Chapter 13 - Alkenes

• Are alkenes saturated or unsaturated hydrocarbons?

  unsaturated due to C=C double bond
• What is the general formula of alkenes?

  CnH2n
• What is the functional group of alkenes?

  C=C
• Describe and explain the formation of a pi-bond

  -each carbon atom in the double bond forms 3 sigma bonds, with one electron left over in a p-orbital

  -the 2 p-orbitals overlap, forming the pi-bond

  -the pi-electron density is concentrated above and below the sigma-bond

  
• Why can't the double bond rotate?

  the pi-bond locks 2 carbon atoms in position
• What is the shape and bond angle around each carbon atom in the double bond?

  -trigonal planar

  -120

  
• What is a stereoisomer?

  A compound with the same structural formula, but with a different arrangements of atoms in space
• What are the 2 types of stereoisomerism?

  -E-Z isomerism

  -Optical isomerism
• What conditions must compounds have to undergo E/Z isomerism?

  -C=C double bond

  -different groups attached to each carbon atom in the double bond
• What conditions must compounds have to undergo Cis-Trans isomerism?

  -one of the attached groups on each carbon atom in the double bond must be a hydrogen atom
• What is the Cis isomer the same as?

  Z isomer
• What is the Trans isomer the same as?

  E isomer
• What is the Cahn-Ingold-Prelog principle?

  the atoms attached to the carbon atoms in the double bond are assigned a priority based on their atomic number

  
• What is a Z isomer?

  the groups of higher priority (higher atomic number) are on the same side of the double bond
• What is an E isomer?

  the groups of higher priority (higher atomic number) are diagonally placed across the double bond
• What is priority dependent on?

  atomic number
• Why are alkenes more reactive than alkanes?

  the pi-electrons are more exposed than electrons in the sigma-bond, resulting in the pi-bond readily breaking
• What is an addition reaction?

  a reaction in which a small molecule adds across the double bond, causing the pi-bond to break and for a saturated product to form
• Describe the test for unsaturation

  -bromine water is added dropwise to an unsaturated compound

  -bromine adds across the double bond

  -in an unsaturated compound, the solution turns colourless

  -in a saturated compound, the solution remains orange

  
• Explain how hydrogenation of alkenes takes place

  alkene is mixed with hydrogen and passed over a nickel catalyst at 423K. Hydrogen adds across the double bond, forming a saturated compound

  
• What is meant by the halogenation of alkenes?

  alkenes undergo a rapid addition reaction with the halogens

  
• What conditions are needed for addition reactions of alkenes with hydrogen halides?

  alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes
• How many products are formed when an unsymmetrical alkene reacts with an unsymmetrical compound?

  2
• What conditions are needed for alkenes to be converted into alcohols?

  -steam

  -phosphoric acid catalyst or concentrated sulfuric acid catalyst

  
• What is electrophilic addition?

  A type of addition reaction in which an electrophile is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond
• What is an electrophile?


  an atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair
• What do curly arrows represent?

  Movement of a pair of electrons
• What is a carbocation?

  An organic ion containing a positively charged carbon atom
• What is Markownikoff's rule?

  In electrophilic addition to alkenes, the major product is formed via the more stable carbocation
• List the carbocations in order of increasing stability

  primary-secondary-tertiary

  
• Why are tertiary carbocations the most stable?

  each alkyl group pushes electrons towards the positive charge of the carbocation, spreading the positive charge over the alkyl groups
• Describe and explain the electrophilic addition of ethene with hydrogen bromine

  1) Br is more electronegative than H, so permanent dipoles are created

  2) Electron pair in pi-bond is attracted to partially positive hydrogen atom, breaking the double bond

  3) Bond forms between H and C

  4) H-Br bond breaks by heterolytic fission, with the electron pair going to Br

  5) Br- ion and a carbocation are formed

  6) Br- reacts with carbocation to form bromoethane

  
• Describe and explain the electrophilic addition of propene with bromine

  1) Br is non-polar and when it approaches an alkene, the pi-electrons interact with the Br-Br bond, causing polarisation and creating an induced dipole

  2) Electron pair in pi-bond is attracted to partially positive Br, causing the double bond to break

  3) Bond forms between Br and C. Br-Br bond breaks by heterolytic fission

  4) Electron pair goes to partially negative Br, forming Br- and a carbocation

  5) Br- reacts with carbocation, forming 1,2-dibromopropane

  
• What are polymers?

  extremely large molecules formed from many thousands of monomers
• What type of polymerisation do unsaturated alkenes undergo?

  addition polymers

  
• What is a repeat unit?

  the specific arrangement of atoms in the polymer molecule that repeats over and over again
• What monomer is poly(ethene) made from?

  ethene
• What can poly(ethene) be used for?

  supermarket bags, shampoo bottles, children's toys
• What is PVC used for?

  pipes, films and sheeting, insulation

  
• What are the environmental concerns of alkene-based polymers?

  -most are non-biodegradable

  -some are difficult to recycle

  -extraction of oil can damage habitats