Chemistry/Chapter 10: Hydroxy Compounds
10.3 Acidity of water, phenol and ethanol
The main idea is that the acidity of a compound is determined by the stability of its conjugate base
Explanation level
In the previous sections, we discussed the preparation of esters and phenol. Now, we will explore a fundamental property of hydroxy compounds: their acidity. The acidity of a compound depends on its ability to donate a proton (H^+) and the stability of the resulting conjugate base. In this section, we compare the acidities of water, phenol, and ethanol, three compounds that all contain an -OH group but show very different acidic strengths.
Main Idea
The main idea is that the acidity of a compound is determined by the stability of its conjugate base. When an acid donates a proton, it forms a conjugate base. The more stable the conjugate base, the stronger the acid.
- Phenol is a stronger acid than water because the phenoxide ion (its conjugate base) is stabilized by resonance (the negative charge is delocalized into the aromatic ring).
- Water is a stronger acid than ethanol because the hydroxide ion (its conjugate base) is smaller and less polarizable than the ethoxide ion, making it more stable in aqueous solution.
- Ethanol is the weakest acid because the ethoxide ion is destabilized by the electron-donating inductive effect of the ethyl group and is less stabilized by solvation.
Core Concept
- Resonance: Delocalization of negative charge stabilizes the conjugate base.
- Inductive Effect: Electron-withdrawing groups stabilize the conjugate base; electron-donating groups destabilize it.
- Solvation: The interaction of the conjugate base with solvent molecules (e.g., hydrogen bonding) can stabilize it.
Detailed Explanation
The textbook states that acidity refers to the stability of the conjugate base formed by proton donation. Water is an acid and base. It is autoionized, but the autoionization constant of water is very low (Kw = 1 × 10-14). Therefore, water is a weak acid. Phenol is a stronger acid than water. The reason for this is that the phenoxide ion which is formed after the proton is lost, undergoes resonance stabilization. The negative charge of the phenoxide ion is delocalized into the ring. Therefore, phenoxide ions are stable. So, Ka of phenol (1.3 × 10-10) is higher than that of water (1.8 × 10-16).
Comparison of Acidities
| Compound | Conjugate Base | Ka (approx.) | pKa | Relative Acid Strength |
|---|---|---|---|---|
| Phenol (C_6H_5OH) | Phenoxide ion (C_6H_5O^-) | 1.3 × 10-10 | ~9.9 | Strongest |
| Water (H_2O) | Hydroxide ion (OH^-) | 1.8 × 10-16 | 15.7 | Intermediate |
| Ethanol (C_2H_5OH) | Ethoxide ion (C_2H_5O^-) | ~10-16 (similar to water) | ~16 | Weakest |
(Note: The Ka of ethanol is often quoted as ~10-16, which is similar to water. However, in terms of pKa, ethanol is slightly less acidic than water.)
Why is Phenol More Acidic Than Water?
- Resonance Stabilization: When phenol loses a proton, it forms the phenoxide ion (C_6H_5O^-). The negative charge on the oxygen atom is delocalized into the aromatic ring through resonance. This delocalization stabilizes the phenoxide ion, making it more stable than the hydroxide ion (which has no such resonance stabilization).
- Hybridization of the Oxygen Atom: The oxygen atom in phenol is attached to an sp² hybridized carbon, which is more electronegative than the sp³ hybridized carbon in water and ethanol. This makes the O-H bond in phenol more polar, facilitating proton loss.
Why is Water More Acidic Than Ethanol?
- Inductive Effect: In ethanol, the ethyl group (C_2H_5) is electron-donating (by inductive effect). This electron density pushes towards the oxygen atom, making the O-H bond less polar and making the ethoxide ion less stable (the negative charge is intensified). In water, there is no such electron-donating group.
- Solvation: The hydroxide ion is smaller and more strongly solvated by water molecules (through hydrogen bonding) than the larger ethoxide ion, making it more stable in aqueous solution.
Summary of Trends
- Acidity Order: Phenol > Water > Ethanol.
- Reason: Resonance stabilization of the phenoxide ion makes phenol the strongest acid. The electron-donating ethyl group destabilizes the ethoxide ion, making ethanol the weakest acid.
