Tutorial 7 — Acidity of Phenols

Learning Outcomes

Compare acidity of phenols with alcohols
Understand effect of substituents on phenol acidity
Analyze pKa values and acidity trends
Apply resonance and inductive effects

Part A: Phenol vs Alcohol Acidity

Question 1

a) Compare the acidity of phenol and cyclohexanol. Explain your answer using structural considerations.

Extracted answer: Phenol is more acidic than cyclohexanol because pKa value for phenol is lower than pKa value for cyclohexanol.

b) Write equations showing the ionization of phenol and cyclohexanol.

c) Explain how resonance stabilization of the phenoxide ion contributes to the enhanced acidity of phenol.

Question 2

Given pKa values:

Phenol: 9.9
Cyclohexanol: ~16

a) Calculate the Ka values for both compounds.

b) Which compound is more than 10⁶ times more acidic? Show your calculation.

Part B: Effect of Halogen Substituents

Question 3

Consider the pKa values of chlorophenols:

o-chlorophenol: ~8.6
m-chlorophenol: ~9.1
p-chlorophenol: ~9.4

a) What trend is observed in the pKa values?

Extracted answer: The trend observed is pKa value increasing from o- to m-, to p-chlorophenol. It means that o- is the most acidic among those three and p- is the least acidic.

b) Explain the observed trend using:

Inductive effects
Resonance effects (for ortho and para positions)

c) Why is the ortho isomer more acidic than the para isomer?

Question 4

a) Predict the order of acidity for the following compounds:

Phenol
2,4-dichlorophenol
2,4,6-trichlorophenol
Pentachlorophenol

b) Explain how multiple chlorine substituents affect acidity.

Part C: Effect of Nitro Substituents

Question 5

Consider the pKa values of nitrophenols:

o-nitrophenol: ~7.2
m-nitrophenol: ~8.3
p-nitrophenol: ~7.1

a) What trend is observed?

Extracted answer: The trend observed is pKa value for o- and p- are same and pKa value for m- is the highest among those three which means the m-nitrophenol is the least acidic than o- and p-.

b) Explain why ortho- and para-nitrophenols have similar acidity.

c) Explain why meta-nitrophenol is the least acidic of the three isomers.

d) Draw resonance structures showing the stabilization of the p-nitrophenoxide ion.

Question 6

Compare 4-chloro-2,6-dinitrophenol and 2-chloro-4,6-dinitrophenol:

Extracted answer: The pKa value for 4-chloro-2,6-dinitrophenol is higher than 2-chloro-4,6-dinitrophenol. It means that the acidity of 2-chloro-4,6-dinitrophenol is stronger than 4-chloro-2,6-dinitrophenol.

a) Explain why the position of the substituents affects acidity even when the same groups are present.

b) Which positions (ortho, meta, or para) have the greatest effect on phenol acidity? Why?

Part D: General Substituent Effects

Question 7

a) Classify the following substituents as electron-withdrawing or electron-donating:

-NO₂
-Cl
-CH₃
-OCH₃
-COOH

b) Predict how each substituent would affect phenol acidity at the ortho position.

c) Arrange the following in order of increasing acidity:

Phenol
p-methylphenol (p-cresol)
p-nitrophenol
p-methoxyphenol

Part E: Applications

Question 8

a) Explain why phenol derivatives are used as antiseptics and disinfectants.

b) How does the acidity of phenols relate to their biological activity?

c) Why is picric acid (2,4,6-trinitrophenol) classified as a strong acid?

Summary Table: pKa Values of Phenol Derivatives

Compound	pKa	Relative Acidity
Picric acid (2,4,6-trinitrophenol)	~0.4	Very strong
2,4-dinitrophenol	~4.1	Strong
p-nitrophenol	~7.1	Moderate
o-nitrophenol	~7.2	Moderate
Phenol	~9.9	Weak
m-nitrophenol	~8.3	Moderate
p-chlorophenol	~9.4	Weak
Cyclohexanol	~16	Very weak

Key Concepts

Phenol — Aromatic hydroxyl compound (Ar-OH)
pKa — Negative logarithm of acid dissociation constant
Inductive Effect — Electron withdrawal/donation through σ bonds
Resonance Effect — Electron delocalization through π bonds
Phenoxide Ion — Conjugate base of phenol
Substituent Effects — How functional groups affect properties

FAD1018 Tutorial 7 — Acidity of Phenols