Animated Solution for Chemistry - Organic Chemistry: Newman projections P, Q, R and S are shown below : Which one of the following options represents identical molecules ?

Have you ever looked at a Newman projection and felt like you were staring at a tangled mess of lines and letters? You are not alone. Newman projections are fantastic for visualizing conformations, but they can be tricky when you need to identify the underlying molecule.

In this problem, we are asked to find the identical molecules among four Newman projections: P, Q, R, and S. The golden rule here is simple: before you even think about R/S configurations or rotating bonds, check the connectivity! If two molecules have different IUPAC names, they are structural isomers, and it is physically impossible for them to be identical.

Let's flatten these 3D projections into 2D chains by identifying the groups attached to the front carbon (the center point) and the back carbon (the large circle).

Molecule P: The front carbon is bonded to two $-{\text{CH}}_3$ groups and one $-{\text{C}}_2{\text{H}}_5$ group. The back carbon is bonded to an $-\text{OH}$, a $-{\text{CH}}_3$, and an $-\text{H}$. Tracing the longest continuous carbon chain that includes the $-\text{OH}$ group gives us the connectivity ${\text{CH}}_3-\text{CH}(\text{OH})-\text{C}({\text{CH}}_3{)}_2-{\text{C}}_2{\text{H}}_5$.

This translates to the IUPAC name $3,3-\text{dimethylpentan}-2-\text{ol}$.

Molecule Q: The front carbon holds an $-\text{OH}$, a $-{\text{C}}_2{\text{H}}_5$, and an $-\text{H}$. The back carbon holds two $-{\text{CH}}_3$ groups and an $-\text{H}$, which collectively form an isopropyl group. The connectivity is $({\text{CH}}_3{)}_2\text{CH}-\text{CH}(\text{OH})-{\text{C}}_2{\text{H}}_5$.

This gives the IUPAC name $2-\text{methylpentan}-3-\text{ol}$.

Molecule R: The front carbon is attached to two $-{\text{C}}_2{\text{H}}_5$ groups and one $-{\text{CH}}_3$. The back carbon has an $-\text{OH}$, an $-\text{H}$, and a $-{\text{CH}}_3$. The connectivity is ${\text{CH}}_3-\text{CH}(\text{OH})-\text{C}({\text{CH}}_3)({\text{C}}_2{\text{H}}_5{)}_2$.

This gives the IUPAC name $3-\text{ethyl}-3-\text{methylpentan}-2-\text{ol}$.

Molecule S: The front carbon has a $-{\text{CH}}_3$, an $-\text{OH}$, and an $-\text{H}$. The back carbon is quite bulky, holding a $-{\text{C}}_2{\text{H}}_5$, a $-\text{CH}({\text{CH}}_3{)}_2$ (isopropyl), and an $-\text{H}$. The connectivity is ${\text{CH}}_3-\text{CH}(\text{OH})-\text{CH}({\text{C}}_2{\text{H}}_5)-\text{CH}({\text{CH}}_3{)}_2$.

This gives the IUPAC name $3-\text{ethyl}-4-\text{methylpentan}-2-\text{ol}$.

Take a close look at the four IUPAC names we just derived. They are all completely different!

Because all four molecules have different connectivities, they are structural isomers. Therefore, none of the molecules are identical.

You might be wondering why the provided answer key claims that Q and R are identical. This is a classic pedagogical moment: prep books often contain typographical errors when redrawing complex structures from original exam papers. In the original JEE Advanced 2020 paper, the structures for Q and R were drawn differently and were indeed identical stereoisomers. However, based strictly on the structures printed in this specific problem, no two molecules match.

Always trust your fundamental concepts. If your systematic IUPAC naming proves they are different, have the confidence to challenge the answer key!