Homologous Series
Alkanes
Alkanes are saturated hydrocarbons or paraffins (para means little, affin means affinity). Their general formula is CₙH₂ₙ₊₂, where ‘n’ is number of carbon atoms. In case of alkanes ‘n’ ranges from 1 to 40. In this way, alkanes form the most important homologous series of compounds.
Hydrocarbons like alkanes are regarded as parent organic compounds. All other compounds are considered to be derived from them by substituting one or more hydrogen atoms of a hydrocarbon by one or more reactive atom or group of atoms (functional group).
Definition of Homologous Series
Organic compounds are classified into classes based on the chemical properties. Each group or class is called homologous series which is a family or set of similar organic compounds having a same functional group which follows a regular structural pattern in which each successive members have a common difference of methylene (>CH₂) or by molecular mass of 14. Each member of series is called a Homologue of the other”. (in Greek, homo means same and logous means ratio).
A series of organic compounds
in which the various members have similar structural features having same
functional group and consecutive members differ from each other by methylene
group (–CH₂–) in their structures (or by molecular mass of 14) is called as
homologous series. Each individual
member of the series is called a Homologue of
the other”. (in Greek, homo means same
and logous means ratio) and the
phenomenon itself is called Homology.
OR
“The members of the same class of organic
compounds arranged in order of ascending molecular masses having same
functional group whose successive members differ from each other by an integral
number of methylene groups (–CH₂–) in their molecular formulae (or by molecular
mass of 14) are said to form a Homologous Series.”
OR
A set or series or group of structurally similar organic compounds in the increasing order of carbon atoms having same elements, same functional group, same general formula and identical chemical properties, show a gradation in physical properties, and any two adjacent members of which differ by –CH₂– units (methylene groups) is called Homologous Series. The individual members of the series are called homologues and the phenomenon itself is called Homology.
For example
Alkanes (hydrocarbon), alkenes (hydrocarbon), alkyl halides (halogen
derivative of alkanes) and alcohols are four common homologous series having
similar chemical properties showing a common difference of methylene group
between their successive members.
It is obvious from above formulae that
successive members of each family differ from its neighbour by methylene group
(–CH₂–). There is a gradual change in their physical properties with increasing
molecular weight but chemical properties are same. Such series is called
Homologous Series. It is a series of compounds in which adjacent members differ
by a –CH₂– unit.
Same features of All
Homologous Series
same elements, same functional group, same general formula, identical chemical properties, same general methods of preparation
Same features of All
Homologous Series
different molar masses, different physical properties
Aromatic Homologous Series
For arenes, successive members differ from its neighbour by –C4H2–group.
Examples of Common Homologous Series
2. Representation of each member by General Molecular
Formula
3. Synthesis of all Homologues by General Method of
Preparation
4. Identical Chemical Properties
5. Gradation in Physical Properties with increasing molar mass
1. Identical structures and common difference in composition
All members of a homologous series have identical structures. Successive members of a series always differ in composition by an integral number of methylene groups (–CH₂–). The difference between the molecular weights of two successive members of a series will be of 14 amu (u). The name of each member of a series begins either with a common prefix or suffix.
2. General Molecular Formula
The composition of all
members of a homologous series can be expressed by a general molecular formula.
3. General Method of Preparation
All members of a homologous series can be
prepared by a common general method of preparation
e.g. all members of alkanes can be prepared by
reduction of alkyl halide by nascent hydrogen [H]:
4. Identical Chemical Properties
All the members of
homologous series show similar chemical properties due to the presence of
common functional group (though the first homologue may considerably different
chemical properties). The elements and functional group present in compound of
a homologous series are same. Homologues show almost identical chemical
properties due to the presence of identical functional group.
e.g.
all alkanes are unreactive under ordinary conditions. They undergo combustion and substitution reaction with halogens.
5. Gradation in Physical Properties
The physical properties (such as solubility,
m.p, b.p, density) of the members of a series change gradually as the number of
carbon atom increases. There is a smooth gradation in the physical properties
of the members of a series such as physical state, solubility, melting and
boiling point etc. as the number of carbon atoms per molecules increases.
e.g. m.p, b.p and densities increase in alkane
series with increase of carbon atoms.
Summary
What is a Homologous Series?
A homologous series is a group of compounds that have:
-
The same functional group.
-
Similar chemical properties.
-
Gradually changing physical properties (like boiling point).
-
A general formula that applies to all members.
Example:
Alkanes form a homologous series:
-
Methane: CH₄
-
Ethane: C₂H₆
-
Propane: C₃H₈
Characteristics of Homologous Series
-
Members differ by a CH₂ unit.
-
Same functional group.
-
Similar chemical reactions.
