why ortho nitrophenol are more volatile than para nitrophenol
Could it be that intermolecular forces in the para are lessened due to the location of the nitros?
Along the same lines as Bob Pursley suggested but with a slghtly different view, the ortho forms a 5-membered ring with the positive H of the OH and the negative O of the NO2 group. This is a strong INTRAmolecular force (within the same molecule). The p-nitrophenol is free to form the usual INTERmolecular bonds (H bonding between the OH of one molecule and the NO2 group of another molecule) which strengthens the crystal structure of the para isomer and that leads to a higher melting point for the para isomer. The ortho isomer lacks the forces between molecules that the para isomer has and it melts lower. My first year in grad school I was given a mixture of organic chemicals to separate and identify. o-nitrophenol was one of the compounds which I separated. It was near the end of the class period, I had to go to another class, I just put the solid on a piece of filter paper, placed it in my desk and went about my other classes. When I came back two days later to tackle the job of identification, ALL of it (and I do mean ALLLLL) was gone (it had completely volatilized) and I had to go through another 3-4 hours work to separate another batch. Tough lesson to learn BUT notice that the experience stayed me for the last 60 years and I remember to this day that o-nitrophenol is both smelly AND volatile. :-)
Ortho-nitrophenol is more volatile than para-nitrophenol due to the difference in their molecular structures and the position of the nitro group (-NO2) on the phenol ring.
To understand why ortho-nitrophenol is more volatile than para-nitrophenol, we need to consider the factors that influence volatility, such as intermolecular forces and molecular interactions.
1. Molecular structure: Ortho and para refer to the positions of the nitro group relative to the hydroxyl group (-OH) on the phenol ring. In ortho-nitrophenol, the nitro group is located adjacent to the hydroxyl group, while in para-nitrophenol, the nitro group is positioned opposite to the hydroxyl group.
2. Intermolecular forces: When a substance evaporates, intermolecular forces between its molecules must be overcome. These forces can include hydrogen bonding, dipole-dipole interactions, and van der Waals forces.
In ortho-nitrophenol, the spatial arrangement of the nitro group and hydroxyl group allows for stronger hydrogen bonding between neighboring molecules. The hydrogen atom in the hydroxyl group of one molecule can form a hydrogen bond with the oxygen atom in the nitro group of a neighboring molecule, enhancing the intermolecular attractions. As a result, more energy is required to break these stronger intermolecular forces, making ortho-nitrophenol less volatile.
On the other hand, in para-nitrophenol, the positioning of the nitro and hydroxyl groups prevents such strong hydrogen bonding. The hydrogen atom in the hydroxyl group is located far away from the oxygen atom in the nitro group of its neighboring molecule, limiting the formation of strong hydrogen bonds. Consequently, it requires less energy to break the intermolecular forces in para-nitrophenol, making it more volatile than ortho-nitrophenol.
In summary, ortho-nitrophenol is less volatile than para-nitrophenol due to the stronger intermolecular forces resulting from the spatial arrangement of the nitro and hydroxyl groups, which promote stronger hydrogen bonding.