Chemistry/Chapter 11: Carbonyl Compounds
11.5.2 Comparison of Hydrolysis of Acyl chlorides, Alkyl chlorides, and Aryl chlorides
The main idea is that the ease of hydrolysis of these compounds follows the order: \[ \text{Acyl chlorides} > \text{Alkyl chlorides} > \text
Explanation level
In the previous section, we learned about the reactions of acyl chlorides, including their hydrolysis with water. Now, we will compare the ease of hydrolysis of three classes of chlorine-containing organic compounds: acyl chlorides, alkyl chlorides (halogenoalkanes), and aryl chlorides (halogenoarenes). This comparison highlights how the structure of the carbon-chlorine bond influences reactivity.
Main Idea
The main idea is that the ease of hydrolysis of these compounds follows the order: Acyl chlorides > Alkyl chlorides > Aryl chlorides This trend can be explained by the strength of the carbon-chlorine (C-Cl) bond and the stability of the intermediate formed during the reaction. Acyl chlorides react most readily because the carbonyl carbon is highly electrophilic and the chloride is a good leaving group. Aryl chlorides are the least reactive because the C-Cl bond has partial double bond character due to resonance, making it very strong and difficult to break.
Core Concept
The core concept is nucleophilic acyl substitution (for acyl chlorides) versus nucleophilic substitution (for alkyl and aryl chlorides). The rate of hydrolysis depends on how easily the nucleophile (water) can attack the carbon atom and how easily the chloride ion can leave.
- Acyl chlorides: The carbonyl carbon is highly electrophilic (attached to two electronegative atoms, O and Cl). The chloride ion is a good leaving group, and the reaction proceeds rapidly through a tetrahedral intermediate.
- Alkyl chlorides: The carbon is attached to only one electronegative atom (Cl). It is less electrophilic than the acyl carbon, so hydrolysis requires a stronger nucleophile or more vigorous conditions.
- Aryl chlorides: The lone pair on the chlorine atom is delocalized into the aromatic ring through resonance, giving the C-Cl bond partial double bond character. This makes the bond very strong and the chlorine very poor leaving group. Hydrolysis is extremely difficult.
Detailed Explanation
The textbook explains the trend in hydrolysis based on the strength of the C-Cl bond.
1. Acyl Chlorides
In acyl chlorides, the carbonyl carbon atom is attached to two electronegative atoms (O and Cl), pulling electrons away. The carbonyl carbon is highly electron deficient. The nucleophile can easily attack it and displace the chlorine atom. So, the hydrolysis occurs most readily.
R-COCl + H2O → R-COOH + HCl
- Reason: The carbonyl carbon is highly electrophilic. The reaction proceeds via a nucleophilic acyl substitution mechanism through a tetrahedral intermediate.
- Rate: Very fast at room temperature.
2. Alkyl Chlorides
In alkyl chlorides, the carbon atom is attached to only one electronegative atom, which pulls electrons away. The carbon atom is therefore less electron deficient than the carbonyl carbon atom. Hydrolysis requires a stronger nucleophile than water.
R-CH2Cl + OH- → R-CH2OH + Cl-
- Reason: The carbon is less electrophilic than the acyl carbon. The reaction is a nucleophilic substitution (SN1 or SN2), which may require heating with aqueous alkali.
- Rate: Slower than acyl chlorides; requires more vigorous conditions (e.g., heating with NaOH).
3. Aryl Chlorides
In aryl chlorides, the lone pair of chlorine is involved in resonance and is delocalized over the ring. The C-Cl bond, therefore, has some double bond character. As a result, the C-Cl bond is difficult to break, even by a strong nucleophile. So, hydrolysis will not occur.
Ar-Cl + H2O → No reaction
- Reason: The C-Cl bond has partial double bond character due to resonance. The carbon is less electrophilic, and the chloride is a poor leaving group.
- Rate: Extremely slow or no reaction under normal conditions. Requires very harsh conditions (e.g., high temperature and pressure with NaOH) to undergo hydrolysis.
