The increasing order of acid strength of 4-Methoxyphenol (I), 4Nitrophenol (II) and 4-Methylphenol (III) is (a) \(\mathrm{III}<\mathrm{I}<\mathrm{II}\) (b) \(\mathrm{I}<\mathrm{II}<\mathrm{III}\) (c) \(I<\mathrm{III}<\mathrm{II}\) (d) \(\mathrm{III}<\mathrm{II}<\mathrm{I}\)

Understanding the impact of different substituents on acidity is crucial for predicting the behavior of organic molecules in different environments. Essentially, a substituent can either be an electron withdrawing group (EWG) or an electron donating group (EDG), affecting the molecule's acidity. An EWG, relative to hydrogen, makes the molecule more acidic by stabilizing the negative charge on the conjugate base through resonance or inductive effects. Conversely, an EDG donates electron density to the conjugate base, making it less stable and the parent molecule less acidic.

When substituents are attached to a benzene ring in phenolic compounds, they influence the acidity of the phenol group. For instance, nitro groups being EWGs increase acidity, while methoxy and methyl groups as EDGs tend to decrease it. Through this understanding, we can predict that substituents in different positions will alter the acidity of phenols differently, which is critical knowledge in fields such as medicinal chemistry and material science.

Importance of Electron Withdrawing Groups

Electron withdrawing groups play a pivotal role in determining the acid strength of a molecule. They exert a pull on the electron cloud, especially when attached to aromatic systems such as phenols. Groups such as -NO2 have a strong -I (inductive) effect and -R (resonance) effect, drawing electrons away from the oxygen atom in the hydroxyl group. This electron withdrawal stabilizes the negative charge that forms on the oxygen atom when the hydrogen atom is removed as a proton, thereby making the molecule more acidic.

For instance, the presence of a nitro group in 4-nitrophenol considerably increases its acidity compared to phenol itself. Understanding these effects is extremely useful for designing synthetic routes in organic chemistry and designing molecules with desired acidity for various applications.

Phenol's acidity is somewhat unique among alcohols due to the aromatic ring's ability to delocalize the negative charge after deprotonation, creating a phenoxide ion. This resonance stabilization makes phenol much more acidic compared to aliphatic alcohols. However, the acidity of phenol can be further altered by substituents. If a substituent can donate a lone pair into the ring, such as a methoxy group, it will decrease the acidity by enhancing the negative charge on the oxygen in its phenoxide form. Meanwhile, a substituent without this ability, or one that withdraws electron density, enhances phenol's acidity.

Therefore, variations in the substituents directly influence the acid-base properties, affecting solubility, reactivity, and interactions of phenolic compounds. In biochemistry, these properties are pivotal as they influence the behavior of phenolic substances within biological systems and their pharmaceutical potential.

Comparing the acid strength of different compounds requires a good understanding of the factors influencing acidity. The key principle to consider is the stability of the conjugate base; the more stable the conjugate base, the stronger the acid. In phenolic compounds, substituents that are able to stabilize the negative charge on the oxygen atom increase the acidity of the compound. For instance, comparing the acid strength of methoxyphenol, nitrophenol, and methylphenol involves examining the influence of methoxy, nitro, and methyl groups, respectively.

As seen in the exercise, 4-nitrophenol (with an EWG) is the strongest acid among the given compounds, followed by 4-methoxyphenol and 4-methylphenol, which both contain EDGs. This order is correctly reflected in 4-methylphenol being the least acidic, demonstrating that the nature and position of substituents are significant determinants of a molecule's acid strength.