Hydrogenate the benzene ring
Add hydrogen across the aromatic ring using a platinum or nickel catalyst and heat to form cyclohexane.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 30. Follow the listed reagents, catalysts, temperatures and reaction scope for benzene and methylbenzene. Directing effects are limited to –NH₂, –OH, –R, –NO₂, –COOH and –COR. Written practice is self-checked
Before you begin
- Use the Topic 29 model of benzene as a planar aromatic ring with a delocalised π system, and recognise electrophiles, nucleophiles and organic functional groups.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the product of hydrogenating benzene under the stated conditions?
What you will learn
- Recall hydrogen, catalyst and heat conditions
- Predict cyclohexane from benzene hydrogenation
- Contrast hydrogenation with substitution reactions
Add hydrogen to the ring under catalytic conditions
The syllabus states that benzene can be hydrogenated using H₂ and a Pt or Ni catalyst with heat to form a cyclohexane ring.
The overall equation is C₆H₆ + 3H₂ → C₆H₁₂. Each of the three equivalent π-bond units is hydrogenated in the overall stoichiometry; the aromatic ring is no longer present in the product.
This is a genuine addition reaction
Unlike electrophilic substitution, hydrogenation adds hydrogen to the ring and removes the aromatic delocalised π system. The product is saturated cyclohexane.
Do not confuse the catalyst or conditions with alkene hydrogenation questions; for this syllabus outcome explicitly state Pt or Ni and heat.
Pause and explain
How many H₂ molecules appear in the balanced equation per benzene molecule?
Worked example
Predict the product when benzene reacts with hydrogen over nickel on heating.
Show the worked solution
- Hydrogen adds to the aromatic ring.
- Three H₂ molecules are required per benzene molecule in the overall equation.
- The product is cyclohexane.
Answer C₆H₆ + 3H₂ → C₆H₁₂ using Ni catalyst and heat.
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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.
Write the balanced equation and conditions for hydrogenating benzene to cyclohexane.
Give me a hint
Balance hydrogen atoms and include either listed metal catalyst plus heat.
Compare my reasoning
- Cyclohexane has formula C₆H₁₂.
- Benzene has six H; add six more as 3H₂.
- State Pt or Ni and heat.
C₆H₆ + 3H₂ → C₆H₁₂, using Pt or Ni catalyst and heat.
Look for these in your work
- Product formula correct
- Hydrogen balanced
- Conditions stated
Why is hydrogenation not electrophilic substitution?
Give me a hint
Track whether a ring hydrogen leaves or hydrogen adds to the ring.
Compare my reasoning
- Hydrogen adds to the ring.
- No ring hydrogen is replaced by an electrophile.
- The product is saturated cyclohexane.
It is addition because H₂ adds to the ring and converts it to saturated cyclohexane; no ring hydrogen is replaced.
Look for these in your work
- Addition identified
- No substitution stated
- Product saturation linked
A student writes C₆H₆ + H₂ → C₆H₈ and names cyclohexane. What needs correcting?
Give me a hint
Check both the hydrogen count and the product formula.
Compare my reasoning
- Cyclohexane is C₆H₁₂.
- Six additional hydrogen atoms are needed.
- Use 3H₂ in the balanced equation.
The equation must be C₆H₆ + 3H₂ → C₆H₁₂; one H₂ is insufficient to form cyclohexane.
Look for these in your work
- Product formula checked
- Hydrogen difference calculated
- Coefficient corrected
Common mistakes
- Predicting bromobenzene instead of cyclohexane
- Using one H₂ molecule in the balanced overall equation
- Calling hydrogenation substitution
- Leaving out the catalyst or heat
What can you explain now?
What is the product of hydrogenating benzene under the stated conditions?
Compare with the explanation
Cyclohexane
Hydrogenation saturates the ring to form cyclohexane.
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Make it stick
Which reagent and conditions convert benzene into a saturated six-carbon ring?
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.