + 1}}}}{\rm{X}}.\)

They are further classified as primary, secondary, or tertiary according to the nature of carbon to which halogen is attached. When naming an alkyl halide, the name of the alkyl residue or the longest parent chain carbon is written first, followed by the respective halide name. For example, \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{Cl}}\) is a three-carbon chain; therefore, the alkyl residue is propyl, and the halide is chloride and the name of the compound is propyl chloride.

Alkyl halide	Carbon number	Formula
Structure	\(1\)	\({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{Br}}\)
ethyl bromide	\(2\)	\({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{Br}}\)
propyl bromide	\(3\)	\({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{Br}}\)
butyl bromide	\(4\)	\({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{Br}}\)

Methyl bromide, ethyl bromide, propyl bromide, and butyl bromide have chemical formulas of \(\mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\), and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\), respectively.
This homologous series of alkyl bromides differs from each successive compound by a \({\rm{ – C}}{{\rm{H}}_2}\) group.

Alcohols in Homologous Series

Alcohols have the same general formula as alkanes except that they have the functional group \({\rm{ – OH}},\) called the hydroxyl group. The most common alcohol, known as ethanol, is used in alcoholic drinks, as fuel (gasoline), as a preservative for biological specimens, and as a solvent for paints and drugs.

In naming alcohols, the suffix of the name \({\rm{ – ol}},\) is added to the parent chain of the alkane name. The position of the \({\rm{ – OH}}\) functional group is indicated in the name. The numbering of the parent chain is done at the end closest to where the \({\rm{ – OH}}\) is located.
The general formula of alcohols homologous series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n + 1}}}}{\rm{OH,}}\) where \({\rm{n}} = 1,2,3..\)

The example of homologous series in alcohol can be methanol, ethanol, propanol, and butanol with chemical formulas of \(\mathrm{CH}_{3} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\), and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\), respectively, where each successive compound differs from the previous one by a \(- {\rm{C}}{{\rm{H}}_{\rm{2}}}\) group.

Aldehydes and Ketones in Homologous Series

The aldehydes and ketones contain a carbonyl group. A carbonyl group is a carbon atom double-bonded to an oxygen atom \(\mathrm{C}=\mathrm{O}\) while the carbon is also bonded to two other atoms or groups.
In an aldehyde, the carbonyl group is always on an end carbon.

To name aldehydes, use the longest parent chain name of the alkane and replace ‘e’ of ‘ane’ with the suffix ‘-al’. Thus, the name of \({\rm{C}}{{\rm{H}}_3}{\rm{CHO}}\) is ethanal, and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) is propanal. A homologous series for aldehydes is methanal, ethanal, propanal, and butanal with chemical formulas of \(\mathrm{HCHO}, \mathrm{CH}_{3} \mathrm{CHO}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\), and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\), respectively, where each successive compound differs from the previous one by a \({\rm{ – C}}{{\rm{H}}_2}\) group.

Aldehyde	Carbon number	Formula	Structure
Methanal	1	\(\mathrm{HCHO}\)
Ethanal	2	\(\mathrm{CH}_{3} \mathrm{CHO}\)
Propanal	3	\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\)

The general formula of the homologous aldehyde series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n}}}}{\rm{O}},\) where \(\mathrm{n}=1,2,3 \ldots\)
In a ketone, the carbonyl group is never on an end carbon. Acetone, for example, is a ketone and is the main component in nail polish remover, while cumin aldehyde is an aldehyde present in cumin seeds.
In order to name ketones, use the longest parent chain name of the alkane and replace ‘e’ of ‘-ane’ by the suffix ‘-one’. For parent chains with more than four carbon atoms, the position of the carbonyl group must be indicated.

For example, the name of the compound \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) is \(2\) -hexanone. The longest parent chain is six carbon atoms; thus, the name of the parent chain alkane is hexane, and since it is a ketone, the suffix -one is added in place of ‘e’ in ‘-ane’.

Now, the position of the ketonic group is determined to be in the second position as it is attached to the second carbon in the chain. Therefore, while writing the name of a ketone, first indicate the position of the ketone group, followed by a hyphen, and then write the parent chain name with the suffix ‘-one’.

A homologous series of ketones can include propanone, butanone, and \(2\) -pentanone with chemical formulas of \(\mathrm{CH}_{3} \mathrm{COCH}_{3}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{3}\), and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COCH}_{3}\), respectively, where each successive compound differs from the previous one by a \({\rm{ – C}}{{\rm{H}}_2}\) group.

