Question

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?

Short answer

In summary, 2,4-dinitrophenol and 2,4,6-trinitrophenol are soluble in aqueous sodium bicarbonate due to their increased acidity compared to most other phenols. The presence of nitro groups in their molecular structure increases their acidity by withdrawing electrons, allowing them to react with sodium bicarbonate and form water-soluble salts and carbon dioxide gas. Most other phenols, with lower acidity, do not react readily with sodium bicarbonate and remain insoluble in the aqueous solution.

Step-by-step solution

Understanding Molecular Structure of Phenols

Phenols are a class of organic compounds with a hydroxyl (-OH) group bonded to a benzene ring. 2,4-dinitrophenol and 2,4,6-trinitrophenol are phenols with nitro (-NO2) groups attached to the benzene ring at specific positions. In the case of 2,4-dinitrophenol, there are two nitro groups at the 2nd and 4th carbon positions. For 2,4,6-trinitrophenol, there are three nitro groups, one at each of the 2nd, 4th, and 6th carbon positions.

Understanding Acidity of Phenols

The acidity of phenols is determined by their ability to donate a proton (H+) from the hydroxyl group. The electron-withdrawing nature and position of substituents on the benzene ring will affect the acidity of the phenol. In 2,4-dinitrophenol and 2,4,6-trinitrophenol, the nitro groups are strong electron-withdrawing groups, making the oxygen atom in the hydroxyl group more electron-deficient and thus, more willing to donate a proton. This increases the acidity of these phenols compared to other, less acidic phenols.

Solubility in Aqueous Sodium Bicarbonate

Sodium bicarbonate (NaHCO3) is a weak base that can react with acidic compounds to form water-soluble salts. When the acidic phenols (2,4-dinitrophenol and 2,4,6-trinitrophenol) are dissolved in aqueous sodium bicarbonate, they undergo an acid-base reaction forming water-soluble salts and carbon dioxide gas. The reaction can be represented by the following equation: Phenol (in an acidic form) + NaHCO3 -> Phenol (in a salt form) + H2O + CO2 The increased acidity of 2,4-dinitrophenol and 2,4,6-trinitrophenol, due to the electron-withdrawing nature of the nitro groups, allows them to react with sodium bicarbonate and become soluble in water. Most other phenols have a lower acidity and do not react readily with sodium bicarbonate, so they remain insoluble in the aqueous solution.

Conclusion

In summary, 2,4-dinitrophenol and 2,4,6-trinitrophenol are soluble in aqueous sodium bicarbonate due to their increased acidity compared to most other phenols. The nitro groups in their molecular structure act as strong electron-withdrawing groups, making the phenols more willing to donate protons and react with sodium bicarbonate. This reaction forms water-soluble salts, explaining their solubility in the aqueous solution.

Key concepts

Acidity of Phenols

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.

Phenol Molecular Structure

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

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.