Alkenes

• What is the general formula for alkenes?

  CnH2n
• What type of bond must alkenes contain?

  At least one double carbon bond
• Are alkenes saturated or unsaturated molecules?

  Unsaturated
• What two types of bonds form a carbon double bond?

  One sigma bond and one pi bond
• How are sigma bonds formed?

  From the head-on overlap of s orbitals
• How are pi bonds formed?

  From the sideways overlap of p orbitals
• What does p-orbital overlap result in?

  Two regions of electron density
• What type of rotation do pi bonds have?

  Restricted rotation
• What geometry do carbons in a C=C bond have?

  Trigonal planar
• What are the bond angles of a trigonal planar geometry?

  120o
• Why do bonding pairs arrange themselves a maximum distance apart?

  To minimise repulsion between them
• What is the definition of stereoisomers?

  Same structural formula, different arrangements in space
• What type of isomers can alkenes form?

  Cis/trans or E/Z isomers
• What causes cis/trans and E/Z isomers?

  Restricted rotation around the double carbon bond
• When can alkenes be labelled as cis/trans?

  If both carbons have at least one group in common
• Where are the groups located in cis isomers?

  On the same side of the double bond
• Where are the matching groups located in trans isomers?

  On different sides of the double bond
• When can alkenes be labelled as E/Z isomers?

  When they don’t have two groups in common
• How are priority values assigned to groups in E/Z isomers?

  By adding up the molecular masses (Mr)
• How does molecular mass relate to priority?

  The larger the Mr, the higher the priority
• Where are the two highest priority groups located in Z isomers?

  On the same side of the double carbon bond
• Where are the two highest priority groups located in E isomers?

  On opposite sides of the double carbon bond
• What is the first criteria for molecules to display E/Z isomerism?

  Contain a double carbon bond
• Why does a double carbon bond mean a molecule can display E/Z isomerism?

  Restricted rotation about the C=C bond
• What is the second criteria for molecules to display E/Z isomerism?

  Each carbon needs to be attached to two different groups
• Why are alkenes more reactive than alkanes?

  The pi bond is a region of high electron density
• What type of molecules can the region of concentrated negative charge attract?

  Polar molecules
• Why is the double carbon bond easier to break?

  It has a lower bond enthalpy
• What type of reactions can alkenes take part in?

  Addition reactions
• What happens in addition reactions?

  The double carbon bond breaks and other atoms attach
• What is the atom economy of addition reactions?

  100%
• What do hydrogen and alkenes react to form?

  Alkanes
• What conditions are required for hydrogenation reactions?

  Nickel catalyst and temperatures of 150 degrees
• What do halogens and alkenes react to form?

  Halogenoalkanes
• What is the reaction of halogens with alkenes called?

  Halogenation
• What conditions are required for halogenation?

  Room temperature and pressure
• What mechanism is involved in halogenation?

  Electrophilic addition mechanism
• Why is the C=C bond particularly negative?

  The extra pi electrons
• What happens when a halogen molecule approaches the C=C bond?

  The bonding electrons in the halogen-halogen bond are repelled
• What does the repulsion of bonding electrons in the halogen molecule create?

  An induced dipole