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.

1. Cis – 2, 3 –dichloro-2-butene
2. Cis – 1,2 –dichloroethene
3. Cis-1,2-dibromo-1,2-dichloroethene
4. 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, R_­2C=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

1. Metamerism

2. Functional isomerism

3. Positional isomerism

4. All

Solution:  Alcohols show position isomerism; Ethers show metamerism; Alcohol and ethers shows functional isomerism. Hence, the correct option is 4. 

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