ISOMERISM IN ORGANIC COMPOUNDS
Isomerism is defined as the occurrence of two or more compounds with the same molecular formular but different molecular structures.
The different molecular structures are known as Isomers.
Isomers may belong to the same homologous series ( having the same functional group) or may belong to different homologous series (having different functional group)
Isomers with the same functional group, (i.e. belonging to the same homologous series) have similar chemical properties while isomers having different functional group (i.e. belonging to different homologous series) have different physical and chemical properties.
Example:- the formula C2H6O has two isomers
1. Ethanol an Alkanol with molecular formula CH3CH2OH has a melting point of 78 degree celçius and is a liquid at room temperature
H H
| |
H--C--C--OH
| |
H. H
and
2. an Alkoxy compound with molecular formula CH3OCH3 which is a gaseous at room temperature
H H
| |
H--C--O--C--H
| |
H H
TYPES OF ISOMERISM
(i) Structural isomerism
(ii) Stereo or geometric Isomerism
(iii) Optical isomerism
(I) STRUCTURAL ISOMERISM
This is the occurrence of two or more compounds with the same molecular formula but different molecular structures
TYPES OF STRUCTURAL ISOMERISM
(a) Chain isomerism: in this case, the isomer differs in the way the carbon atoms are arranged in the molecule. Eg
CH3CH2CH2CH3 CH3CHCH3
n-butane |
CH3
2-methylpropane (isobutane)
(‘n’ stands for the normal compound while ‘iso’ means the isomer of the normal compound).
NOTE: - Isomerism begins from the 4th member of the alkane homologous series, that is, butane, and as the number of carbon atoms increases the number of isomers also increases. For example, butane has two isomers normal-butane and 2-methyl propane, pentane (C5H12) has three isomers: n-pentane, 2-methylbutane and 2,2-dimethylpropane.
I. CH3CH2CH2CH2CH3. (n-pentane)
CH3
|
II. CH3CHCH2CH3 (2-methylbutane)
III. CH3
|
CH3-C-CH3
|
CH3 2,2-dimethylpropane
Three isomers for pentane
Another example is hexane (C6H14)
I. CH3CH2CH2CH2CH2CH3
n-hexane
CH3
|
II. CH3CHCH2CH2CH3 2-methylpentane
III. CH3
|
CH3CH2CHCH2CH3 3-methylpentane
CH3
|
IV. CH3CCH2CH3 2,2-dimethylbutane
|
CH3
CH3
|
V. CH3CHCHCH3 2,3-dimethylbutane
|
CH3
Hexane has 5 isomers
(b) position isomerism: Position isomers are those that have a substituent in different position on the same carbon skeleton. Example
CH3CH2CH2OH CH3CHCH3
propan-1-ol |
OH
propan-2-ol
(c) Functional Group isomerism: These are isomers having the same molecular formular but different functional groups. They belong to different homologous series.
e.g.
H
/
(i) CH3CH2C CH3-CCH3
\\ ‖
O O
Propanal (an aldehyde) propan-2-one (a ketone)
(ii) CH3CH2OH CH3-O-CH3
Ethanol ( Alkanol) Methoxymethane (dimethyl ether) (Alkoxy compounds)
(2) STEREO ISOMERISM OR GEOMETRIC ISOMERISM
In this type of isomerism, compounds have the same molecular formular but differ only in the way their atoms are arranged in space.
TYPES STEREO ISOMERISM
(a) Geometrical isomerism: This type of isomerism is found in compounds having either a double bond, triple bond or a ring structure. These multiple bonds prevent the free rotation about a carbon=carbon atom. that is, the carbon atoms are not cylindrically symmetrical. There are two forms of geometric isomers, the Cis and Trans isomer. For example, but-2-ene has two geometric isomer which are trans-but-2-ene and cis-but-2-ene as shown below.
when naming geometric isomers, you look at the substituents, where the substituents (group other than Carbon atoms) are on the same side of the double bond, that isomer is named the Cis isomer and when the substituents are on opposite sides of the double bond, we put a trans- isomer, but the names are the same
H CH3 H H
\ / \ /
C=C C=C
/ \ ∕ ∖
CH3 H CH3 CH3
Trans-but-2-ene Cis-but-2-ene
H Br H H
\ / \ /
C=C C=C
/ \ / \
Br H Br Br
Trans-1,2-dibromoethene Cis-1,2-dibromoethene
C1OOH. C4OOH H COOH
\ / \ /
C2=C3 C=C
∕ ∖ / \
COOH H H H
Trans-butenedioic acid Cis-butenedioic acid
(b) Optical isomerism: Optical isomers are isomers that have the same molecular and structural formular but cannot be superimposed on each other. In other words, an optical isomer is one which is not super-imposable on its mirror image. An optical isomer has at least one carbon atom which has four different groups or atoms attached to it. Such a carbon atom (surrounded by four different atoms /group) is called chiral carbon or chiral center. E.g. 2-hydroxylpropanoic acid (Lactic acid).
