Question

How many ring isomers are possible in ${{C}_{3}}{{H}_{4}}C{{l}_{2}}$ ?
A. 1
B. 2
C. 3
D. 0

Answer 1

Always try to draw the structure of the compound before finding the isomers. After drawing the main structure, assign different positions to the functional groups present in the compound. That makes different compounds. Also before drawing the compound, find the DBE to know the presence of multiple bonds.

Complete step by step answer:
-Isomers are the compounds that have the same molecular formula of the compound but they differ in the properties and characteristics due to the difference in the structure of the compounds so formed.
-Organic compounds show mainly 2 types of isomerism namely structural isomerism and stereoisomerism. Structural isomers have different structures of the compounds and are subdivided into chain, position, ring and functional isomers.
-Stereoisomers have the same molecular formula also and same structure also. They differ in their orientation along the three dimensions and are classified as geometrical isomers and optical isomers. All of the isomers are counted as different compounds.
-To find the isomers, we first need to find the DBE which is called the double bond equivalent of a compound. It suggests the presence of a ring, double bond or triple bond. Its formula is
DBE = C+1-$\dfrac{\left( H+X-N \right)}{2}$
Where C=no. of carbon atoms
H=no. of hydrogen atoms
X=no. of monovalent atoms
N= no. of trivalent atoms

-DBE=1 suggests the presence of a ring or a double bond. Here we have to find the ring isomers. So we find the DBE of ${{C}_{3}}{{H}_{4}}C{{l}_{2}}$
-According to the formula, DBE will be equal to 3+1-$\left( \dfrac{4+2}{2} \right)$ =1. So there is only 1 ring and no double or triple bond in the compound. There are 3 carbon atoms and so the ring will be a 3-carbon ring.
-There are 2 chlorine atoms that can be placed at 3 carbon atoms in 2 different ways only. One way is keeping both chlorine atoms on the same carbon and the other way is keeping both chlorine atoms on different atoms. So there are two structural isomers present. They can be shown as

-Before concluding for the answer, we need to check for the presence of all the stereoisomers possible. It is possible for geometrical isomers in cis and trans form and optical isomers if there is a presence of a chiral centre. The second compound forms geometrical isomers also and so there will be 3 isomers in total. The isomer can be shown as

So, the correct answer is “Option C”.

Note: The DBE for a triple bond is counted as 2 and not 1. DBE represents the presence of pi bonds as well as the presence of rings. The isomers differ in their physical as well as chemical properties and so are very important to be distinguished.