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A homologous series in organic chemistry refers to different sets of compounds grouped in accordance to their similarity in chemical structure, chemical properties and functional groups. The French chemist Charles Gerhardt originally suggested the concept of homologous series in 1843. One member of a homologous series can be changed into the following member by a chemical reaction called a homologation reaction.
The homologous series organizes a wide range of organic compounds in a very comprehensive manner. This series also helps us to understand the physical and chemical properties as well as the chemical behavior of organic compounds from the molecular point of view.
Read more: Classification of organic compounds
| Table of Content |
KeyTerms: Homologous series, Compounds, Alkanes, Carbon-carbon bond, Hydrogen, Methylene group.
What is a homologous series?
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A homologous series is actually a series of organic compounds which possess the same functional group. The molecular mass and hence the size of the compounds in a homologous series increases gradually due to addition of methylene group (CH2) at each successive level.
Methylene group
Each functional group forms its own homologous series. The successor or predecessor of a compound can be easily found out by adding or subtracting a methylene group respectively.
There are various series of compounds in organic chemistry- alkanes, alkenes and alkynes. Alkanes have a single carbon-carbon bond, alkenes have a double carbon-carbon bond, and alkynes have a triple carbon-carbon bond.
A particular homologous series is denoted by its general name and has a general formula.
Examples of homologous series
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For example, look at the case of simple organic compounds consisting of only carbon and hydrogen atoms joined by a C-C single bond. All such compounds have been categorized as alkanes. In the category of alkanes there can be many compounds such as methane, ethane, propane, butane, and so on. For the ease of remembrance and for the sake of organization all such compounds have been arranged in increasing order of their molecular masses and this increase is due to the addition of a methylene (CH 2 ) group at each successive level. This level of organization gives rise to the Homologous series.
In the same way numerous homologous series (Ketones, aldehydes, carboxylic acids, alcohols, amines, benzyl compounds, etc.) have been prepared. These are provided in the table given below:
| S.No. | Name of the series | General formula | Functional group | I-homologue | II-homologue |
|---|---|---|---|---|---|
| 1 | Alkane | C n H 2n+2 | NIL | CH4 | CH3 – CH3 |
| 2 | Alkene | C n H 2n | C=C | CH2 = CH2 | CH3 -CH=CH2 |
| 3 | Alkyne | C n H 2n-2 | C≡C | CH≡CH | CH3 -CH≡CH |
| 4 | Halo alkane | C n H 2n+1 X | (X → F, Cl, Br, I) | CH3X | CH3 –CH2 -X |
| 5 | Alcohol | C n H 2n+2 O | -OH | CH3OH | CH3 –CH2 -OH |
| 6 | Ether | C n H2n+2 O | -C-O-C- | CH3 – O – CH3 | CH3 –CH2–O-CH3 |
| 7 | Aldehyde | C n H 2n O | O ǁ -C- H | CH3 –C -H ǁ O | CH3 –CH2 - C -H ǁ O |
| 8 | Ketone | C n H 2n O | O ǁ -C- | O ǁ CH3 –C- CH3 | O ǁ CH3 –CH2 –C- CH3 |
| 9 | Carboxylic acids | C n H 2n O2 | O ǁ -C-OH | O ǁ CH3 -C-OH | O ǁ CH3CH2 -C-OH |
| 10 | Ester | C n H 2n O2 | O ǁ -O -C- | O ǁ CH3 –C- O-CH3 | O ǁ CH3- CH2–C- O-CH3 |
| 11 | Amide | C n H 2n+1 NO | -CONH2 | CH3 –CONH2 | CH3-CH2 –CONH2 |
| 12 | Nitro alkane | C n H 2n+1 NO | -N=O ↓ O | CH3-N=O ↓ O | CH3CH2-N=O ↓ O |
| 13 | Amine | C n H 2n+3 N | -NH2 | CH3 –NH2 | CH3-CH2 –NH2 |
Examples of homologous series
Characteristics of homologous series
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Each member of the series differs from the preceding one by the addition of CH3 group and by molecular mass of 14 a.m.u. 2.A homologous series has a similar formula that all of its members share. (Same elements and same functional group). For example, the general formula for alkane is C n H 2n+2 alkene is C n H 2n.
As the number of carbon atoms per molecule increases, or as molecular mass increases, the physical properties of the members change gradually, i.e
They exhibit gradation in properties.
For example melting point, boiling point and the density of the successive members of the homologous series increase with the increase in molecular mass.
- Members of a homologous series have similar chemical properties. For example: Methane reacts with chlorine to form methyl chloride.
| CH4 + Cl2 → CH,CI + HCl |
Similarly, ethane reacts to form ethyl chloride:
| C2H6 + Cl2 → C2HCl + HCl. |
- The same general method of preparation can be used to prepare each member of a homologous series.
For example: Alcohols are prepared by alkyl halides.
