9701 · 36.1
Organic synthesis flashcards
Revision flashcards for Cambridge 9701 Organic synthesis (syllabus 36.1). Flip, recall, then mark a real past-paper question.
Card
What is retrosynthesis?
A problem-solving technique for planning organic syntheses. It involves working backwards from the target molecule to simpler, commercially available starting materials.
Card
What reagent and conditions are used to oxidise a primary alcohol to a carboxylic acid?
Potassium dichromate(VI) ($K_2Cr_2O_7$) with concentrated sulfuric acid ($H_2SO_4$) under reflux. Reflux ensures the intermediate aldehyde is also oxidised.
Card
What reagent and conditions are used to oxidise a primary alcohol to an aldehyde?
Potassium dichromate(VI) ($K_2Cr_2O_7$) with dilute sulfuric acid ($H_2SO_4$), with immediate distillation of the product as it forms to prevent further oxidation.
Card
What is the key difference between the reducing agents $NaBH_4$ and $LiAlH_4$?
$NaBH_4$ (sodium borohydride) is a milder reducing agent that reduces aldehydes and ketones. $LiAlH_4$ (lithium aluminium hydride) is a powerful reducing agent that reduces aldehydes, ketones, carboxylic acids, and esters. $LiAlH_4$ requires a dry ether solvent and reacts violently with water.
Card
How do you form a C-C bond starting from a haloalkane?
React the haloalkane with potassium cyanide ($KCN$) in ethanol under reflux (nucleophilic substitution). This replaces the halogen with a -CN group, extending the carbon chain by one.
Card
How do you convert a nitrile (-CN) to a carboxylic acid (-COOH)?
Acid hydrolysis: Heat under reflux with a dilute acid (e.g., $HCl(aq)$ or $H_2SO_4(aq)$). This adds two water molecules across the triple bond.
Card
How do you convert a nitrile (-CN) to a primary amine (-CH₂NH₂)?
Reduction: Use a strong reducing agent like $LiAlH_4$ in dry ether, or catalytic hydrogenation ($H_2$ with a Ni catalyst).
Card
What is a 'protecting group' in organic synthesis?
A temporary modification of a functional group to prevent it from reacting in a subsequent step. It is added, the desired reaction is performed elsewhere on the molecule, and then it is removed.
Card
What are the reagents and conditions for esterification?
React a carboxylic acid with an alcohol in the presence of a strong acid catalyst (e.g., concentrated $H_2SO_4$) and heat under reflux.
Card
How can you hydrolyse an ester?
Acid hydrolysis (reversible): Heat under reflux with dilute acid. Base hydrolysis (saponification, irreversible): Heat under reflux with aqueous alkali (e.g., $NaOH(aq)$), which produces a carboxylate salt.
Card
How do you convert a secondary alcohol to a ketone?
Oxidation using acidified potassium dichromate(VI) ($K_2Cr_2O_7 / H_2SO_4$) and heating under reflux.
Card
What is retrosynthetic analysis?
A strategy for planning an organic synthesis by working backwards from the target molecule to simpler, commercially available starting materials. It uses 'disconnections' to identify potential precursors.
Card
What is the purpose of a 'disconnection' in retrosynthesis?
It is an imaginary bond-breaking step, shown with a retrosynthetic arrow (=>), that simplifies the target molecule. It must correspond to a known, reliable bond-forming reaction in the forward synthesis.
Card
Give an example of a reaction that extends a carbon chain by one carbon atom.
The reaction of a haloalkane with potassium cyanide (KCN) in ethanol. This is a nucleophilic substitution that replaces the halogen with a -CN group, which can then be hydrolysed to a carboxylic acid.
Card
What is Functional Group Interconversion (FGI)?
A reaction that converts one functional group into another without changing the carbon skeleton of the molecule, for example, the oxidation of a primary alcohol to a carboxylic acid using acidified potassium manganate(VII).
Card
What is the three-step process for using a protecting group?
1. Protect: Convert the sensitive functional group into an unreactive form. 2. React: Carry out the desired transformation elsewhere in the molecule. 3. Deprotect: Remove the protecting group to restore the original functional group.