Intermediate Level
The acidity of a compound depends on the stability of its conjugate base. Phenol is more acidic than water because the phenoxide ion is stabilized by resonance, delocalizing the negative charge into the aromatic ring. Water is more acidic than ethanol because the ethyl group in ethanol is electron-donating by inductive effect, which destabilizes the ethoxide ion. Additionally, the hydroxide ion is smaller and more strongly solvated than the ethoxide ion, making it more stable. The Ka values reflect these differences: phenol (1.3 × 10-10) > water (1.8 × 10-16) > ethanol (~10-16). This order can be explained by the relative stability of the conjugate bases.
Important Points
- - Acidity order: Phenol > Water > Ethanol.
- - Phenol: Strongest acid due to resonance stabilization of the phenoxide ion.
- - Water: Intermediate acidity; hydroxide ion is more stable than ethoxide.
- - Ethanol: Weakest acid; ethoxide ion is destabilized by the electron-donating ethyl group.
- - Ka values: Phenol (1.3 × 10-10) > Water (1.8 × 10-16) > Ethanol (~10-16).
- - Resonance: Delocalization of negative charge stabilizes the conjugate base.
- - Inductive effect: Electron-donating groups destabilize the conjugate base.
Remember
- - Phenol is more acidic than water.
- - Water is more acidic than ethanol.
- - Phenol > Water > Ethanol.
- - Reason: Resonance stabilization of phenoxide.
- - Reason: Ethyl group is electron-donating (destabilizes ethoxide).
- - Ka of phenol: 1.3 × 10-10.
- - Ka of water: 1.8 × 10-16.
Common Mistakes
- - Thinking that all compounds with an -OH group have the same acidity. The acidity depends on the stability of the conjugate base, which varies with structure.
- - Forgetting that phenol is more acidic than water. The resonance stabilization of the phenoxide ion is the key reason.
- - Assuming ethanol is more acidic than water. The electron-donating ethyl group makes ethanol less acidic.
- - Confusing Ka and pKa values. A higher Ka means a stronger acid; a lower pKa means a stronger acid.
- - Neglecting the role of solvation. The solvation of the conjugate base can significantly affect acidity, especially in aqueous solutions.
Quick Revision
The acidity of a compound depends on the stability of its conjugate base. Among water, phenol, and ethanol, phenol is the strongest acid because its conjugate base (the phenoxide ion) is stabilized by resonance, which delocalizes the negative charge into the aromatic ring. Water is a stronger acid than ethanol because the ethyl group in ethanol is electron-donating by inductive effect, which destabilizes the ethoxide ion. Additionally, the hydroxide ion is smaller and more strongly solvated than the ethoxide ion. The Ka values are: phenol (1.3 × 10-10) > water (1.8 × 10-16) > ethanol (~10-16). This order reflects the relative stability of the conjugate bases.
60-Second Revision
Phenol is the strongest acid among phenol, water, and ethanol because its conjugate base (phenoxide) is stabilized by resonance. Water is more acidic than ethanol because the ethyl group in ethanol is electron-donating, destabilizing the ethoxide ion. The acidity order is Phenol > Water > Ethanol. The Ka of phenol is 1.3 × 10-10, and that of water is 1.8 × 10-16.
Frequently Asked Questions (FAQs)
Q1. Why is phenol more acidic than water? Answer: Phenol is more acidic than water because the phenoxide ion (the conjugate base of phenol) is stabilized by resonance. The negative charge on the oxygen atom is delocalized into the aromatic ring, making the phenoxide ion more stable than the hydroxide ion. This stabilization increases the tendency of phenol to donate a proton, making it a stronger acid.
Q2. Why is water more acidic than ethanol? Answer: Water is more acidic than ethanol because the hydroxide ion (the conjugate base of water) is more stable than the ethoxide ion (the conjugate base of ethanol). The ethyl group in ethanol is electron-donating by inductive effect, which intensifies the negative charge on the oxygen atom and destabilizes the ethoxide ion. Additionally, the smaller hydroxide ion is more strongly solvated by water molecules than the larger ethoxide ion.
- Q3. What is the acidity order of phenol, water, and ethanol?
- Answer: The acidity order is:
- Phenol > Water > Ethanol
- Phenol is the strongest acid, and ethanol is the weakest.