Summary of the Trend
| Compound Type | C-Cl Bond Strength | Electrophilicity of Carbon | Leaving Group Ability | Ease of Hydrolysis |
|---|---|---|---|---|
| Acyl chloride | Weak | High | Good | Very fast |
| Alkyl chloride | Moderate | Moderate | Moderate | Slow (requires heat/base) |
| Aryl chloride | Strong (partial double bond) | Low | Poor | Very slow / no reaction |
Intermediate Level
The ease of hydrolysis of the three types of chlorides depends on the strength of the C-Cl bond and the electrophilicity of the carbon atom. Acyl chlorides hydrolyze most readily because the carbonyl carbon is highly electrophilic (attached to two electronegative atoms) and the chloride is a good leaving group. The reaction is a nucleophilic acyl substitution that proceeds rapidly at room temperature. Alkyl chlorides are less reactive because their carbon is less electrophilic, and the reaction requires a stronger nucleophile (like OH⁻) and heating. The mechanism is SN1 or SN2. Aryl chlorides are the least reactive because the C-Cl bond has partial double bond character due to resonance, making it very strong. Hydrolysis of aryl chlorides is extremely difficult and requires harsh conditions. The order of ease of hydrolysis is: Acyl chlorides > Alkyl chlorides > Aryl chlorides.
Important Points
- - Order of hydrolysis: Acyl chlorides > Alkyl chlorides > Aryl chlorides.
- - Acyl chlorides: Fast hydrolysis due to highly electrophilic carbonyl carbon and good leaving group.
- - Alkyl chlorides: Slower hydrolysis, requires heat and strong nucleophile (e.g., OH⁻).
- - Aryl chlorides: Very slow or no hydrolysis due to partial double bond character of the C-Cl bond.
- - Reason: C-Cl bond strength and carbon electrophilicity.
- - Aryl chlorides: Resonance stabilization of the C-Cl bond.
Remember
- - Acyl chlorides hydrolyze fastest.
- - Alkyl chlorides hydrolyze slowly (require heat/base).
- - Aryl chlorides do not hydrolyze easily (resonance).
- - Reason: C-Cl bond strength and electrophilicity of carbon.
- - Order: Acyl > Alkyl > Aryl.
- - Aryl chlorides: C-Cl bond has partial double bond character.
- - Alkyl chlorides: Require strong nucleophile and heat.
Common Mistakes
- - Thinking that all C-Cl bonds are equally reactive. The reactivity depends on the type of carbon (acyl, alkyl, or aryl).
- - Forgetting that aryl chlorides have resonance stabilization. This is the main reason for their low reactivity.
- - Assuming that alkyl chlorides hydrolyze as fast as acyl chlorides. Alkyl chlorides require more vigorous conditions.
- - Not recognizing the role of the carbonyl group in acyl chlorides. The carbonyl carbon is highly electrophilic and makes the chloride a good leaving group.
- - Confusing the mechanisms. Acyl chlorides undergo nucleophilic acyl substitution; alkyl and aryl chlorides undergo nucleophilic substitution (if at all).
Quick Revision
The ease of hydrolysis of chlorine-containing organic compounds follows the order: acyl chlorides > alkyl chlorides > aryl chlorides. Acyl chlorides are the most reactive because the carbonyl carbon is highly electrophilic (bonded to two electronegative atoms) and the chloride is a good leaving group. The reaction proceeds rapidly at room temperature via a nucleophilic acyl substitution mechanism. Alkyl chlorides are less reactive because their carbon is less electrophilic; hydrolysis requires a strong nucleophile (like OH⁻) and heat. Aryl chlorides are the least reactive because the lone pair on the chlorine atom is delocalized into the aromatic ring, giving the C-Cl bond partial double bond character. This makes the bond very strong and resistant to cleavage, requiring harsh conditions for hydrolysis.
60-Second Revision
The ease of hydrolysis of chlorides is Acyl > Alkyl > Aryl. Acyl chlorides are most reactive because the carbonyl carbon is highly electrophilic and Cl⁻ is a good leaving group. Alkyl chlorides require strong nucleophiles and heat. Aryl chlorides are least reactive because the C-Cl bond has partial double bond character due to resonance, making it very strong.
Frequently Asked Questions (FAQs)
Q1. Why are acyl chlorides more reactive than alkyl chlorides towards hydrolysis? Answer: Acyl chlorides are more reactive because the carbonyl carbon is bonded to two electronegative atoms (oxygen and chlorine), making it highly electrophilic. This allows water (a weak nucleophile) to attack easily. In alkyl chlorides, the carbon is bonded to only one electronegative atom (chlorine), so it is less electrophilic, and hydrolysis requires a stronger nucleophile (like OH⁻) and heat.
