Chemistry (AS Level)·Hydrocarbons · NSSCAS 4.2.2
Alkenes
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Alkenes are hydrocarbons with a reactive carbon–carbon double bond. You'll learn their general formula CₙH₂ₙ, how to draw and name them up to six carbons, and how to deduce a molecular formula. Then we tackle their chemistry: addition of hydrogen, steam, hydrogen halides and halogens — including Markovnikov addition — plus oxidation by manganate(VII), the electrophilic addition mechanism with curly arrows, carbocation stability, and addition polymers such as poly(ethene) and PVC together with the waste problem they create.
What you'll learn in this lesson
By the end you should be able to (NSSCAS Chemistry (AS Level) 4.2.2):
- Interpret and use general (CₙH₂ₙ), structural, displayed and skeletal formulae of alkenes
- Name alkenes up to six carbons, e.g. but-1-ene, but-2-ene
- Deduce the molecular formula of an alkene from a given formula
- Describe the reactions of ethene and propene: addition of hydrogen (H₂/Ni), steam (H₂O, H₃PO₄), hydrogen halides (HBr) and halogens (Br₂), including Markovnikov addition using propene + HBr
- Describe oxidation by cold dilute acidified manganate(VII) to a diol (and the decolourising test for C=C)
- Describe oxidation by hot concentrated acidified manganate(VII) rupturing the C=C to locate the double bond
- Describe heterolytic fission
- Describe the mechanism of electrophilic addition using Br₂ + ethene and HBr + propene (curly arrows, carbocation intermediate)
- Explain the inductive effect of alkyl groups on carbocation stability (tertiary > secondary > primary)
- Describe addition polymerisation exemplified by poly(ethene) and PVC; deduce the repeat unit from a monomer and the monomer from a polymer
- Recognise the difficulty of disposing of poly(alkene)s and discuss biodegradable and water-soluble polymers
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