Ketone	Carbon number	Formula
Propanone	1	\(\mathrm{CH}_{3} \mathrm{COCH}_{3}\)
Butanone	2	\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{3}\)
Pentanone	3	\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COCH}_{3}\)

The general formula of the homologous ketone series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n}}}}{\rm{O}},\) where \({\rm{n}} = 1,2,3 \ldots \)

Carboxylic Acids in Homologous Series

The organic acids (or carboxylic acids) contain the carboxyl group. A carboxyl group is a carbon atom double-bonded to an oxygen atom \(\mathrm{C}=\mathrm{O}\), and a hydroxyl group \(-\mathrm{OH}\), attached to a hydrogen atom or an alkyl group. Vinegar, chemical formula \(\mathrm{CH}_{3} \mathrm{COOH}\), is a common carboxylic acid called ethanoic acid and known as acetic acid.

When naming organic acids, use the parent chain name of the alkane, and add the suffix ‘oic acid’ by replacing the ‘e’ of the ‘-ane’.
For example, the organic compound with the chemical formula of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\) is propanoic acid, and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) is butanoic acid. The first compound contains a three-carbon chain; therefore, the parent chain name of the alkane is propane. Since it is a carboxylic acid, the ‘e’ of ‘-ane’ is replaced by ‘-oic acid’, making the name of the compound propanoic acid. Similarly, the name of the carboxylic acid with the four-carbon chain is butanoic acid.

Carboxylic acid	Carbon number	Formula
Methanoic acid	1	\(\mathrm{HCOOH}\)
Ethanoic acid	2	\(\mathrm{CH}_{3} \mathrm{COOH}\)
Propanoic acid	3	\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\)
Butanoic acid	4	\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\)

The general formula of the carboxylic acid homologous series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}}}}{{\rm{O}}_2},\) where \(\mathrm{n}=1,2,3 \ldots\)
A homologous series of carboxylic acids can include pentanoic acid, hexanoic acid, and heptanoic acid with chemical formulas of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\), respectively, where each successive compound differs from the previous one by a \({\rm{ – C}}{{\rm{H}}_2}\) group.

Esters in Homologous Series

An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one \( – {\rm{OH}}\) hydroxyl group is replaced by an \({\rm{ – O – alkyl}}\) (alkoxy) group, as in the substitution reaction of a carboxylic acid and an alcohol.

Esters are named as if the alkyl chain from the alcohol is a substituent. No number is assigned to this alkyl chain. This is followed by the name of the parent chain from the carboxylic acid part of the ester with an -e removed and replaced with the ending -oate.

Ester	Formula
Methyl acetate	\(\mathrm{CH}_{3} \mathrm{COOCH}_{3}\)
Ethyl acetate	\(\mathrm{CH}_{3} \mathrm{COOCH}_{2} \mathrm{CH}_{3}\)
Ethyl propionate	\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOCH}_{2} \mathrm{CH}_{3}\)

The general formula of the homologous ester series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n}}}}{{\rm{O}}_{\rm{2}}}\) where \(\mathrm{n}=1,2,3 \ldots\)
In the table given above, ethyl acetate differs from methyl acetate by a \( – {\rm{C}}{{\rm{H}}_{\rm{2}}}\) unit. Hence, they belong to the same homologous series.

Amides in Homologous Series

Amides are derived from carboxylic acids. A carboxylic acid contains the \( – {\rm{COOH}}\) functional group, and in an amide, the \( – {\rm{OH}}\) part of that group is replaced by an \( – {\rm{N}}{{\rm{H}}_{\rm{2}}}\) group. So, amides contain the \( – {\rm{CON}}{{\rm{H}}_{\rm{2}}}\) group.

Amides	Cardon number	Formula
Methanamide	1	\(\mathrm{HCONH}_{2}\)
Ethanamide	2	\(\mathrm{CH}_{3} \mathrm{CONH}_{2}\)
Propanamide	3	\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONH}_{2}\)

The general formula of the homologous amide series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n + 1}}}}{\rm{NO}},\) where \(\mathrm{n}=1,2,3 \ldots\)
A homologous series of amides include methanamide, ethanamide and propanamide, where each successive compound differs from the previous one by a \( – {\rm{C}}{{\rm{H}}_{\rm{2}}}\) group.

Nitroalkane in Homologous Series

The organic compounds that contain one or more nitro \(\left( { – {\rm{N}}{{\rm{O}}_{\rm{2}}}} \right)\) functional groups are called nitroalkanes. These compounds are named by placing the ‘nitro’ as a prefix to the alkanes.