Q4. What is the Ka value of phenol? Answer: The Ka value of phenol is approximately 1.3 × 10-10. This is higher than the Ka of water (1.8 × 10-16), indicating that phenol is a stronger acid.
- Q5. What is the Ka value of water?
- Answer: The Ka value of water is approximately 1.8 × 10-16. This is very small, indicating that water is a weak acid.
Q6. Explain the resonance stabilization of the phenoxide ion and how it contributes to the acidity of phenol. Answer: When phenol loses a proton, it forms the phenoxide ion (C_6H_5O^-). The negative charge on the oxygen atom is delocalized into the aromatic ring through resonance, with the charge being distributed over the ortho and para positions. This delocalization stabilizes the phenoxide ion, making it more stable than the hydroxide ion (where the charge is localized on a single oxygen atom). Because the conjugate base is more stable, phenol is more willing to donate a proton, making it a stronger acid than water.
Q7. Why is ethanol a weaker acid than water? Answer: Ethanol is a weaker acid than water because the ethyl group (C_2H_5) is electron-donating by inductive effect. This electron density pushes towards the oxygen atom, intensifying the negative charge on the ethoxide ion and destabilizing it. In contrast, water has no such electron-donating group, so the hydroxide ion is more stable. Additionally, the hydroxide ion is smaller and more strongly solvated by water molecules.
- Phenol is the strongest acid because its conjugate base (phenoxide) is resonance-stabilized.
- Water is intermediate because its conjugate base (hydroxide) is smaller and more strongly solvated than ethoxide, and there is no destabilizing electron-donating group.
- Ethanol is the weakest acid because the ethyl group is electron-donating, which destabilizes the ethoxide ion.
Q9. What is the relationship between the stability of the conjugate base and the strength of the acid? Answer: The stronger the acid, the more stable its conjugate base. A stable conjugate base can better accommodate the negative charge left behind after the proton is lost. Factors that stabilize the conjugate base (such as resonance, electron-withdrawing groups, and solvation) increase the acidity of the corresponding acid.
Multiple Choice Questions (MCQs)
- Q1. Which of the following is the strongest acid?
- A. Water
- B. Ethanol
- C. Phenol
- D. Methanol
- Correct Answer: C
- Explanation: Phenol is the strongest acid among the given options because its conjugate base (phenoxide ion) is stabilized by resonance.
- Q2. Why is phenol more acidic than water?
- A. Because it has a higher molecular weight.
- B. Because the phenoxide ion is resonance-stabilized.
- C. Because it has a lower boiling point.
- D. Because it is a solid at room temperature.
- Correct Answer: B
- Explanation: The phenoxide ion is stabilized by resonance, making phenol a stronger acid than water.
- Q3. Which of the following has the lowest Ka value?
- A. Phenol
- B. Water
- C. Ethanol
- D. All have the same Ka
- Correct Answer: C
- Explanation: Ethanol has the lowest Ka value, making it the weakest acid among the options.
- Q4. What is the approximate Ka value of phenol?
- A. 1.3 × 10-10
- B. 1.8 × 10-16
- C. 1.0 × 10-7
- D. 1.0 × 10-14
- Correct Answer: A
- Explanation: The Ka of phenol is approximately 1.3 × 10-10.
- Q5. What is the approximate Ka value of water?
- A. 1.3 × 10-10
- B. 1.8 × 10-16
- C. 1.0 × 10-7
- D. 1.0 × 10-14
- Correct Answer: B
- Explanation: The Ka of water is approximately 1.8 × 10-16.
- Q6. Which of the following is the correct order of acidity?
- A. Ethanol > Water > Phenol
- B. Phenol > Water > Ethanol
- C. Water > Phenol > Ethanol
- D. Phenol > Ethanol > Water
- Correct Answer: B
- Explanation: The acidity order is Phenol > Water > Ethanol.
- Q7. Why is ethanol a weaker acid than water?
- A. Because it has a higher molecular weight.
- B. Because the ethyl group is electron-donating.
- C. Because it has a lower boiling point.
- D. Because it is a liquid at room temperature.
- Correct Answer: B
- Explanation: The ethyl group is electron-donating, which destabilizes the ethoxide ion.
