9701 · 21.1
Organic synthesis flashcards
Revision flashcards for Cambridge 9701 Organic synthesis (syllabus 21.1). Flip, recall, then mark a real past-paper question.
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What is retrosynthesis?
A strategy for planning organic syntheses. You start with the target molecule and work backwards one step at a time to identify suitable starting materials. The special arrow used is ⇒.
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Reagent and conditions to convert an alkene to an alkane?
Reagent: Hydrogen ($H_2$). Conditions: Nickel (Ni) catalyst, high temperature (e.g., 150°C) and pressure. Reaction type: Hydrogenation (or electrophilic addition).
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How do you convert a primary alcohol to a carboxylic acid?
Reagent: Acidified potassium dichromate(VI) ($K_2Cr_2O_7 / H_2SO_4$). Conditions: Heat under reflux. This ensures complete oxidation by preventing volatile intermediates from escaping.
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How do you stop the oxidation of a primary alcohol at the aldehyde stage?
Reagent: Acidified potassium dichromate(VI) ($K_2Cr_2O_7 / H_2SO_4$). Conditions: Gently heat and distil the product as it forms. The aldehyde has a lower boiling point than the alcohol, allowing its removal before it can be further oxidised.
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What is the key feature of a nucleophilic substitution reaction?
A reaction where a nucleophile (an electron-pair donor) attacks an electron-deficient carbon atom (δ+), replacing a leaving group. Example: $CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-$.
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How do you convert a halogenoalkane to a nitrile?
Reagent: Ethanolic potassium cyanide (KCN in ethanol). Conditions: Heat under reflux. This is a nucleophilic substitution that increases the carbon chain length by one.
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How do you convert a nitrile to a carboxylic acid?
Reaction: Acid hydrolysis. Reagent: Dilute acid (e.g., $HCl(aq)$). Conditions: Heat under reflux. The overall reaction is $R-CN + 2H_2O + H^+ \rightarrow R-COOH + NH_4^+$.
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What is the product when an aldehyde reacts with $NaBH_4$?
A primary alcohol is formed. $NaBH_4$ (sodium borohydride) is a reducing agent that reduces aldehydes to primary alcohols and ketones to secondary alcohols.
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What is a 'protecting group'?
A chemical group temporarily added to a reactive functional group to 'mask' it and prevent it from reacting, while another part of the molecule is modified. It is removed in a later step.
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Why is planning a synthesis backwards (retrosynthesis) often more effective than forwards?
It simplifies the problem by focusing on the final bond formation or functional group change. This helps identify the immediate precursor molecule, making the choice of reactions more logical and reducing the number of possible but unproductive pathways.
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What reagent is used to convert an acyl chloride to a primary amide?
Concentrated aqueous ammonia ($NH_3(aq)$). The reaction is a vigorous nucleophilic addition-elimination, producing dense white fumes of ammonium chloride if the acyl chloride is volatile.