9701 · 13.2
Characteristic organic reactions flashcards
Revision flashcards for Cambridge 9701 Characteristic organic reactions (syllabus 13.2). Flip, recall, then mark a real past-paper question.
Card
What is a free radical?
A species with an unpaired electron, typically formed by homolytic fission of a covalent bond. It is highly reactive.
Card
What are the three stages of a free-radical chain reaction?
1. Initiation: Formation of radicals using energy (e.g., UV light). 2. Propagation: A radical reacts to form a new molecule and a new radical. 3. Termination: Two radicals combine to form a stable molecule.
Card
What is an electrophile?
An electron-pair acceptor. It is a species that is attracted to regions of high electron density, such as a C=C double bond. Examples include H⁺, Br⁺ (in Br₂), and NO₂⁺.
Card
What is a nucleophile?
An electron-pair donor. It is a species with a lone pair of electrons that is attracted to an electron-deficient atom (e.g., a δ⁺ carbon). Examples include OH⁻, CN⁻, and NH₃.
Card
What is the chemical test for an alkene?
Add aqueous bromine (bromine water) and shake. An alkene will decolourise the orange/brown solution via an electrophilic addition reaction.
Card
State Markovnikov's rule.
When an unsymmetrical reagent (e.g., HBr) adds to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom of the double bond that is already bonded to the greater number of hydrogen atoms.
Card
What is the difference between Sₙ1 and Sₙ2 mechanisms?
Sₙ1 is a two-step mechanism via a carbocation intermediate, favoured by tertiary halogenoalkanes. Sₙ2 is a single-step mechanism involving a transition state, favoured by primary halogenoalkanes.
Card
What are the reagents and conditions to oxidise a primary alcohol to an aldehyde?
Reagent: Acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄(aq). Conditions: Gentle heating with immediate distillation of the aldehyde product to prevent further oxidation.
Card
What are the reagents and conditions to oxidise a primary alcohol to a carboxylic acid?
Reagent: Acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄(aq). Conditions: Heat under reflux to ensure the reaction goes to completion.
Card
Why are tertiary alcohols resistant to oxidation?
The carbon atom bonded to the hydroxyl (-OH) group is not bonded to any hydrogen atoms. Oxidation would require breaking a strong C-C bond, which does not happen under typical laboratory conditions.
Card
What colour change is observed when a primary or secondary alcohol is oxidised with acidified potassium dichromate(VI)?
The solution changes from orange, the colour of the dichromate(VI) ion (Cr₂O₇²⁻), to green, the colour of the chromium(III) ion (Cr³⁺).