Use electrophilic substitution and addition–elimination terminology
Classify organic mechanisms by the species involved and whether a substituent is replaced or added.
Section 29 · An introduction to A Level organic chemistry
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 29. Follow the formula, naming and optical-isomerism limits stated in the syllabus. Benzene-ring displayed formulas are not expected; use the supplied convention for aromatic skeletal formulas. Do not extend the optical-isomerism scope to naming diastereoisomers or meso compounds. Written practice is self-checked
Before you begin
- Use covalent formula conventions, functional groups, reaction terminology, and the AS organic chemistry foundations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In electrophilic substitution, what happens to a group on the molecule?
What you will learn
- Define electrophilic substitution
- Define addition–elimination
- Use each term to describe an organic transformation
Substitution replaces a group
In electrophilic substitution, an electrophile reacts with a molecule and replaces a hydrogen or another substituent while the main framework is restored. In the A Level course, arenes such as benzene undergo electrophilic substitution; the aromatic system is retained after substitution.
The term describes the overall reaction type. When a question asks for a mechanism, show the required electron-pair movement and intermediates as well; naming the type alone is not a mechanism explanation.
Addition–elimination has two linked stages
In addition–elimination, a species first adds to a polar multiple bond, then another species is eliminated. Acyl chlorides show this pattern: a nucleophile adds at the carbonyl carbon, followed by elimination of chloride as the carbonyl is re-formed.
Do not classify acyl chloride reactions as simple substitution just because one group is replaced overall. Cambridge specifically uses the term addition–elimination for reactions of acyl chlorides in Topic 33.
Pause and explain
Which sequence defines addition–elimination at an acyl chloride?
Worked example
Classify benzene nitration and reaction of an acyl chloride with water.
Show the worked solution
- Nitration replaces a ring hydrogen with a nitro group: electrophilic substitution.
- Water attacks the acyl chloride carbonyl in an addition step.
- Chloride is eliminated as the carbonyl reforms: addition–elimination.
Answer Benzene nitration is electrophilic substitution; acyl chloride hydrolysis is addition–elimination.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Why is bromination of benzene called electrophilic substitution?
Give me a hint
State what the electrophile does to the ring.
Compare my reasoning
- The reacting species is an electrophile.
- A hydrogen on the ring is replaced by bromine.
- The aromatic ring framework remains.
It is electrophilic substitution because an electrophile replaces a ring hydrogen while the aromatic framework is restored.
Look for these in your work
- Electrophile identified
- Replacement described
- Ring retained
Describe the two stages of acyl chloride hydrolysis using the term addition–elimination.
Give me a hint
Track attack at the carbonyl followed by departure of chloride.
Compare my reasoning
- Water acts as a nucleophile and adds to the carbonyl carbon.
- A tetrahedral intermediate can form.
- Chloride is eliminated as the carbonyl reforms.
Water adds to the carbonyl carbon, then chloride is eliminated as C=O reforms; the mechanism is addition–elimination.
Look for these in your work
- Addition stage stated
- Intermediate context included
- Elimination stage stated
A learner calls acyl chloride reaction with an alcohol electrophilic substitution because Cl is replaced. Explain why that label is wrong.
Give me a hint
Mechanism names depend on the electron-flow pathway, not only the net group change.
Compare my reasoning
- The alcohol's oxygen nucleophile attacks the carbonyl carbon.
- Addition to C=O occurs first.
- Chloride then leaves as the carbonyl reforms.
The net reaction replaces Cl, but its mechanism is addition–elimination: nucleophilic addition to the carbonyl is followed by chloride elimination. It is not electrophilic substitution.
Look for these in your work
- Mechanistic pathway used
- Net outcome distinguished
- Correct term justified
Common mistakes
- Calling every overall replacement a substitution mechanism
- Describing addition–elimination as one unexplained step
- Saying an electrophile donates an electron pair
- Claiming aromatic substitution destroys the ring framework
What can you explain now?
In electrophilic substitution, what happens to a group on the molecule?
Compare with the explanation
It is replaced by an electrophile-derived group
Replacement defines substitution.
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Make it stick
How can the mechanism sequence distinguish addition–elimination from electrophilic substitution when both may involve a group leaving?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.