13 - alkenes

• Are alkenes saturated or unsaturated?

  Unsaturated.
• What do alkene molecules contain?

  At least one carbon-carbon double bond.
• What is the general formula of aliphatic alkenes?

  CnH2n
• What are the first three members of the alkene homologous series?

  Ethene, Propene and Butene.
• What bonds are present in alkenes?

  The carbon with a double bond forms 3 σ-bonds and 1 π-bond,
• Where is the π-bond located in an alkene?

  The π-bond is one of the shared pairs of electrons in the carbon-carbon double bond.
• How is the π-bond formed in an alkene?

  Sideways overlap of the p-orbitals of each carbon atom.

  
• Where is the π-electron density concentrated?

  Above and below the plane (line) of the two carbon atoms.
• Why is the geometry of an alkene different from an alkane?

  The π-bond of an alkene locks the two carbon atoms in position and prevents them from rotating around the double bond. This is different from alkenes where rotation is possible around every atom.
• What is the shape around a each of the carbon atoms in a double bond?

  Trigonal planar
• How many regions of electron density are there around each carbon atom in a double bond?

  3
• What is the bond angle around each carbon atom in a double bond?

  120 degrees.
• Why is there a trigonal planar shape around each of the carbon atoms in a double bond?


  - There are three regions of electron density around each of the carbon atoms.

  - The three regions repel each other as far apart as possible, so the bond angle around each carbon atom is 120°.

  - All of the atoms are in the same plane.
• Stereoisomers


  Molecules with the same structural formula but a different arrangement of atoms in space.
• What are the two types of stereoisomerism?

  E/Z isomerism and optical isomerism
• What type of compounds does E/Z isomerism occur in?


  It only occurs in compound with a C=C double bond
• Why does stereoisomers occur around double bonds?


  Because rotation about the double bond is not possible due to the presence of the π-bond.
• What are the two conditions a molecule must satisfy to have E/Z isomerism?


  1. A C=C double bond

  2. Different groups attached to each carbon atom of the double bond.
• Why does cis-trans ismoerism occur in unsaturated compounds?


  - The groups attached to the C=C carbons remain in their fixed position.

  - This is because free rotation around the C=C bond is not possible due to the presence of the π-bond.
• Why does cis-trans isomerism not occur in saturated compounds?


  The atoms/groups attached to the single bonded carbons are not fixed in their position due to free rotation about the C-C σ-bond.
• Cis isomers


  Cis isomers have two of the same groups on the same side of the double bond. I.e. both above or both below the C=C bond.
• Trans isomers


  Trans isomers have two of the same groups on opposite sides of the double bond. I.e. one above and one below the C=C bond.
• How to identify the cis and trans isomer in cis-trans isomerism?

  Cis = same side of a double bond

  Trans = opposite side of a double bond

  
• How are the sides of the double bond divided?

  Horizontally

  
• How to identify Z isomers of E/Z isomerism?


  Z isomers have the higher priority molecules attached on the same (zame) side of the double bond.

  
• How to identify the E isomers of E/Z isomerism?


  E isomers have the higher priority molecules on opposite sides of the double bond.

  
• How did Cahn-Ingold-Prelog rule which groups attached to a double bond were higher priority?


  The higher the atomic number of the group, the higher the priority. This refers to the single atom attached to the carbon=carbon double bond.
• Why are alkenes more reactive than alkanes?


  Because of the presence of the π-bond.
• What is bond enthalpy of a C-C single bond in ethene?

  347 kJ mol⁻¹
• What is the bond enthalpy of a C=C double bond in ethene?

  612 kJ mol⁻¹
• Which bond in a C=C double bond is weaker and broken more readily?


  The π-bond.
• What may alkenes undergo addition reactions with?


  - Hydrogen (in the presence of a nickel catalyst).

  - Halogens

  - Hydrogen halides

  - Steam (in the presence of an acid catalyst)
• What happens in addition reactions?

  The π-bond of the C=C double bond breaks and new bonds form.
• What does hydrogenation of alkenes form?

  Alkane.
• Reagents for hydrogenation of alkenes

  Hydrogen gas.
• Conditions for hydrogenation of alkenes


  - Nickel catalyst

  - 200°C

  - 1000 kPa
• What is an important application of hydrogenation of alkenes?

  The production of margarine from vegetable oils.
• How is margarine made from vegetable oils?


  - Vegetable oils are unsaturated.

  - They can be hydrogenated to make margarine.

  - Margarine has a higher melting point that due to stronger London forces.
• What does halogenation of alkenes form?


  Dihaloalkane.
• What is the mechanism of halogenation of alkenes?

  Electrophilic addition.