CH3
|
HO — *C— H
|
COOH
The Asterix C-atom is called a chiral C-atom because it is surrounded by four (4) different groups.
In glucose, there are several chiral carbon atoms.
H
∣
C = O
|
H — *C —OH
|
OH —*C —H
|
H — *C— OH
|
H —*C —OH
|
H — C—H
|
OH
Optical isomers are also called Enantiomers.
Isomers that belong to the same homologous series (same functional group) have similar chemical properties but may have different physical properties. But isomers which belong to different homologous series (different functional group) have different chemical and physical properties.
OBJECTIVE QUESTIONS
1. Isomerism is the phenomenon in which compounds have
A. same molecular mass only
B. same empirical formula only
C. same molecular formula but different structures
D. different molecular formula but same structure
2. Which of the following pairs are isomers?
A. CH₄ and C₂H₆
B. C₂H₆ and C₃H₈
C. C₄H₁₀ and C₄H₁₀
D. C₂H₄ and C₂H₆
3. Butane and 2-methylpropane are examples of
A. position isomers
B. functional isomers
C. chain isomers
D. geometric isomers
4. The two main classes of isomerism are
A. structural and stereoisomerism
B. chain and position
C. optical and functional
D. alkane and alkene
5. Which compound shows chain isomerism?
A. C₂H₆
B. C₃H₈
C. C₄H₁₀
D. CH₄
6. Intermediate Level
Position isomerism occurs due to different
A. molecular formula
B. carbon chain length
C. functional groups
D. positions of functional group or substituent
7. Ethanol and dimethyl ether are
A. chain isomers
B. position isomers
C. functional group isomers
D. optical isomers
8. Which of the following shows functional isomerism?
A. Propane and propene
B. Ethanol and ethanoic acid
C. Propanal and propanone
D. Butane and butene
9. Geometric isomerism is also known as
A. structural isomerism
B. optical isomerism
C. cis–trans isomerism
D. functional isomerism
10. Which compound can show geometric isomerism?
A. Ethane
B. Propane
C. But-2-ene
D. Methane
11. Which of these has the highest number of chain isomers?
A. C₃H₈
B. C₄H₁₀
C. C₅H₁₂
D. C₂H₆
12. Optical isomerism is caused by
A. double bonds
B. presence of aromatic ring
C. chiral carbon atom
D. branching
13. The pair CH₃CH₂CHO and CH₃COCH₃ are
A. chain isomers
B. position isomers
C. functional isomers
D. geometric isomers
14. Which statement is true about isomers?
A. They always have the same boiling point
B. They always have identical properties
C. They may have different physical and chemical properties
D. They must belong to different homologous series
15. The number of structural isomers for C₄H₈ (alkenes only) is
A. 1
B. 2
C. 3
D. 4
THEORY QUESTIONS
1.(a)(i)Define isomerism.
(ii). What are isomers?
(iii). State two differences between structural and stereoisomerism.
1(b)(i). Explain chain isomerism with one example.
(ii)Explain position isomerism with one example.
(iii). What is functional group isomerism?
1(c)(i). Define geometric isomerism.
State the condition necessary for geometric isomerism.
What is optical isomerism?
Give two examples of compounds that show functional isomerism.
Structured Questions
(a) Define structural isomerism
(b) List three types of structural isomerism
(c) Give one example each
(a) What is chain isomerism?
(b) Draw the structural formulas of the isomers of C₄H₁₀
(a) Explain functional isomerism
(b) Show the structures of ethanol and dimethyl ether
(a) Distinguish between cis and trans isomers
(b) Use but-2-ene to illustrate
Draw and name all the structural isomers of C₅H₁₂.
Long Essay Questions
(a) Define isomerism
(b) Describe the different types of structural isomerism
(c) Give two examples for each type
Explain geometric isomerism using suitable diagrams and discuss its importance in chemistry.
Write short notes on:
(i) Chain isomerism
(ii) Position isomerism
(iii) Functional group isomerism
(iv) Optical isomerism
List the structural isomers of C₄H₈ and classify them.
Describe the differences between structural and stereoisomerism with examples.
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