CH,Br + KOH boil, CH,OH + KBr. (aqueous)
C2H,Br + KOH (aqueous) boil →CH2OH + KBг.
Read more:
| Relevant Concepts | ||
|---|---|---|
| Haloalkanes & Haloarenes | Diethyl Ether | Finkelstein Reaction |
| Ester | Chlorate Formula | Change of State |
Significance of homologous series
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Homologous series is a method of classification of organic compounds which is widely used throughout the world and has generated convenience in the study and research. Here the significance of homologous series is listed below:-
- If the properties of the first few members of that series are known, it is possible to determine the nature of any compound in that family.
- Even the series' unprepared members can benefit from being predicted in terms of their characteristics.
- Knowledge of homologous series is useful for the systematic study of organic chemistry, since it saves learning time.
Things to remember
- Organic Compounds with similar physical and chemical properties are grouped into different groups depending upon the nature of the functional groups. Each such group is known as a homologous series.
- In each series the compounds are arranged in increasing order of the number of methylene groups (-CH2-).
- Each homologous series is denoted by its general name and its general formula.
- Some common examples of homologous series are- alkanes, alkene, alkynes, alcohols, ether, ester, carboxylic acid, ketone, aldehydes, amines, etc.
- One member of a homologous series can be changed into the following member by a chemical reaction called a homologation reaction.
Sample questions
Ques. What is the significance of the term homologous series? (1 mark)
Ans. In the homologous series, carbon compounds are dictated by the same functional group regardless of the length of the carbon chain. They are referred to as a "homologous series of carbon compounds."
Ques. What are the advantages of homologous series? (4 marks)
Ans. The advantages of homologous series are as follows:
- The compounds in a homologous series all have the same functional group, so the chemical characteristics of members of a homologous series are the same.
- This series has given scientists and engineers the chance to thoroughly examine a number of chemical compounds.
- Moreover, it gives an idea of the various chemical characteristics of compounds belonging to the same homologous series.
- By knowing the chemical behavior of one homologue we can predict the chemical behavior of another homologue.
Ques. What is a generic aldehyde formula? (2 marks)
Ans. The carbonyl group in aldehydes has one hydrogen atom linked to it, as well as a second hydrogen atom or a hydrogen group that can be an alkyl group or one that contains a benzene ring. Aldehyde has the general formula CnH2n+1 CHO or CnH2nO.
Ques. What does it mean when one series is homologous? (1 mark)
Ans. A homologous series is a collection of hydrocarbons that have the same general formula and have similar chemical properties. Alkanes, alkenes, and cycloalkanes are the three hydrocarbon series homologous to each other. Only hydrogen and carbon make up hydrocarbons.
Ques. Write the name of the first two compounds of the homologous series of amines. (3 marks)
Ans. A homologous series is a group of organic compounds having the same general formula and similar chemical properties in which the successive compounds differ by a (-CH2-) group.
The first homologue of amine is methanamine: (CH3-NH2)
The second homologue of amine is ethanamine: (CH3- CH2-NH2)
Ques. How to find out the formula of the lower and higher homologues of an alkene with a specific number of carbon atoms? (1 mark)
Ans. For a given compound of a specific number of carbon atoms, it's easy to find the lower homologue by subtracting (-CH2-)group and to find out the formula of its higher homologue add a (-CH2-) Group from the given compound.
Ques. Molecules in a homologous series have different boiling points because of differences in _____. (2 marks)
(a) methane groups
(b) London dispersion forces
(c) silanes
(d) hydrogen bonding
Ans. (a) methane group
Reason: It is found that the greater the number of methyl groups present in a compound the higher its boiling point is. The cause is rooted in the fact that increase in the number of methyl groups increases the molecular mass of the compound, leading to an increase in the boiling point.
Ques. Why is the first homologue of KETONE Proponone and not Methanone? (2 marks)
Ans. The structure of propanone is as follows:
In one molecule of propanone there are three carbon atoms and a ketonic compound is not possible with two or one carbon atom, hence the first homologue of ketone is propanone (according to the latest conventions of IUPAC).
Ques. Classify the following carbon compounds into two homologous series and name them. (5 marks)
(a) CH4
(b) C3H6
(c) C4H6
(d) C5H8
(e) C5H10
Ans. The compound given in the list are hydrocarbons:
- C3H4 is an alkyne.
- C3H6 is an alkene.
- C4H6 is an alkyne.
- C5H8 is also an alkyne.
- C5H10 is an alkene.
| Alkene (Cn H2n) | Alkyne (Cn H2n-2) |
| C3H6 | C3H4 |
| C5H10 | C4H6 |
| C5H8 |
Ques. Homologous differ from each other by one _____ group. (2 marks)
(a) Methylene
(b) Alcohol
(c) Ketone
(d) Amine
Ans. (a) Methylene
Reason: In a homologous series, the compounds are arranged in increasing order of their molecular masses. The molecular masses increase with addition of methylene group at each successive, thus the homologues differ from each other by one methylene group.
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