- Q8. What is the conjugate base of phenol?
- A. Phenoxide ion
- B. Hydroxide ion
- C. Ethoxide ion
- D. Benzene
- Correct Answer: A
- Explanation: The conjugate base of phenol is the phenoxide ion (C_6H_5O^-).
- Q9. Which of the following is the weakest acid?
- A. Phenol
- B. Water
- C. Ethanol
- D. All are equally acidic
- Correct Answer: C
- Explanation: Ethanol is the weakest acid among the given options.
- Q10. What is the main factor that stabilizes the phenoxide ion?
- A. Inductive effect
- B. Resonance
- C. Hydrogen bonding
- D. Solvation
- Correct Answer: B
- Explanation: Resonance stabilizes the phenoxide ion by delocalizing the negative charge into the aromatic ring.
Multiple Choice Questions (MCQs)
- Q1. Which of the following is the strongest acid?
- A. Water
- B. Ethanol
- C. Phenol
- D. Methanol
- Correct Answer: C
- Explanation: Phenol is the strongest acid.
- Q2. Why is phenol more acidic than water?
- A. Because it has a higher molecular weight.
- B. Because the phenoxide ion is resonance-stabilized.
- C. Because it has a lower boiling point.
- D. Because it is a solid at room temperature.
- Correct Answer: B
- Explanation: The phenoxide ion is resonance-stabilized.
- Q3. Which of the following has the lowest Ka value?
- A. Phenol
- B. Water
- C. Ethanol
- D. All have the same Ka
- Correct Answer: C
- Explanation: Ethanol has the lowest Ka value.
- Q4. What is the approximate Ka value of phenol?
- A. 1.3 × 10-10
- B. 1.8 × 10-16
- C. 1.0 × 10-7
- D. 1.0 × 10-14
- Correct Answer: A
- Explanation: The Ka of phenol is 1.3 × 10-10.
- Q5. What is the approximate Ka value of water?
- A. 1.3 × 10-10
- B. 1.8 × 10-16
- C. 1.0 × 10-7
- D. 1.0 × 10-14
- Correct Answer: B
- Explanation: The Ka of water is 1.8 × 10-16.
- Q6. Which of the following is the correct order of acidity?
- A. Ethanol > Water > Phenol
- B. Phenol > Water > Ethanol
- C. Water > Phenol > Ethanol
- D. Phenol > Ethanol > Water
- Correct Answer: B
- Explanation: The acidity order is Phenol > Water > Ethanol.
- Q7. Why is ethanol a weaker acid than water?
- A. Because it has a higher molecular weight.
- B. Because the ethyl group is electron-donating.
- C. Because it has a lower boiling point.
- D. Because it is a liquid at room temperature.
- Correct Answer: B
- Explanation: The ethyl group is electron-donating.
- Q8. What is the conjugate base of phenol?
- A. Phenoxide ion
- B. Hydroxide ion
- C. Ethoxide ion
- D. Benzene
- Correct Answer: A
- Explanation: The conjugate base is the phenoxide ion.
- Q9. Which of the following is the weakest acid?
- A. Phenol
- B. Water
- C. Ethanol
- D. All are equally acidic
- Correct Answer: C
- Explanation: Ethanol is the weakest acid.
- Q10. What is the main factor that stabilizes the phenoxide ion?
- A. Inductive effect
- B. Resonance
- C. Hydrogen bonding
- D. Solvation
- Correct Answer: B
- Explanation: Resonance stabilizes the phenoxide ion.
Multiple Choice Questions (MCQs)
- Q1. What is the conjugate base of water?
- A. Hydroxide ion
- B. Phenoxide ion
- C. Ethoxide ion
- D. Hydronium ion
- Correct Answer: A
- Explanation: The conjugate base of water is the hydroxide ion (OH^-).
- Q2. What is the conjugate base of ethanol?
- A. Hydroxide ion
- B. Phenoxide ion
- C. Ethoxide ion
- D. Hydronium ion
- Correct Answer: C
- Explanation: The conjugate base of ethanol is the ethoxide ion (C_2H_5O^-).
- Q3. Which of the following is the strongest acid?