Q2. Why are aryl chlorides resistant to hydrolysis? Answer: Aryl chlorides are resistant to hydrolysis because the lone pair of electrons on the chlorine atom is delocalized into the aromatic ring through resonance. This gives the C-Cl bond partial double bond character, making it stronger and harder to break. The chloride ion is also a poor leaving group. Therefore, hydrolysis requires very harsh conditions.
- Q3. What is the order of ease of hydrolysis for acyl chlorides, alkyl chlorides, and aryl chlorides?
- Answer: The order is:
- Acyl chlorides > Alkyl chlorides > Aryl chlorides
Q4. What is the mechanism of hydrolysis of an acyl chloride? Answer: The hydrolysis of an acyl chloride proceeds via a nucleophilic acyl substitution mechanism. Water (the nucleophile) attacks the electrophilic carbonyl carbon, forming a tetrahedral intermediate. The intermediate then collapses, expelling the chloride ion as a leaving group, and a proton is lost to yield the carboxylic acid.
Q5. What is the mechanism of hydrolysis of an alkyl chloride? Answer: The hydrolysis of an alkyl chloride proceeds via a nucleophilic substitution mechanism (SN1 or SN2). The nucleophile (e.g., OH⁻) attacks the carbon atom bonded to the chlorine, and the chloride ion is displaced as a leaving group. This reaction is slower than the hydrolysis of acyl chlorides and requires heat.
Q6. Why does the hydrolysis of aryl chlorides require harsh conditions? Answer: The C-Cl bond in aryl chlorides has partial double bond character due to resonance, making it very strong and difficult to break. The sp² hybridized carbon is also less electrophilic. Therefore, hydrolysis requires high temperature and pressure, and often a strong base, to proceed.
- Q7. What is the product of the hydrolysis of an acyl chloride?
- Answer: The product is a carboxylic acid and hydrogen chloride:
- R-COCl + H2O → R-COOH + HCl
- Q8. What is the product of the hydrolysis of an alkyl chloride?
- Answer: The product is an alcohol and a chloride ion (or hydrogen chloride, depending on the conditions):
- R-CH2Cl + OH- → R-CH2OH + Cl-
- Q9. What is the product of the hydrolysis of an aryl chloride?
- Answer: Under harsh conditions, the product is a phenol and hydrogen chloride:
- Ar-Cl + H2O → Ar-OH + HCl
- However, under normal conditions, no reaction occurs.
Multiple Choice Questions (MCQs)
- Q1. Which of the following compounds is the most reactive towards hydrolysis?
- A. Acyl chloride
- B. Alkyl chloride
- C. Aryl chloride
- D. All are equally reactive
- Correct Answer: A
- Explanation: Acyl chlorides are the most reactive due to the highly electrophilic carbonyl carbon and the good leaving group.
- Q2. Which of the following compounds is the least reactive towards hydrolysis?
- A. Acyl chloride
- B. Alkyl chloride
- C. Aryl chloride
- D. All are equally reactive
- Correct Answer: C
- Explanation: Aryl chlorides are the least reactive due to the partial double bond character of the C-Cl bond.
- Q3. What is the reason for the low reactivity of aryl chlorides towards hydrolysis?
- A. The C-Cl bond is weak.
- B. The C-Cl bond has partial double bond character due to resonance.
- C. The chlorine is not electronegative.
- D. The ring is electron-deficient.
- Correct Answer: B
- Explanation: The C-Cl bond has partial double bond character due to resonance, making it strong.
- Q4. What is the order of ease of hydrolysis?
- A. Acyl > Alkyl > Aryl
- B. Alkyl > Acyl > Aryl
- C. Aryl > Alkyl > Acyl
- D. Acyl > Aryl > Alkyl
- Correct Answer: A
- Explanation: The order is Acyl > Alkyl > Aryl.
- Q5. What is the product of the hydrolysis of an acyl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: A
- Explanation: The product is a carboxylic acid.
- Q6. What is the product of the hydrolysis of an alkyl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: B
- Explanation: The product is an alcohol.
- Q7. What is the product of the hydrolysis of an aryl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: C
- Explanation: The product is a phenol.
