AQA CHEMISTRY

• Alkene definition

  Unsaturated hydrocarbons.
• General formula of alkenes

  CnH2n
• What is required somewhere in the structure of alkenes?

  A carbon-carbon double bond.
• What is the arrangement of bonds around the C=C? (Shape and bond angle)

  Planar, 120 degrees.
• What is stereoisomerism?

  Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms.
• Why do E-Z isomers exist?

  Due to restricted rotation about the C=C bond.
• When do E/Z stereoisomers arise?

  (a) There is restricted rotation around the C=C double bond.

  (b) There are two different groups/atoms attached both ends of the double bond.
• What type of bonds easily rotate?

  Single carbon-carbon covalent bonds.
• Z isomer

  High priority groups on same side of double bond
• E isomer

  The isomer with the high-priority groups on opposite sides of the double bond
• What is the priority group?

  The atom with the bigger Atomic number is classed as the priority atom.
• Physical properties of alkenes

  Double bond doesn't affect properties. This means the physical properties are similar to alkanes. Melting and boiling point increase with number of carbon atoms.

  Not soluble in water.
• Electrophile definition

  An electron pair acceptor.
• Test for double bond

  Add a few drops of bromine solution, and the solution should decolourise because the products are colourless.
• Combustion

  Alkenes will burn in air, however are not used as fuels, can be used for other purposes.
• What type of reaction do alkenes undergo?

  Electrophilic Addition reactions.
• Why do alkenes experience addition reactions?

  The double bonds in alkenes are areas with high electron density (has 4 electrons). This attracts electrophiles and the alkenes undergo addition reactions.
• Reaction of bromine with alkenes

  What is the conditions?

  Room temperature (not in UV light).
• Reaction of bromine with alkenes

  What is the mechanism?

  Electrophilic addition.
• Reaction of bromine with alkenes

  What is the type of reagent?

  Electrophile Br+.
• Reaction of bromine with alkenes

  What happens to the alkene in terms of polarity when this reaction takes place?

  As the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond. This INDUCES a DIPOLE. Br2 becomes polar and ELECTROPHILIC (Brδ+ ).
• Reaction of bromine with alkenes

  What is the intermediate formed called?

  The INTERMEDIATE formed, which has a positive charge on a carbon atom is called a CARBOCATION.
• Reaction of hydrogen bromide with alkenes

  What is the change in functional group?

  Alkene → halogenoalkane
• Reaction of hydrogen bromide with alkenes

  What is the conditions?

  Room temperature.
• Reaction of hydrogen bromide with alkenes

  What is the mechanism?

  Electrophilic addition
• Reaction of hydrogen bromide with alkenes

  What is the type of reagent?

  Electrophile Hδ+
• Reaction of hydrogen bromide with alkenes

  Explanation of what happens with the bonds.

  HBr is a polar molecule because Br is more electronegative than H. The H δ + is attracted to the electron-rich pi bond.
• Reaction of hydrogen bromide with alkenes

  What can this reaction lead to?

  This reaction can lead to two products when the alkene is unsymmetrical.
• Reaction of hydrogen bromide with alkenes

  What is Markownikoff's Rule in relation to this reaction?


  In most cases, bromine will be added to the carbon with the fewest hydrogens attached to it.
• Reaction of hydrogen bromide with alkenes

  What happens if the alkene is unsymmetrical?


  If the alkene is unsymmetrical, addition of hydrogen bromide can lead to two isomeric products.
• In electrophilic addition to alkenes, how is the major product formed?

  In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate.
• Reaction of hydrogen bromide with alkenes

  But-2-ene reaction with HBr mechanism example

  
• Reaction of hydrogen bromide with alkenes

  But-2-ene reaction with HBr major and minor product

  
• Reaction of hydrogen bromide with alkenes

  But-2-ene reaction with HBr why is the major product, the major product?

  This carbocation intermediate is more stable because the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it.
• Alkyl groups

  Have a tendency to release electrons, this stabilises the positive charge of the intermediate carbocation. The more alkyl groups, the more stable to carbocation.

  -CH3 or -C2H5 are examples
• Tertiary carbocations

  Three alkyl groups

  Attached to a C+

  
• Secondary carbocation

  A carbocation which has two carbon atoms directly bonded to the positively charged carbon.

  
• Primary carbocation

  A carbocation which has one carbon atom directly bonded to the positively charged carbon.

  
• Order of stability for carbocations

  tertiary > secondary > primary
• Reaction of Sulphuric acid with Alkenes

  What is the change in functional group?

  Alkene → alkyl hydrogensulphate