- A. C_6H_5OH
- B. H_2O
- C. C_2H_5OH
- D. CH_3OH
- Correct Answer: A
- Explanation: Phenol (C_6H_5OH) is the strongest acid.
- Q4. Which of the following is the weakest acid?
- A. C_6H_5OH
- B. H_2O
- C. C_2H_5OH
- D. HCl
- Correct Answer: C
- Explanation: Ethanol (C_2H_5OH) is the weakest acid.
- Q5. What is the pKa of phenol approximately?
- A. 5
- B. 10
- C. 15.7
- D. 16
- Correct Answer: B
- Explanation: The pKa of phenol is approximately 10.
- Q6. What is the pKa of water?
- A. 5
- B. 10
- C. 15.7
- D. 16
- Correct Answer: C
- Explanation: The pKa of water is 15.7.
- Q7. What is the pKa of ethanol approximately?
- A. 5
- B. 10
- C. 15.7
- D. 16
- Correct Answer: D
- Explanation: The pKa of ethanol is approximately 16.
- Q8. Which of the following is the correct order of pKa values?
- A. Phenol < Water < Ethanol
- B. Phenol > Water > Ethanol
- C. Water < Phenol < Ethanol
- D. Ethanol < Water < Phenol
- Correct Answer: A
- Explanation: The pKa order is Phenol (10) < Water (15.7) < Ethanol (16). A lower pKa means a stronger acid.
- Q9. What is the main reason for the difference in acidity between phenol and water?
- A. Resonance stabilization of the phenoxide ion.
- B. The electron-donating effect of the ethyl group.
- C. The difference in molecular weight.
- D. The difference in boiling points.
- Correct Answer: A
- Explanation: Resonance stabilization of the phenoxide ion is the main reason.
- Q10. What is the main reason for the difference in acidity between water and ethanol?
- A. Resonance stabilization of the ethoxide ion.
- B. The electron-donating effect of the ethyl group.
- C. The difference in molecular weight.
- D. The difference in boiling points.
- Correct Answer: B
- Explanation: The electron-donating effect of the ethyl group destabilizes the ethoxide ion, making ethanol less acidic than water.
Multiple Choice Questions (MCQs)
- Q1. What is the conjugate base of water?
- A. Hydroxide ion
- B. Phenoxide ion
- C. Ethoxide ion
- D. Hydronium ion
- Correct Answer: A
- Explanation: The conjugate base of water is the hydroxide ion.
- Q2. What is the conjugate base of ethanol?
- A. Hydroxide ion
- B. Phenoxide ion
- C. Ethoxide ion
- D. Hydronium ion
- Correct Answer: C
- Explanation: The conjugate base of ethanol is the ethoxide ion.
- Q3. Which of the following is the strongest acid?
- A. C_6H_5OH
- B. H_2O
- C. C_2H_5OH
- D. CH_3OH
- Correct Answer: A
- Explanation: Phenol is the strongest acid.
- Q4. Which of the following is the weakest acid?
- A. C_6H_5OH
- B. H_2O
- C. C_2H_5OH
- D. HCl
- Correct Answer: C
- Explanation: Ethanol is the weakest acid.
- Q5. What is the pKa of phenol approximately?
- A. 5
- B. 10
- C. 15.7
- D. 16
- Correct Answer: B
- Explanation: The pKa of phenol is approximately 10.
- Q6. What is the pKa of water?
- A. 5
- B. 10
- C. 15.7
- D. 16
- Correct Answer: C
- Explanation: The pKa of water is 15.7.
- Q7. What is the pKa of ethanol approximately?
- A. 5
- B. 10
- C. 15.7
- D. 16
- Correct Answer: D
- Explanation: The pKa of ethanol is approximately 16.
- Q8. Which of the following is the correct order of pKa values?
- A. Phenol < Water < Ethanol
- B. Phenol > Water > Ethanol
- C. Water < Phenol < Ethanol
- D. Ethanol < Water < Phenol
- Correct Answer: A
- Explanation: The pKa order is Phenol (10) < Water (15.7) < Ethanol (16).
- Q9. What is the main reason for the difference in acidity between phenol and water?
- A. Resonance stabilization of the phenoxide ion.
- B. The electron-donating effect of the ethyl group.
