Question

What is the complete IUPAC name of the following stereoisomer ?

• A.
  $\left(1S\right)-2-\text{Ethyl}-3,3-\text{dimethylcyclopentane}$
• B.
  $\left(1R\right)-2-\text{Ethyl}-3,3-\text{dimethylcyclopentane}$
• C.
  $\left(3S\right)-3-\text{Ethyl}-1,1-\text{dimethylcyclopentane}$
• D.
  $\left(3R\right)-3-\text{Ethyl}-1,1-\text{dimethylcyclopentane}$

Answer 1

The correct option is C

$\left(3S\right)-3-\text{Ethyl}-1,1-\text{dimethylcyclopentane}$
The IUPAC numbering of the compound starts from the carbon having two methyl substituents.
The IUPAC name of the compound will be $\left(3S\right)-3-\text{Ethyl}-1,1-\text{dimethylcyclopentane}$ where at $C_3$ position its configuration is $S$. At position $C_1$, there are two same groups(-$C{H}_3$) and so this position is achiral. Now at $C_3$, the fourth group(the group with the least priority) is above the plane, hence, the clockwise rotation for decreasing order of priority of groups would result in $S$-configuration.

Hence, (c) is correct.