-
Gradual change in physical properties (boiling/melting point).
-
Can be represented by a general formula.
Quick Tips for MDCAT/ECAT
-
Always memorize the general formula for each series.
-
Look for CH₂ differences in questions.
-
Remember that chemical properties are the same for the whole series.
-
Use real-life examples to remember series easily.
Remember functional groups → define chemical properties
Physical properties increase gradually with chain length
Practice MCQs for fast recall
MCQ Example for Practice
1.
🟥 A. Methane
🟦 B. Ethene
🟩 C. Ethanol
🟨 D. Propene
Question: Which of these is an alkane?
✅ Answer: A. Methane
2.
🟥 A. Propane
🟦 B. Butene
🟩 C. Methanol
🟨 D. Ethyne
Question: Which of these is a saturated hydrocarbon?
✅ Answer: A. Propane
3.
🟥 A. Ethene
🟦 B. Butane
🟩 C. Propyne
🟨 D. Methanol
Question: Which of these contains a double bond?
✅ Answer: A. Ethene
4.
🟥 A. Butanol
🟦 B. Butane
🟩 C. Ethyne
🟨 D. Methanal
Question: Which of these is an alcohol?
✅ Answer: A. Butanol
5.
🟥 A. Propene
🟦 B. Ethane
🟩 C. Methanol
🟨 D. Butyne
Question: Which of these is an alkene?
✅ Answer: A. Propene
6.
🟥 A. Ethyne
🟦 B. Propanol
🟩 C. Methane
🟨 D. Butanol
Question: Which of these contains a triple bond?
✅ Answer: A. Ethyne
7.
🟥 A. Butane
🟦 B. Methanal
🟩 C. Ethanol
🟨 D. Propene
Question: Which of these is a member of the alkane series?
✅ Answer: A. Butane
8.
🟥 A. Methanol
🟦 B. Ethane
🟩 C. Propene
🟨 D. Butyne
Question: Which of these is an alcohol?
✅ Answer: A. Methanol
9.
🟥 A. Ethane
🟦 B. Ethene
🟩 C. Ethyne
🟨 D. Ethanol
Question: Which of these is saturated?
✅ Answer: A. Ethane
10.
🟥 A. Propanol
🟦 B. Propene
🟩 C. Propane
🟨 D. Propyne
Question: Which of these is a hydrocarbon with no double or triple bonds?
✅ Answer: C. Propane
11.
🟥 A. Butyne
🟦 B. Butane
🟩 C. Butanol
🟨 D. Butene
Question: Which of these contains a triple bond?
✅ Answer: A. Butyne
12.
🟥 A. Ethanol
🟦 B. Ethyne
🟩 C. Ethane
🟨 D. Ethene
Question: Which of these is an alcohol?
✅ Answer: A. Ethanol
13.
🟥 A. Methane
🟦 B. Methanol
🟩 C. Methanal
🟨 D. Methylamine
Question: Which of these is a member of the alkane series?
✅ Answer: A. Methane
14.
🟥 A. Propene
🟦 B. Propyne
🟩 C. Propanol
🟨 D. Propane
Question: Which of these has a double bond?
✅ Answer: A. Propene
15.
🟥 A. Butane
🟦 B. Butene
🟩 C. Butyne
🟨 D. Butanol
Question: Which of these is a saturated hydrocarbon?
✅ Answer: A. Butane
16.
🟥 A. Methanol
🟦 B. Methanal
🟩 C. Methane
🟨 D. Methylamine
Question: Which of these is an alcohol?
✅ Answer: A. Methanol
17.
🟥 A. Ethyne
🟦 B. Ethene
🟩 C. Ethanol
🟨 D. Ethane
Question: Which of these contains a triple bond?
✅ Answer: A. Ethyne
18.
🟥 A. Propane
🟦 B. Propene
🟩 C. Propyne
🟨 D. Propanol
Question: Which of these is saturated?
✅ Answer: A. Propane
19.
🟥 A. Butanol
🟦 B. Butane
🟩 C. Butene
🟨 D. Butyne
Question: Which of these is an alcohol?
✅ Answer: A. Butanol
20.
🟥 A. Ethane
🟦 B. Ethene
🟩 C. Ethyne
🟨 D. Ethanol
Question: Which of these is a member of the alkane series?
✅ Answer: A. Ethane
21.
🟥 A. Pentane
🟦 B. Pentene
🟩 C. Pentyne
🟨 D. Pentanol
Question: Which of these is a saturated hydrocarbon?
✅ Answer: A. Pentane
22.
🟥 A. Hexene
🟦 B. Hexane
🟩 C. Hexanol
🟨 D. Hexyne
Question: Which of these contains a double bond?