- C. The difference in molecular weight.
- D. The difference in boiling points.
- Correct Answer: A
- Explanation: Resonance stabilization of the phenoxide ion is the main reason.
- Q10. What is the main reason for the difference in acidity between water and ethanol?
- A. Resonance stabilization of the ethoxide ion.
- B. The electron-donating effect of the ethyl group.
- C. The difference in molecular weight.
- D. The difference in boiling points.
- Correct Answer: B
- Explanation: The electron-donating effect of the ethyl group destabilizes the ethoxide ion.
Multiple Choice Questions (MCQs)
- Q1. Which of the following compounds is the most acidic?
- A. Phenol
- B. Water
- C. Ethanol
- D. Methanol
- Correct Answer: A
- Explanation: Phenol is the most acidic.
- Q2. Which of the following compounds is the least acidic?
- A. Phenol
- B. Water
- C. Ethanol
- D. Methanol
- Correct Answer: C
- Explanation: Ethanol is the least acidic.
- Q3. What is the Ka of phenol?
- A. 1.3 × 10-10
- B. 1.8 × 10-16
- C. 1.0 × 10-7
- D. 1.0 × 10-14
- Correct Answer: A
- Explanation: The Ka of phenol is 1.3 × 10-10.
- Q4. What is the Ka of water?
- A. 1.3 × 10-10
- B. 1.8 × 10-16
- C. 1.0 × 10-7
- D. 1.0 × 10-14
- Correct Answer: B
- Explanation: The Ka of water is 1.8 × 10-16.
- Q5. Which of the following is the correct order of acidity?
- A. Phenol > Water > Ethanol
- B. Phenol > Ethanol > Water
- C. Water > Phenol > Ethanol
- D. Ethanol > Water > Phenol
- Correct Answer: A
- Explanation: The acidity order is Phenol > Water > Ethanol.
- Q6. What is the conjugate base of phenol?
- A. Phenoxide ion
- B. Hydroxide ion
- C. Ethoxide ion
- D. Benzene
- Correct Answer: A
- Explanation: The conjugate base of phenol is the phenoxide ion.
- Q7. What is the conjugate base of water?
- A. Phenoxide ion
- B. Hydroxide ion
- C. Ethoxide ion
- D. Hydronium ion
- Correct Answer: B
- Explanation: The conjugate base of water is the hydroxide ion.
- Q8. What is the conjugate base of ethanol?
- A. Phenoxide ion
- B. Hydroxide ion
- C. Ethoxide ion
- D. Hydronium ion
- Correct Answer: C
- Explanation: The conjugate base of ethanol is the ethoxide ion.
- Q9. Why is phenol a stronger acid than water?
- A. Because of resonance stabilization of the phenoxide ion.
- B. Because of the electron-donating effect of the ethyl group.
- C. Because of the difference in molecular weight.
- D. Because of the difference in boiling points.
- Correct Answer: A
- Explanation: Resonance stabilization of the phenoxide ion makes phenol a stronger acid.
- Q10. Why is ethanol a weaker acid than water?
- A. Because of resonance stabilization of the ethoxide ion.
- B. Because of the electron-donating effect of the ethyl group.
- C. Because of the difference in molecular weight.
- D. Because of the difference in boiling points.
- Correct Answer: B
- Explanation: The electron-donating effect of the ethyl group destabilizes the ethoxide ion.
Did you understand this topic?
Your answer is saved with your learning record. It does not mark the topic as mastered.
Quick revision
Revision Zone
10.3 Acidity of water, phenol and ethanol
Sticky Notes · Quick Revision
Key Point 1
- Acidity order: Phenol > Water > Ethanol.
Key Point 2
- Phenol: Resonance-stabilized phenoxide ion.
Key Point 3
- Water: Hydroxide ion is more stable than ethoxide.
Key Point 4
- Ethanol: Ethyl group is electron-donating, destabilizes ethoxide.
Key Point 5
- Ka of phenol: 1.3 × 10-10.
Key Point 6
- Ka of water: 1.8 × 10-16.
Key Point 7
- Resonance stabilizes conjugate base.
Key Point 8
- Inductive effect destabilizes conjugate base.
Same supplied content as this Notes page (shared canonical source).