In aromatic organic chemistry, arrange o-cresol, phenol, and o-nitrophenol in order of increasing acidity (lowest to highest) in aqueous solution.

Introduction / Context:
This question is about comparing the acidity of substituted phenols in organic chemistry. Phenol and its derivatives show different acid strengths depending on the nature of substituents attached to the benzene ring. Electron-donating groups such as alkyl groups decrease acidity, while electron-withdrawing groups such as nitro groups increase acidity. Additionally, the position (ortho, meta, para) of the substituent influences resonance and inductive effects. Understanding how these substituents affect acidity helps students predict reaction outcomes, choose appropriate reagents, and rationalise trends in organic reactivity.

Given Data / Assumptions:
- The three compounds to be ordered by increasing acidity are o-cresol (ortho-methyl phenol), phenol, and o-nitrophenol (ortho-nitrophenol).
- The order must be from least acidic to most acidic in aqueous solution.
- It is assumed that you know methyl groups are electron donating and nitro groups are strongly electron withdrawing.
- Basic principles of resonance, inductive effects, and stabilisation of the phenoxide ion apply.

Concept / Approach:
Acidity in phenols is largely determined by the stability of the phenoxide ion formed after deprotonation. Electron-donating groups like the methyl group in o-cresol push electron density towards the ring and the oxygen, destabilising the negative charge and decreasing acidity compared to unsubstituted phenol. Electron-withdrawing groups such as the nitro group in o-nitrophenol pull electron density away via both -I (inductive) and -R (resonance) effects, stabilising the phenoxide ion and increasing acidity relative to phenol. Therefore, in terms of acidity: o-cresol (with an electron-donating methyl group) is least acidic, phenol is intermediate, and o-nitrophenol (with a strongly electron-withdrawing nitro group) is most acidic.

Step-by-Step Solution:
Step 1: Consider o-cresol, which is ortho-methyl phenol. The methyl group is electron donating through its +I effect, increasing electron density on the ring and the oxygen, destabilising the negative charge that would form on oxygen in the phenoxide ion. Step 2: Compare this to phenol, which has no substituents on the ring. Its phenoxide ion is stabilised only by the normal resonance delocalisation of the negative charge over the aromatic ring. Step 3: Now consider o-nitrophenol, where the nitro group is a powerful electron-withdrawing group. It stabilises the phenoxide ion by pulling electron density away via both resonance and inductive effects. Step 4: The greater stabilisation of the phenoxide ion in o-nitrophenol means it more readily loses a proton, making it more acidic than phenol. Step 5: Conversely, the electron-donating methyl group in o-cresol makes its conjugate base less stabilised and therefore less acidic than phenol. Step 6: Putting these effects together, the acidity increases in the order: o-cresol (least acidic) < phenol < o-nitrophenol (most acidic).

Verification / Alternative check:
Experimental pKa values support this trend. Phenols with electron-donating alkyl groups generally have higher pKa values (weaker acids) than phenol, reflecting decreased acidity. Phenols with nitro substituents, especially in the ortho and para positions, show significantly lower pKa values, indicating stronger acidity. In o-nitrophenol, intramolecular hydrogen bonding between the nitro group and the hydroxyl hydrogen further stabilises the molecular form and influences acidity, but the dominant effect is the strong electron withdrawal by the nitro group stabilising the conjugate base. Reference tables consistently place nitrophenols as more acidic than unsubstituted phenol and alkyl-substituted phenols as less acidic, confirming the order o-cresol < phenol < o-nitrophenol.

Why Other Options Are Wrong:
The order phenol < o-nitrophenol < o-cresol implies that o-cresol is the most acidic, which contradicts the electron-donating nature of the methyl group and experimental data. The order o-nitrophenol < o-cresol < phenol suggests that nitro substitution makes the compound less acidic, which is opposite to the known strong electron-withdrawing effect of nitro groups. None of the above is incorrect because there is a correct and well established ordering based on substituent effects. Therefore, options B, C, and D do not match the real acidity trend.

Common Pitfalls:
Students sometimes assume that any substituted phenol must be more acidic than phenol, ignoring whether the substituent is electron donating or withdrawing. Another mistake is to focus only on the number of substituents rather than their nature, or to misremember that methyl is an electron donating group and nitro is electron withdrawing. Confusion can also arise from not distinguishing inductive and resonance effects. To avoid these pitfalls, always analyse whether a substituent donates or withdraws electron density and how that affects the stability of the conjugate base. In this case, methyl decreases acidity, nitro increases acidity, and phenol lies in between.

Final Answer:
The correct order of increasing acidity (lowest to highest) is o-cresol < phenol < o-nitrophenol.