9701 · 14.2
Alkenes flashcards
Revision flashcards for Cambridge 9701 Alkenes (syllabus 14.2). Flip, recall, then mark a real past-paper question.
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What is an alkene?
An unsaturated hydrocarbon containing at least one carbon-carbon double bond (C=C). The general formula for non-cyclic alkenes is $C_n H_{2n}$.
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Describe the bonding in a C=C double bond.
It consists of one σ (sigma) bond formed by the head-on overlap of sp² hybridised orbitals, and one π (pi) bond formed by the sideways overlap of unhybridised p-orbitals.
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Why are alkenes more reactive than alkanes?
The π bond in an alkene has a lower bond enthalpy than a C-C or C-H σ bond. Its electron density is also exposed above and below the plane of the molecule, making it susceptible to attack by electrophiles.
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What is an electrophile?
An electron-pair acceptor. It is a species that is attracted to electron-rich regions, such as positive ions (e.g., $H^+$) or molecules with a partial positive charge (e.g., the $H$ in $H^{\delta+}-Br^{\delta-}$).
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What is an electrophilic addition reaction?
A reaction in which an electrophile attacks an electron-rich C=C double bond, causing the π bond to break and the electrophile to add across the two carbon atoms.
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What is a carbocation?
An ion containing a positively charged carbon atom. It is a key intermediate formed during the electrophilic addition mechanism in alkenes.
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State Markovnikov's rule.
When a hydrogen halide (H-X) 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.
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Explain the basis for Markovnikov's rule.
The rule is based on carbocation stability. The major product is formed via the more stable carbocation intermediate. The stability order is: tertiary > secondary > primary.
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How do you test for unsaturation (the presence of a C=C bond)?
Add aqueous bromine ($Br_2(aq)$), which is orange-brown. If a C=C bond is present, an electrophilic addition reaction occurs and the solution is decolourised.
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What are the conditions for the hydrogenation of an alkene?
Reagent: Hydrogen gas ($H_2$). Conditions: Nickel (Ni) catalyst and a temperature of approximately 150°C.
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What are the conditions for the industrial hydration of ethene to produce ethanol?
Reagent: Steam ($H_2O(g)$). Conditions: High pressure (~60-70 atm), high temperature (~300°C), and a solid phosphoric(V) acid ($H_3PO_4$) catalyst adsorbed on silica.
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What is the difference between a major and minor product in electrophilic addition?
The major product is formed from the most stable carbocation intermediate, while the minor product is formed from a less stable carbocation intermediate. More of the major product is formed.