The correct order of acid strength of the following compounds is
I. Phenol
II. p-cresol
III. m-nitro

Q: Why doesn't the $-M$ effect work at the meta position?

When you draw the resonance structures for the phenoxide ion, the negative charge delocalizes to the ortho and para positions, but never to the meta position. Since the charge never reaches the meta carbon, a substituent there cannot participate in resonance with the oxygen's lone pairs.

Q: Why is p-cresol less acidic than phenol?

The methyl group in p-cresol is an electron-donating group. It pushes electron density into the ring via $+I$ and hyperconjugation ($+H$) effects. This intensifies the negative charge on the phenoxide oxygen, destabilizing the conjugate base and making the parent acid weaker.

Q: What is the difference between $-I$ and $-M$ effects?

The Inductive ($-I$) effect is the pulling of electrons through sigma bonds due to electronegativity differences; it weakens with distance. The Mesomeric or Resonance ($-M$) effect is the delocalization of pi electrons through a conjugated system; it is generally much stronger than the inductive effect and operates effectively at ortho and para positions.

Q: How does hyperconjugation affect acidity in p-cresol?

Hyperconjugation involves the delocalization of sigma electrons from the $C-H$ bonds of the methyl group into the pi system of the benzene ring. This pushes electron density towards the phenoxide oxygen, destabilizing the anion and reducing acidity.

Q: Would o-nitrophenol be stronger or weaker than p-nitrophenol?

o-Nitrophenol is slightly weaker than p-nitrophenol. Although the $-I$ effect is stronger at the ortho position (due to proximity), o-nitrophenol forms an intramolecular hydrogen bond between the $-OH$ group and the $-NO_2$ group. This makes it slightly harder to release the proton compared to p-nitrophenol.

Question

The correct order of acid strength of the following compounds is
I. Phenol
II. p-cresol
III. m-nitrophenol
IV. p-nitrophenol

Options:

(A) III > II > I > IV

(B) IV > III > I > II

(C) II > IV > I > III

(D) I > II > IV > III

Sigma Percentile · Accepted Answer

### The Core Principle: Stability of the Conjugate Base

When comparing the acidic strength of organic compounds, the golden rule is to look at the stability of their conjugate bases. An acid donates a proton ($H^+$), leaving behind a negatively charged ion. If this resulting anion is stable, the parent acid is strong. If the anion is unstable, the parent acid is weak.

For phenols, the conjugate base is the **phenoxide ion**. Our job is to determine how different substituents on the benzene ring affect the stability of this negative charge.

### The Role of Substituents: Friends and Foes

Imagine the negative charge on the oxygen atom as a heavy backpack. 
- **Electron Withdrawing Groups (EWGs)** like $-NO_2$ act as helpful friends. They pull electron density towards themselves via inductive ($-I$) and resonance ($-M$) effects, effectively sharing the burden of the negative charge and stabilizing the ion.
- **Electron Donating Groups (EDGs)** like $-CH_3$ are the opposite. They push more electron density towards the oxygen via $+I$ and hyperconjugation ($+H$) effects, adding more weight to the backpack and destabilizing the ion.

### Analyzing the Contenders

Let's break down our four molecules:

**1. Phenol (I):** This is our baseline. It has no substituents, so its phenoxide ion has a standard level of stability.

**2. p-Cresol (II):** Here, we have a methyl group ($-CH_3$) at the para position. The methyl group is an EDG. Through its $+I$ and $+H$ effects, it intensifies the negative charge on the oxygen atom. Because the phenoxide ion is destabilized, p-cresol is a **weaker acid** than phenol. ($II < I$)

**3. m-Nitrophenol (III):** The nitro group ($-NO_2$) is a powerful EWG. However, it is located at the **meta position**. A critical rule in organic chemistry is that resonance ($-M$ or $+M$) effects do not operate at the meta position. Therefore, the $-NO_2$ group can only stabilize the phenoxide ion through its $-I$ effect. This still makes it a **stronger acid** than phenol. ($III > I$)

**4. p-Nitrophenol (IV):** In this molecule, the $-NO_2$ group is at the para position. From here, it can exert both its $-I$ effect and a strong $-M$ resonance effect. The negative charge on the oxygen can be delocalized all the way into the oxygen atoms of the nitro group. This provides maximum stabilization, making p-nitrophenol the **strongest acid** of the group. ($IV > III$)

### The Final Verdict

Combining our observations, the stability of the phenoxide ions follows the order: IV > III > I > II. Consequently, the acidic strength follows the exact same order.

**Final Order:** $IV > III > I > II$

Animated Solution for Chemistry - Organic Chemistry: The correct order of acid strength of the following compounds is I. Phenol II. p-cresol III. m-nitrophenol IV. p-nitrophenol

Select Answer:

Visualized Solution

$K_{a} \propto Stability of Conjugate Base$

$EWG vs EDG$

$p-Cresol: + I, + H Effects$

$m-Nitrophenol: - I Effect Only$

$p-Nitrophenol: - I, - M Effects$

$Final Order: IV > III > I > II$

$The Way Forward$

The Sigma Insight: Alcohols, Phenols, Ethers

The Core Principle

The Role of Substituents

Analyzing the Contenders

The Final Verdict

Similar Questions

Arrange the following compounds in the order of decreasing acidity I. p-chlorophenol II. p-cresol III. p-nitrophenol IV. p-methoxyphenol

The increasing order of the $p K_{a}$ values of the following compounds is

Arrange the following compounds in increasing order of $C - O H$ bond length : methanol, phenol, p-ethoxyphenol

The increasing order of boiling points of the following compounds is

Which of the following derivative of alcohols is unstable in an aqueous base?

Identify the major products A and B respectively in the following reactions of phenol.

Phenol on treatment with $CO_{2}$ in the presence of NaOH followed by acidification produces compound X as the major product. X on treatment with $(CH_{3} CO)_{2} O$ in the presence of catalytic amount of $H_{2} SO_{4}$ produces:

Which of the following reagents may be used to distinguish between phenol and benzoic acid ?

Which one of the following phenols does not give colour when condensed with phthalic anhydride in presence of conc. $H_{2} SO_{4}$ ?

Phenol is heated with a solution of mixture of KBr and KBrO $_{3}$ . The major product obtained in the above reaction is

Animated Solution for Chemistry - Organic Chemistry: The correct order of acid strength of the following compounds is I. Phenol II. p-cresol III. m-nitrophenol IV. p-nitrophenol

Select Answer:

Visualized Solution

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Ka​∝Stability of Conjugate Base

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } EWG vs EDG

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } p-Cresol: +I,+H Effects

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } m-Nitrophenol: −I Effect Only

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } p-Nitrophenol: −I,−M Effects

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Final Order: IV > III > I > II

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The Way Forward