✅ Answer: A. Hexene
23.
🟥 A. Heptyne
🟦 B. Heptane
🟩 C. Heptanol
🟨 D. Heptene
Question: Which of these contains a triple bond?
✅ Answer: A. Heptyne
24.
🟥 A. Hexanol
🟦 B. Hexane
🟩 C. Hexene
🟨 D. Hexyne
Question: Which of these is an alcohol?
✅ Answer: A. Hexanol
25.
🟥 A. Methane
🟦 B. Ethane
🟩 C. Propane
🟨 D. Butane
Question: Which of these follows the general formula CnH2n+2?
✅ Answer: All options (A, B, C, D) are alkanes
26.
🟥 A. Methanol
🟦 B. Ethane
🟩 C. Propene
🟨 D. Butyne
Question: Which of these is an alcohol?
✅ Answer: A. Methanol
27.
🟥 A. Ethanal
🟦 B. Ethanol
🟩 C. Ethyne
🟨 D. Ethene
Question: Which of these is an aldehyde?
✅ Answer: A. Ethanal
28.
🟥 A. Propanone
🟦 B. Propanol
🟩 C. Propene
🟨 D. Propane
Question: Which of these is a ketone?
✅ Answer: A. Propanone
29.
🟥 A. Methanoic acid
🟦 B. Methane
🟩 C. Methanol
🟨 D. Methylamine
Question: Which of these is a carboxylic acid?
✅ Answer: A. Methanoic acid
30.
🟥 A. Butanol
🟦 B. Butanal
🟩 C. Butanone
🟨 D. Butane
Question: Which of these is an alcohol?
✅ Answer: A. Butanol
31.
🟥 A. Butanal
🟦 B. Butanol
🟩 C. Butyne
🟨 D. Butane
Question: Which of these is an aldehyde?
✅ Answer: A. Butanal
32.
🟥 A. Pentanone
🟦 B. Pentanol
🟩 C. Pentane
🟨 D. Pentene
Question: Which of these is a ketone?
✅ Answer: A. Pentanone
33.
🟥 A. Ethanoic acid
🟦 B. Ethanol
🟩 C. Ethene
🟨 D. Ethyne
Question: Which of these is a carboxylic acid?
✅ Answer: A. Ethanoic acid
34.
🟥 A. Methanol
🟦 B. Methanal
🟩 C. Methane
🟨 D. Methylamine
Question: Which of these has an –OH functional group?
✅ Answer: A. Methanol
35.
🟥 A. Butanone
🟦 B. Butanol
🟩 C. Butanal
🟨 D. Butyne
Question: Which of these has a C=O group inside the chain?
✅ Answer: A. Butanone
36.
🟥 A. Propanoic acid
🟦 B. Propanol
🟩 C. Propanone
🟨 D. Propene
Question: Which of these is a carboxylic acid?
✅ Answer: A. Propanoic acid
37.
🟥 A. Ethanal
🟦 B. Ethanol
🟩 C. Ethyne
🟨 D. Ethene
Question: Which of these contains –CHO group?
✅ Answer: A. Ethanal
38.
🟥 A. Pentanol
🟦 B. Pentanal
🟩 C. Pentanone
🟨 D. Pentane
Question: Which of these is an alcohol?
✅ Answer: A. Pentanol
39.
🟥 A. Butanoic acid
🟦 B. Butane
🟩 C. Butanol
🟨 D. Butanal
Question: Which of these is a carboxylic acid?
✅ Answer: A. Butanoic acid
40.
🟥 A. Hexanone
🟦 B. Hexanol
🟩 C. Hexane
🟨 D. Hexanal
Question: Which of these is a ketone?
✅ Answer: A. Hexanone
41.
🟥 A. Methanal
🟦 B. Methanol
🟩 C. Methane
🟨 D. Methylamine
Question: Which of these is an aldehyde?
✅ Answer: A. Methanal
42.
🟥 A. Propanoic acid
🟦 B. Propanol
🟩 C. Propanone
🟨 D. Propene
Question: Which of these contains –COOH group?
✅ Answer: A. Propanoic acid
43.
🟥 A. Butanol
🟦 B. Butanone
🟩 C. Butanal
🟨 D. Butane
Question: Which of these is an alcohol?
✅ Answer: A. Butanol
44.
🟥 A. Ethanal
🟦 B. Ethanoic acid
🟩 C. Ethanol
🟨 D. Ethene
Question: Which of these is an aldehyde?
✅ Answer: A. Ethanal
45.
🟥 A. Methanoic acid
🟦 B. Methanal
🟩 C. Methanol
🟨 D. Methane
Question: Which of these is a carboxylic acid?
✅ Answer: A. Methanoic acid
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