Organic synthesis: one question to try now
Real past-paper questions, the answer key from the mark scheme, and the explanation that goes with it. No account needed to answer them.
Question 1
The diagram shows a synthetic route to produce 1-methylcyclohexanol. What is reagent Y?
Answer: A.
Follow the route. HBr adds to 1-methylcyclohexene, and Markovnikov's rule puts the bromine on the more substituted carbon, the one already carrying the methyl, because that gives the more stable tertiary carbocation. Product X is 1-bromo-1-methylcyclohexane.
The target replaces that Br with an OH in the same position, so the second step is nucleophilic substitution, and the reagent for hydrolysing a halogenoalkane is aqueous sodium hydroxide with heat.
C, ethanolic NaOH, is the same base in a different solvent, and the solvent is what decides the outcome. In ethanol the hydroxide acts as a base rather than a nucleophile, removing a hydrogen from the neighbouring carbon and eliminating HBr to give an alkene. That would simply undo the first step.
B and D are oxidising agents, and neither converts a halogenoalkane into anything. Cold dilute KMnO4 would make a diol from the original alkene, and hot concentrated KMnO4 would break its ring open.
Aqueous means substitution and ethanolic means elimination, with the same reagent in both. That single pair is worth memorising, because Cambridge offers both in the same list most years.
What this practice covers
These questions are drawn from past CIE 9701 Chemistry papers. You answer, you find out immediately whether you were right, and you get the reasoning for the correct option and for each distractor. Wrong answers go to a mistakes locker so you can come back to exactly those.
Practice is free. You need an account only so your progress and your mistakes are still there next time.
What examiners see students get wrong here
These are the errors that cost marks on organic synthesis, taken from our own topic notes. Read them before you practise and you will recognise the traps in the questions.
- Starting to plan without counting the carbons, and missing that a chain-lengthening step is needed.
- Giving a reagent without its conditions. Both are usually required.
- Using KCN on an alcohol. It reacts with a halogenoalkane, so the alcohol has to be converted first.
- Trying to oxidise a tertiary alcohol, or a ketone, in a route. Neither happens.
- Forgetting that direct esterification is reversible and low-yielding, when an acyl chloride would be the better answer for a good yield.
- Naming a reagent correctly but the reaction type wrongly, when the question asks for both.
Revise it first
If any of the above is unfamiliar, work through the notes before practising: Organic synthesis revision notes.