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Solved Examples on Isomerism Question 1. How many chain isomers does butylene have? Solution: Butylene have two chain isomers, 1. n-Butane : CH3CH2CH=CH2 2. Isobutylene: CH3C(CH3)=CH2 Question 2. How many chain isomers does propyl benzene have? Solution: Question 3. Give the possible chain isomers for ethyl benzene. Solution: Question 4. State whether the following compound is optically active or inactive. The molecule contains one chiral carbon and so must be optically active. Isotopes are regarded as different atom Question 5. The specific rotation of R— (—) –2– bromooctane is –36°. What is percentage composition of a mixture of enantiomers of 2-bromooctane where rotation is +18°? Solution. Let X = mole fraction of R, => (1–X) = mole fraction S. X(–36°) + (1–X) 36° = 18° or, X = 1/4 The mixture has 25% R and 75%, it is 50% racemic and 50% S. Question 6 : C4H8 can have so many isomers. Write their structures. What are different tests to make distinction between them Solution: C4H8 can have following structures: (a) CH3—CH2—Ch=CH2 1-butene (b) CH3—CH=CH—CH3 2-butene, (cis and trans) (c) CH3—C(CH3)=CH2 2-methyl-1-propene (d) Cyclobutane (e) methyl cyclopropane Distinctions: i) Ozonolysis of (a) forms CH3CH2CHO and HCHO or (b) forms CH3CHO of (c) forms (CH3)2C=O and HCHO ii) Question 7 : Arrange the following species in increasing order of dipole moment. Cis – 2, 3 –dichloro-2-butene Cis – 1,2 –dichloroethene Cis-1,2-dibromo-1,2-dichloroethene Trans -1,2 -dichlorethene? Solution: In I, there is addition of vector In II, there will neither be addition nor subtraction of vector In III, there is subtraction of vector In IV, the vectors almost cancel each other. So the increasing order of dipole moment is IV < III < II < I. Question 8: Write all possible structural isomers of the compound with molecular formula C5H12O. Solution: Total number of structural isomers = 7 Question 9: Write the following Alkenes in increasing order of their stability with explanation R2C=CR2, R2C=CHR, R2C=CH2, RCH=CH2, CH2=CH2 Solution: From (a) to (e) number of hyperconjugative structures increases, hence stability of alkene also increases, hence the correct order is a < b < c < d < e. Question 10: The compound having molecular formula C4H10O can show Metamerism Functional isomerism Positional isomerism All Solution: Alcohols show position isomerism; Ethers show metamerism; Alcohol and ethers shows functional isomerism. Hence, the correct option is 4. Click here to refer Revision notes on Organic Chemistry, IIT JEE Organic Chemistry Syllabus and Chemistry books
Question 1. How many chain isomers does butylene have?
Solution: Butylene have two chain isomers,
1. n-Butane : CH3CH2CH=CH2
2. Isobutylene: CH3C(CH3)=CH2
Question 2. How many chain isomers does propyl benzene have?
Solution:
Question 3. Give the possible chain isomers for ethyl benzene.
Question 4. State whether the following compound is optically active or inactive.
The molecule contains one chiral carbon and so must be optically active. Isotopes are regarded as different atom
Question 5. The specific rotation of R— (—) –2– bromooctane is –36°. What is percentage composition of a mixture of enantiomers of 2-bromooctane where rotation is +18°?
Solution.
Let X = mole fraction of R,
=> (1–X) = mole fraction S.
X(–36°) + (1–X) 36° = 18°
or, X = 1/4
The mixture has 25% R and 75%, it is 50% racemic and 50% S.
Question 6 : C4H8 can have so many isomers. Write their structures. What are different tests to make distinction between them
C4H8 can have following structures:
(a) CH3—CH2—Ch=CH2 1-butene
(b) CH3—CH=CH—CH3 2-butene, (cis and trans)
(c) CH3—C(CH3)=CH2 2-methyl-1-propene
(d) Cyclobutane
(e) methyl cyclopropane
Distinctions:
i) Ozonolysis of (a) forms CH3CH2CHO and HCHO or (b) forms CH3CHO of (c) forms (CH3)2C=O and HCHO
ii)
Question 7 : Arrange the following species in increasing order of dipole moment.
In I, there is addition of vector
In II, there will neither be addition nor subtraction of vector
In III, there is subtraction of vector
In IV, the vectors almost cancel each other. So the increasing order of dipole moment is IV < III < II < I.
Question 8: Write all possible structural isomers of the compound with molecular formula C5H12O.
Solution: Total number of structural isomers = 7
Question 9: Write the following Alkenes in increasing order of their stability with explanation
R2C=CR2, R2C=CHR, R2C=CH2, RCH=CH2, CH2=CH2
From (a) to (e) number of hyperconjugative structures increases, hence stability of alkene also increases, hence the correct order is a < b < c < d < e.
Question 10: The compound having molecular formula C4H10O can show
Metamerism
Functional isomerism
Positional isomerism
All
Solution: Alcohols show position isomerism; Ethers show metamerism; Alcohol and ethers shows functional isomerism. Hence, the correct option is 4.
Click here to refer Revision notes on Organic Chemistry, IIT JEE Organic Chemistry Syllabus and Chemistry books
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