Question

IUPAC name of the following compound is

A. Bicyclo $\left [{2, 1, 0} \right]$pentane
B. $1, 2 -$ cyclopropyl cyclobutane
C. $1, 2 -$ methyl cyclobutane
D. Cyclopantane $\left [{4, 3} \right]$annulene

Answer 1

Bicyclo compounds are the compounds that have two joined cyclic rings. The name of bicycle compounds follows the order prefix ‘bicyclo’ then the number of atoms join the bridging atoms in the bracket and then the total number of carbon atoms.

Complete step by step answer:
As the name indicates bicycle compounds contain two rings. The given compound contains two rings, one is three-membered and one is four-membered, so the given compound is a bicycle compound.
The way to the IUPAC name to the bicycle compound is as follows:
First, write the prefix ‘Bicyclo’.
Then determine the bridging atoms, the atoms at which the two rings join.
Then determine the number of atoms that join the two bridging atoms. For this give numbering form one bridging atom to another bridging atom, first follow the longest path and then follow the second-longest path.
Arrange the number of atoms that join the two bridging atoms in descending order in a bracket.
After that write the total number of carbon atoms of the compound.
The given compound has a total of five carbon atoms so the root name is pentane.

Now determine the number of atoms that join the bridging atoms as follows:

The numbering in red colour shows the first-longest path and the numbering in the blue colour shows the second-longest path. The first path has two connecting atoms and the second path has only one connecting atom and the third path has zero connecting atoms.
Arrange the connecting atoms in descending order as$\left[ {2,1,0} \right]$.
So, the name of the given compound is Bicyclo $\left[ {2,1,0} \right]$pentane.
The option (B), (C), and (D) do not have the prefix ‘bicyclo’ in the name whereas the given compound is a bicycle compound so, the options (B), (C) and (D) are incorrect.

Therefore the option (A), Bicyclo $\left[ {2,1,0} \right]$pentane, is correct.

Note:
Fused bicyclic compounds always have zero in brackets because one path is always there which connects the bridging atoms directly. Follow the correct path to determine the number of connecting atoms from longest to shortest.