Animated Solution for Chemistry - Organic Chemistry: Given below are two statements: Statement I $o$-nitrophenol is steam volatile due to intramolecular hydrogen bonding. Statement II $o$-nitrophenol has high melting due to hydrogen bonding. In the light of the above statements, choose the most appropriate answer from the options given below.

To truly understand the physical properties of organic molecules, we must dive into their microscopic geometry. Let's look at the structure of $o$-nitrophenol. In this molecule, the hydroxyl ($-OH$) group and the nitro ($-N{O}_2$) group are situated right next to each other on the benzene ring, occupying the 1 and 2 positions (the ortho positions).

Because these two highly polar groups are in such close proximity, a fascinating quantum mechanical interaction occurs. The partially positive hydrogen atom of the $-OH$ group is strongly attracted to the partially negative oxygen atom of the adjacent $-N{O}_2$ group.

This attraction leads to the formation of a hydrogen bond directly within the same molecule. We call this intramolecular hydrogen bonding. Geometrically, this forms a highly stable, six-membered chelate ring. Because the molecule is essentially "holding hands with itself," its hydrogen bonding sites are internally occupied.

As a result, $o$-nitrophenol molecules do not associate strongly with neighboring molecules. They exist as discrete, independent monomers. Because the intermolecular forces holding these discrete units together are relatively weak (mostly just Van der Waals forces), it takes very little thermal energy to separate them. This makes $o$-nitrophenol highly volatile, allowing it to easily vaporize and be carried over by steam during distillation. Thus, it is steam volatile, making Statement I absolutely true.

Now, let's contrast this with its positional isomer, $p$-nitrophenol. In the para isomer, the $-OH$ and $-N{O}_2$ groups are on completely opposite sides of the benzene ring. Because of this vast spatial separation, they simply cannot reach each other to form an internal bond.

Instead, the hydrogen of one molecule reaches out and bonds with the oxygen of a neighboring molecule. This is known as intermolecular hydrogen bonding. This type of bonding creates a massive, extensive polymeric network, tying thousands of molecules tightly together.

To melt or boil $p$-nitrophenol, you must supply a massive amount of thermal energy to break this extensive intermolecular network. Consequently, $p$-nitrophenol has a significantly higher melting and boiling point.

Returning to $o$-nitrophenol, because it lacks this intermolecular network, it actually possesses a lower melting and boiling point compared to its isomers. Therefore, Statement II, which claims that $o$-nitrophenol has a high melting point due to hydrogen bonding, is fundamentally flawed and false.

Combining these insights, we conclude that Statement I is true, and Statement II is false, leading us directly to the correct option.