- Q8. Why are acyl chlorides more reactive than alkyl chlorides?
- A. Because the carbonyl carbon is more electrophilic.
- B. Because the C-Cl bond is stronger.
- C. Because the chlorine is less electronegative.
- D. Because they are smaller molecules.
- Correct Answer: A
- Explanation: The carbonyl carbon is more electrophilic.
- Q9. What type of mechanism is involved in the hydrolysis of acyl chlorides?
- A. Nucleophilic acyl substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: The mechanism is nucleophilic acyl substitution.
- Q10. What type of mechanism is involved in the hydrolysis of alkyl chlorides?
- A. Nucleophilic acyl substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: The mechanism is nucleophilic substitution.
Multiple Choice Questions (MCQs)
- Q1. Which of the following compounds is the most reactive towards hydrolysis?
- A. Acyl chloride
- B. Alkyl chloride
- C. Aryl chloride
- D. All are equally reactive
- Correct Answer: A
- Explanation: Acyl chlorides are the most reactive.
- Q2. Which of the following compounds is the least reactive towards hydrolysis?
- A. Acyl chloride
- B. Alkyl chloride
- C. Aryl chloride
- D. All are equally reactive
- Correct Answer: C
- Explanation: Aryl chlorides are the least reactive.
- Q3. What is the reason for the low reactivity of aryl chlorides towards hydrolysis?
- A. The C-Cl bond is weak.
- B. The C-Cl bond has partial double bond character due to resonance.
- C. The chlorine is not electronegative.
- D. The ring is electron-deficient.
- Correct Answer: B
- Explanation: The C-Cl bond has partial double bond character.
- Q4. What is the order of ease of hydrolysis?
- A. Acyl > Alkyl > Aryl
- B. Alkyl > Acyl > Aryl
- C. Aryl > Alkyl > Acyl
- D. Acyl > Aryl > Alkyl
- Correct Answer: A
- Explanation: The order is Acyl > Alkyl > Aryl.
- Q5. What is the product of the hydrolysis of an acyl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: A
- Explanation: The product is a carboxylic acid.
- Q6. What is the product of the hydrolysis of an alkyl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: B
- Explanation: The product is an alcohol.
- Q7. What is the product of the hydrolysis of an aryl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: C
- Explanation: The product is a phenol.
- Q8. Why are acyl chlorides more reactive than alkyl chlorides?
- A. Because the carbonyl carbon is more electrophilic.
- B. Because the C-Cl bond is stronger.
- C. Because the chlorine is less electronegative.
- D. Because they are smaller molecules.
- Correct Answer: A
- Explanation: The carbonyl carbon is more electrophilic.
- Q9. What type of mechanism is involved in the hydrolysis of acyl chlorides?
- A. Nucleophilic acyl substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: The mechanism is nucleophilic acyl substitution.
- Q10. What type of mechanism is involved in the hydrolysis of alkyl chlorides?
- A. Nucleophilic acyl substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: The mechanism is nucleophilic substitution.
Multiple Choice Questions (MCQs)
- Q1. Which of the following compounds is the most reactive towards hydrolysis?
- A. Acyl chloride
- B. Alkyl chloride
- C. Aryl chloride
- D. All are equally reactive
- Correct Answer: A
- Explanation: Acyl chlorides are the most reactive.
- Q2. Which of the following compounds is the least reactive towards hydrolysis?
- A. Acyl chloride
- B. Alkyl chloride
- C. Aryl chloride
- D. All are equally reactive
- Correct Answer: C
- Explanation: Aryl chlorides are the least reactive.
- Q3. What is the reason for the low reactivity of aryl chlorides towards hydrolysis?
- A. The C-Cl bond is weak.
- B. The C-Cl bond has partial double bond character due to resonance.
- C. The chlorine is not electronegative.
- D. The ring is electron-deficient.
- Correct Answer: B
- Explanation: The C-Cl bond has partial double bond character.
- Q4. What is the order of ease of hydrolysis?
- A. Acyl > Alkyl > Aryl
- B. Alkyl > Acyl > Aryl
- C. Aryl > Alkyl > Acyl
- D. Acyl > Aryl > Alkyl
- Correct Answer: A
- Explanation: The order is Acyl > Alkyl > Aryl.
- Q5. What is the product of the hydrolysis of an acyl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: A
- Explanation: The product is a carboxylic acid.
