How do you account for the fact that, unlike most phenols, \(2,4-\) dinitrophenol and \(2,4,6\) -trinitrophenol are soluble in aqueous sodium bicarbonate?

Phenols are known for their unique acidic behavior, which sets them apart from alcohols despite both containing the hydroxyl (-OH) group. The acidity of phenols arises primarily from the resonance stabilization of the phenoxide ion formed once the proton is donated. When a phenol loses a hydrogen ion (H+), the negative charge on the oxygen can be delocalized throughout the aromatic ring.

This stabilization makes the loss of the proton more favorable, hence increasing the acidity. Factors such as the presence and position of substituents on the benzene ring greatly influence this acidity. Substituents that are electron-withdrawing, like the nitro groups in 2,4-dinitrophenol and 2,4,6-trinitrophenol, enhance this delocalization by pulling electron density through the ring, making the oxygens even more willing to relinquish their protons.

In layman's terms, think of the phenol molecule like a team playing tug-of-war. When the opposing team (the substituents) is strong and pulls hard (electron-withdrawing), it becomes much easier for the phenol to let go of the rope (donate a proton), resulting in a more acidic compound.

The molecular structure of phenol consists of a benzene ring, a hexagonal ring of carbon atoms with alternating double bonds, and a hydroxyl group (-OH) attached to one of the carbon atoms. Unlike in alcohols, where the hydroxyl group is bonded to a saturated carbon atom (sp3 hybridized), the hydroxyl group in phenol is bonded to an unsaturated carbon atom (sp2 hybridized) of the aromatic benzene ring.

This connection allows the electrons from the oxygen to resonate with the pi-electrons of the benzene ring, creating a system where the charges are more spread out over the structure. This spreading out, or delocalization, contributes to the unique properties of phenols, such as their acidity.

Moreover, when substituents like nitro groups are attached to the benzene ring, they can influence the electron density across the molecule. In our specific cases of 2,4-dinitrophenol and 2,4,6-trinitrophenol, the nitro groups are strategically positioned to have a maximized electron-withdrawing effect, which is essential for understanding their reactivity and solubility.

Electron-withdrawing groups (EWGs) are substituents that, through various mechanisms, draw electron density away from other parts of a molecule. These groups, such as nitro (-NO2), cyano (-CN), and carbonyl (-C=O) functionalities, have a profound impact on the chemical reactivity of the compounds they are part of. In the case of phenols, EWGs directly attached to the aromatic ring enhance the acidity by stabilizing the negative charge on the oxygen after deprotonation.

Their electron-withdrawing action can be attributed to two main effects: the inductive effect and the resonance effect. The inductive effect relates to the electronegativity of atoms pulling electron density along sigma bonds, while the resonance effect involves the contribution of EWGs to the delocalization of electrons via the pi system of the aromatic ring.

In summary, these groups act like 'electron thieves,' pulling away electron density and leaving behind a more acidic hydrogen that is easier for a base, such as sodium bicarbonate, to snatch away. The very property that allows EWGs to enhance acidity also explains why phenols with these groups are more likely to dissolve in basic solutions, creating the solubility observed in our exercise example.