State the general formula of a non-cyclic alkene and explain what is meant by the term “unsaturated”.
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CₙH₂ₙ; contains a C=C double bond / can undergo addition reactions
Alkene Structure Isomerism And Reactions
Describe how the σ and π bonds in ethene are formed.
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σ bond formed by head-on/end-on overlap of sp² orbitals between the two carbon atoms; π bond formed by sideways overlap of p-orbitals; π bond lies above and below the plane of the molecule
Explain why but-2-ene shows geometric isomerism but but-1-ene does not.
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but-2-ene has two different groups (H and CH₃) on each C=C carbon; restricted rotation around C=C locks groups in fixed positions; but-1-ene has two H atoms on one C=C carbon so flipping gives the same molecule
State the structural feature responsible for restricted rotation in an alkene.
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the π bond / the C=C double bond
State why the cis–trans naming system is not always sufficient for naming geometric isomers.
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cis–trans only works when each C=C carbon has one H and one other group / a clear main chain; fails when each carbon has three or four different substituents
Define what is meant by the labels E and Z.
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Z means the two higher-priority groups (by atomic number) are on the same side of the C=C; E means they are on opposite sides
State the reagents and conditions for the conversion of propene into propan-2-ol.
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steam / H₂O(g); concentrated phosphoric acid catalyst (H₃PO₄), 300 °C and 60 atm
Write a balanced equation for the reaction of but-2-ene with bromine.
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CH₃CH=CHCH₃ + Br₂ → CH₃CHBrCHBrCH₃; correctly balanced with all atoms present
Describe what is observed when ethene is bubbled through cold, dilute, acidified potassium manganate(VII), and name the organic product formed.
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purple solution decolourises / turns colourless; product is ethane-1,2-diol (CH₂(OH)CH₂(OH))
Describe what is observed when bromine water is shaken with hex-1-ene.
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orange/yellow bromine water decolourises / turns colourless; (because bromine adds across the C=C)
Hexane and hex-1-ene are both colourless liquids. State a chemical test that distinguishes them.
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add bromine water and shake; hex-1-ene decolourises bromine water, hexane does not
Define the term electrophile.
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an electron-pair acceptor / a species that accepts a pair of electrons to form a covalent bond
Explain why 2-bromopropane is the major product when HBr reacts with propene.
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HBr adds across the C=C via electrophilic addition; H attaches to the carbon with more H atoms / C1 this places the positive charge on C2 forming a secondary carbocation the alternative pathway forms a primary carbocation on C1 the secondary carbocation is more stable due to the +I / electron-donating effect of two alkyl groups, so it forms preferentially
Discuss the mechanism of the reaction between bromine and ethene, including how a non-polar Br₂ molecule can act as an electrophile. In your answer you should: explain how the Br₂ becomes polarised; describe the role of the π electrons; identify the intermediate and the final product
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as Br₂ approaches the alkene, the π electrons of the C=C repel the bonding electrons in Br₂ this induces a dipole, with δ⁺ on the closer Br atom and δ⁻ on the further Br the π electrons attack the δ⁺ Br, forming a new C–Br σ bond the Br–Br bond breaks heterolytically, releasing a Br⁻ ion a carbocation intermediate (CH₂⁺–CH₂Br) is formed the Br⁻ ion donates a lone pair to the positively charged carbon, forming 1,2-dibromoethane
Draw the repeat unit of the polymer formed from chloroethene (CH₂=CHCl).
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–[CH₂–CHCl]ₙ– drawn with bonds extending through the brackets; subscript n shown outside the brackets
State the type of polymerisation undergone by alkenes and explain why no small molecule is eliminated during the process.
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addition polymerisation; the π bond of the C=C breaks and the carbons link directly so all atoms of the monomer are retained in the polymer
Explain why most addition polymers cause environmental disposal problems.
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they contain only strong, non-polar C–C and C–H bonds; these are not broken down (hydrolysed) by microorganisms, so polymers persist in landfill / are non-biodegradable
State one toxic gas produced when poly(chloroethene) is incinerated and describe how it can be removed from flue gases.
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HCl (hydrogen chloride); passed through an alkaline scrubber containing CaO / Ca(OH)₂ which neutralises the HCl
Suggest one advantage and one disadvantage of incinerating waste polymers compared with sending them to landfill.
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advantage — recovers energy as heat / electricity / reduces volume of waste; disadvantage — releases CO₂ contributing to climate change / produces toxic gases needing treatment