Organic 1 Section 3 - Unit 14: Alkenes
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Explain how you would test for alkenes (2 marks)
• Shake with bromine water - The colour will change from orange to colourless
Explain why tertiary carbocations are more stable than primary carbocations (3)
• Alkyl groups on either side of the positive carbon are electron releasing • So they reduce the charge • And the carbocation is stable overall
Change in functional group, reagent and conditions for reaction of alkenes with hydrogen halides (3)
• Alkene ➡ Halogenoalkane • HBr or HCl (Hδ+) • Room Temperature
Change in functional group, reagent and conditions for reaction of alkenes with halogens (3)
• Alkene ➡ Dihalogenoalkane • Br2 (Brδ+) • Room Temperature
Change in functional group, reagent and mechanism for reaction of alkenes with sulphuric acid (6)
Step 1: - Alkene ➡ Alkyl Hydrogensulfate - Concentrated H2SO4 - Electrophilic Addition Step 2: - Alkyl Hydrogensulfate ➡ Alcohol - Warm water - Hydrolysis reaction
Change in functional group and conditions for reaction of alkenes with water (4)
• Alkene ➡ Alcohol • 300°C to 600°C • 70 atm (high pressure) • Concentrated H3PO4 (catalyst)
Explain why alkenes have lower melting and boiling points than alkanes (2)
• Less electrons in alkenes so weaker Van der Waals forces - So less energy is needed to overcome them
Explain what is meant by the term ‘electrophile’ (1 mark)
• An electron pair acceptor
Explain why, even though bromine molecules are non-polar, they react with the double bonds in alkenes (2 marks)
• Double bond have high electron densities - So bromine forms an induced dipole
Explain what is meant by an addition reaction (1 mark)
• Two reactants form one product
Explain how a bromine molecule is able to act polar even though it is normally non-polar (1 mark)
• The electrons of the double bond polarise the Br-Br bond - And Brδ+ becomes electrophilic