Question

Structural isomerism and stereoisomerism should be considered in answering this question. Compound $J$ is reacted with $KOH$ dissolved in ethanol. Three isomeric alkenes with molecular formula ${C_4}{H_8}$ are formed.
What is $J$?
A.${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{Br}}$
B.${\text{C}}{{\text{H}}_{\text{3}}}{\text{CH(Br)C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{3}}}$
C.${\text{C}}{{\text{H}}_{\text{3}}}{\text{CH(C}}{{\text{H}}_{\text{3}}}{\text{)C}}{{\text{H}}_{\text{2}}}{\text{Br}}$
D.$C{H_3}C(C{H_3})(C{H_3})Br$

Answer 1

Isomerism: This is defined as the phenomenon in which more than one compounds have the same chemical formula but they have different chemical structures.
Structural isomerism: They are defined as the compounds which differ in the connectivity between the atoms that constitute the compound.
Stereoisomerism: The compounds which have the same molecular formula but differ in the arrangements of atoms in three-dimensional structure.

Complete step by step answer:
Now we will talk about the question. In the question it is given that the compound $J$ is reacted with $KOH$ dissolved in ethanol. Three isomeric alkenes with molecular formula ${C_4}{H_8}$ are formed. So we know that when haloalkanes reacts with $KOH$ dissolved in ethanol then they form alkenes. And which alkene will form is decided by the number of hydrogen atoms at the adjacent carbon atoms. The alkene which will have the largest number of hydrogen atoms at the adjacent carbon atom will form in large quantity and other will also produce but in small quantity. So in the question it is given that the reaction forms three alkenes so the reactant must be ${\text{C}}{{\text{H}}_{\text{3}}}{\text{CH(Br)C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{3}}}$ because this compound on reaction with $KOH$ dissolved in ethanol will form three compounds i.e.
${\text{C}}{{\text{H}}_{\text{3}}}{\text{CH = CHC}}{{\text{H}}_{\text{3}}},{\text{C}}{{\text{H}}_2}{\text{ = CHC}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{3}}},{\text{C}}{{\text{H}}_{\text{3}}}{\text{CH(C}}{{\text{H}}_3}{\text{)C}}{{\text{H}}_{\text{3}}}$. Option A will produce only one alkene i.e. ${\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_{\text{2}}}{\text{CH = C}}{{\text{H}}_{\text{2}}}$.
Option C will produce only one alkene i.e. ${\text{C}}{{\text{H}}_{\text{3}}}{\text{C(C}}{{\text{H}}_{\text{3}}}{\text{) = CH}}$.
Option D will produce only one alkene i.e. $C{H_3}C(C{H_3}) = C{H_2}$.

Hence option B is correct.

Note:
Alkanes have the general formula as ${C_n}{H_{(2n + 2)}}$, where $n$ is the number of carbon atoms in the hydrocarbon. Alkenes have the general formula as ${C_n}{H_{(2n)}}$, where $n$ is the number of carbon atoms in the hydrocarbon. And similarly alkynes have the general formula as ${C_n}{H_{(2n - 2)}}$, where $n$ is the number of carbon atoms in the hydrocarbon.