Nitroalkanes	Carbon number	Formula	Structure
Nitromethane	1	\(\mathrm{CH}_{3} \mathrm{NO}_{2}\)
Nitroethane	2	\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NO}_{2}\)
Nitropropane	3	\({\left( {{\rm{C}}{{\rm{H}}_3}} \right)_2}{\rm{CHC}}{{\rm{H}}_2}{\rm{N}}{{\rm{O}}_2}\)

The general formula of the nitroalkanes homologous series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n + 1}}}}{\rm{N}}{{\rm{O}}_2},\) where \(\mathrm{n}=\) \(1,2,3 \ldots\)
A homologous series of nitroalkanes include nitromethane, nitroethane, nitropropane, where each successive compound differs from the previous one by a \( – {\rm{C}}{{\rm{H}}_{\rm{2}}}\) group.

Amines in Homologous Series

Amines are formally derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. The amines are named as a derivative of the alkane (or cycloalkane), having the appropriate number of carbon atoms by deleting the terminal \( – {\rm{e}}\) of the alkane and replacing it with the suffix –amine.

Amines	Carbon number	Formula	Structure
Methenamine	1	\(\mathrm{CH}_{3} \mathrm{NH}_{2}\)
Ethanamine	2	\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\)
Propanamine	3	\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\)

The general formula of the homologous amine series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n + 3}}}}{\rm{N}},\) where \(\mathrm{n}=1,2,3 \ldots\)

Order in Physical and Chemical Properties

What will happen to a train if an extra compartment is added to it? With every addition of a compartment to the train, its speed decrease by \(15\% \) but its use remains the same. In a similar way, the homologous series is a series of molecules that have similar chemical properties but show a trend in their physical properties.

1. The physical properties of members of a homologous series change gradually as the number of carbon atoms increases.

2. Each homologous series has its own functional group. These functional groups determine the chemical reactions that an organic molecule can undergo. Hence, all members of a homologous series have very similar chemical properties.
For example,

1. The carbon-carbon double bond allows all members of the homologous alkene series to undergo addition reactions.
2. Acidic properties of carboxylic acids are due to the presence of the carboxyl group,
3. The order in the physical and chemical properties of organic molecules in a homologous series makes it easy to predict the properties and behaviour of unknown members of the series. By studying the properties of a few members of a homologous series, it is possible to deduce the properties of the other members in the same series.

Characteristic Properties of Homologous Series

A homologous series is a group or family of organic compounds that have certain characteristics:
(a) Members of the series can be represented by a general formula.
(b) Successive members differ from each other by \({\rm{ – C}}{{\rm{H}}_2}\) or \(14\) amu.
(c) Physical properties change gradually with an increasing number of carbon atoms per molecule.
(d) Members have similar chemical properties because they have the same functional group.
(e) Members can be prepared by similar methods.

Summary

Alkanes, alkenes, alcohols, carboxylic acids and esters belong to different homologous series. Organic compounds’ organisation into the various homologous series makes the study of organic chemistry more systematic, orderly and effective. The members of any homologous series have properties that vary in a regular and predictable manner. There is order in all homologous series with respect to the nomenclature, physical properties and chemical properties. Furthermore, each homologous series has its own functional group. It helps in determining the chemical reactions that an organic molecule can undergo.

Study Classification of Organic Compounds

FAQs on Homologous Series

Q.1. What are the four characteristics of homologous series?
Ans: Four characteristics of a homologous series are:
(i) The general formula of all compounds in the series is the same.
(ii) They have the same functional group.
(iii) Their physical properties, such as melting point, boiling point, density, generally show a gradual change with an increase of molecular formula in the series.

Q.2. What is the importance of homologous series?
Ans: Homologous series is the characteristic feature of carbon compounds in which carbon and hydrogen atoms in hydrocarbons are varying by a single parameter. Homologous series helps in the determination of the structure of the successive member of the series, and the property of those members can also be predicted by their series.

Q.3. Why are cyclopropane and cyclopentane members of the same homologous series?
Ans: Even though they have the same general formula as the alkenes, the cycloalkanes are saturated and contain only single bonds. This means that they do not quickly decolourise bromine solution. Cyclopropane and cyclopentane differ by two \({\rm{ – C}}{{\rm{H}}_2}\) units. Hence, they belong to the same homologous series.

Q.4. How many compounds are present in the homologous series of nitroalkanes?
Ans: A homologous series of nitroalkanes involve nitromethane, nitropropane, nitroethane, where each successive compound differs from the previous one by a \( – {\rm{C}}{{\rm{H}}_{\rm{2}}}\) group.

Q.5. List down the examples of Homologous series.
Ans: Some of the examples of homologous series are methane, ethane, propane, and butane.

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