- Q6. What is the product of the hydrolysis of an alkyl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: B
- Explanation: The product is an alcohol.
- Q7. What is the product of the hydrolysis of an aryl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: C
- Explanation: The product is a phenol.
- Q8. Why are acyl chlorides more reactive than alkyl chlorides?
- A. Because the carbonyl carbon is more electrophilic.
- B. Because the C-Cl bond is stronger.
- C. Because the chlorine is less electronegative.
- D. Because they are smaller molecules.
- Correct Answer: A
- Explanation: The carbonyl carbon is more electrophilic.
- Q9. What type of mechanism is involved in the hydrolysis of acyl chlorides?
- A. Nucleophilic acyl substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: The mechanism is nucleophilic acyl substitution.
- Q10. What type of mechanism is involved in the hydrolysis of alkyl chlorides?
- A. Nucleophilic acyl substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: The mechanism is nucleophilic substitution.
Multiple Choice Questions (MCQs)
- Q1. Which of the following compounds is the most reactive towards hydrolysis?
- A. Acyl chloride
- B. Alkyl chloride
- C. Aryl chloride
- D. All are equally reactive
- Correct Answer: A
- Explanation: Acyl chlorides are the most reactive.
- Q2. Which of the following compounds is the least reactive towards hydrolysis?
- A. Acyl chloride
- B. Alkyl chloride
- C. Aryl chloride
- D. All are equally reactive
- Correct Answer: C
- Explanation: Aryl chlorides are the least reactive.
- Q3. What is the reason for the low reactivity of aryl chlorides towards hydrolysis?
- A. The C-Cl bond is weak.
- B. The C-Cl bond has partial double bond character due to resonance.
- C. The chlorine is not electronegative.
- D. The ring is electron-deficient.
- Correct Answer: B
- Explanation: The C-Cl bond has partial double bond character.
- Q4. What is the order of ease of hydrolysis?
- A. Acyl > Alkyl > Aryl
- B. Alkyl > Acyl > Aryl
- C. Aryl > Alkyl > Acyl
- D. Acyl > Aryl > Alkyl
- Correct Answer: A
- Explanation: The order is Acyl > Alkyl > Aryl.
- Q5. What is the product of the hydrolysis of an acyl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: A
- Explanation: The product is a carboxylic acid.
- Q6. What is the product of the hydrolysis of an alkyl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: B
- Explanation: The product is an alcohol.
- Q7. What is the product of the hydrolysis of an aryl chloride?
- A. Carboxylic acid
- B. Alcohol
- C. Phenol
- D. Ketone
- Correct Answer: C
- Explanation: The product is a phenol.
- Q8. Why are acyl chlorides more reactive than alkyl chlorides?
- A. Because the carbonyl carbon is more electrophilic.
- B. Because the C-Cl bond is stronger.
- C. Because the chlorine is less electronegative.
- D. Because they are smaller molecules.
- Correct Answer: A
- Explanation: The carbonyl carbon is more electrophilic.
- Q9. What type of mechanism is involved in the hydrolysis of acyl chlorides?
- A. Nucleophilic acyl substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: A
- Explanation: The mechanism is nucleophilic acyl substitution.
- Q10. What type of mechanism is involved in the hydrolysis of alkyl chlorides?
- A. Nucleophilic acyl substitution
- B. Nucleophilic substitution
- C. Electrophilic addition
- D. Free radical substitution
- Correct Answer: B
- Explanation: The mechanism is nucleophilic substitution.
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
11.5.2 Comparison of Hydrolysis of Acyl chlorides, Alkyl chlorides, and Aryl chlorides
Sticky Notes · Quick Revision
Key Point 1
- Hydrolysis order: Acyl > Alkyl > Aryl.
Key Point 2
- Acyl chloride: Fast (carbonyl carbon is electrophilic).
Key Point 3
- Alkyl chloride: Slow (requires OH⁻ and heat).
Key Point 4
- Aryl chloride: Very slow (resonance stabilizes C-Cl bond).
Key Point 5
- C-Cl bond strength: Aryl > Alkyl > Acyl.
Key Point 6
- Aryl chloride: Partial double bond character.
Key Point 7
- Acyl chloride: Good leaving group (Cl⁻).
Same supplied content as this Notes page (shared canonical source).