Key Learning Objectives
- Draw the nucleophilic addition-elimination mechanism for ethanoyl chloride with ethanol.
- Explain curly arrow movements, lone pair attack on carbonyl carbon, tetrahedral intermediate, and elimination of chloride.
- Contrast the reactivity of acyl chlorides with carboxylic acids in esterification.
Topic Summary & Core Notes
Master the nucleophilic addition-elimination mechanism for acyl chlorides reacting with alcohols to synthesise esters and white steamy fumes of HCl.
Key Rules, Formulas & Definitions
Acyl Chloride Esterification
R-COCl + R'-OH → R-COOR' (Ester) + HCl(g) [Room temperature, no acid catalyst needed]
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Drawing the Mechanism
Question: Outline the mechanism for the reaction between ethanoyl chloride (CH3COCl) and methanol (CH3OH).
Step-by-Step Solution:
- Step 1: Nucleophilic attack: Oxygen lone pair on methanol attacks the delta-positive carbonyl carbon; C=O pi bond electrons move onto oxygen.
- Step 2: Tetrahedral intermediate: Carbonyl oxygen bears negative charge; methanol oxygen bears positive charge.
- Step 3: Elimination: Oxygen lone pair reforms C=O double bond, chloride ion (Cl-) is eliminated.
- Step 4: Deprotonation: Chloride ion removes the proton from the protonated ester, yielding methyl ethanoate and HCl.
Final Answer: Nucleophilic addition-elimination yielding methyl ethanoate and steamy white fumes of HCl.
Senior Examiner Insight: Always show partial charges (delta+, delta-) and full lone pairs on both oxygen atoms.
Worked Example 2
Worked Example 2: Reactivity Comparison
Question: Explain why ethanoyl chloride reacts faster with alcohols than ethanoic acid does.
Step-by-Step Solution:
- Step 1: Electronegativity: The chlorine atom in ethanoyl chloride is strongly electron-withdrawing, making the carbonyl carbon much more delta-positive.
- Step 2: Leaving group: Chloride (Cl-) is a much better leaving group than hydroxide (OH-).
Final Answer: The electron-withdrawing chlorine makes the carbonyl carbon more electrophilic, and chloride is a better leaving group.
Senior Examiner Insight: Credit points for higher delta-positive carbon AND better chloride leaving group.
Common Pitfalls & Examiner Warnings
Common Mistake: Calling the mechanism nucleophilic substitution
The reaction proceeds via addition to form a tetrahedral intermediate followed by elimination, so the correct name is Nucleophilic Addition-Elimination.
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
What visible observation confirms the reaction of an acyl chloride with an alcohol or water?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Steamy white fumes of hydrogen chloride (HCl)
Detailed Explanation: Hydrogen chloride gas is released, forming misty acidic fumes in moist air.
Mark Scheme & Scoring Points: 1 mark for Steamy white fumes of HCl.
Question 2 of 3
What is the formal IUPAC name of the mechanism for acyl chlorides reacting with nucleophiles?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Nucleophilic addition-elimination
Detailed Explanation: The mechanism begins with nucleophilic addition across C=O, followed by chloride elimination.
Mark Scheme & Scoring Points: 1 mark for Nucleophilic addition-elimination.
Question 3 of 3
What organic product is formed when benzoyl chloride reacts with ethanol?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Ethyl benzoate
Detailed Explanation: Acyl chloride + Alcohol → Ester: Benzoyl chloride + Ethanol → Ethyl benzoate + HCl.
Mark Scheme & Scoring Points: 1 mark for Ethyl benzoate.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
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