Alkenes

• General formula of alkenes?

  CnH2n
• the functional group contains a double covalent bond which an area of high electron density
• Unsaturated?

  contains one or more carbon carbon double bond
• the shape around each C atom in the carbon carbon double bond is Trigonal planar and the bond angle is 120
• The C=C group has a pi bond, which forms when p-orbitals overlap sideways
• the presence of the pi bond means there is restricted rotation about the planar (trigonal) C=C bond
• Alkenes are named based on the longest unbranched carbon chain with the suffix ene
• in alkenes with 4 or more carbon atoms in the longest chain, the position of the C=C bond must be indictated
• Some unsaturated molecules contain more than one carbon- carbon double bond
• A diene has two C=C bonds
• A triene has three C=C bonds
• cycloalkanes have two less hydrogen atoms than the open chain alkene and are named according to the largest ring containing the C=C bond
• Structural Isomers?


  compounds with the same molecular formula but a different structural formula
• Strereoisomers?

  compounds with the same structural formula but different arrangement of atoms in space
• E/Z isomerism occurs as a result of restricted rotation about the planar carbon-carbon double bond
• For an alkene to exhibit E/Z isomerism each carbon atom of the carbon-carbon double bond must be bonded to 2 different groups
• To determine whether an alkene is the E or Z isomer

  • Identify the highest priority group on each C atom of the C=C
  • If both highest priority groups are on the same side of the C=C bond it is the Z isomer
  • If the highest priority groups are on opposite sides of the C=C bond it is the E isomer
• Cahn-Ingold-Prelog Priority Rules?

  The CIP rules allow the highest priority group attached to a C atom to be determined easily
• CIP rules?

  • The atom with the highest atomic number has the highest priority
  • If two atoms have the same atomic number, the next atom along the chain should be compared. The first point of difference will be used to assign priority.
• Alkenes are reactive due to the high electron density of the C=C double bond
• Alkenes undergo addition reactions in which a small molecule is added across the C=C group, forming a saturated molecule
• Electrophile?

  an electron pair acceptor
• Alkenes react with electrophiles because electrophiles are attached to the high electron density in the C=C bond
• During the reaction, covalent bonds are both broken and formed producing a reactive positively charges intermediate called a carbocation
• a mechanism shows the breaking and forming of covalent bonds using curly arrows
• Curly arrows?

  shows the movement of a pair of electrons
• Curly arrows must be drawn accurately and start from either

  • a lone pair
  • a covalent bond
• alkenes are reactive due to the high electron density of the C=C double bond
• alkenes undergo addition reactions in which a small molecule is added across the C=C group forming a saturated molecule
• Electrophile?

  an electron pair acceptor
• organic hydrogensulfates are ready hdrolysed with warm water to form an alcohol
• Hydrolysis?

  breaking of a bond using water
• alcohols can be prepared by the hydration of alkenes with steam in teh presence of a phosphoric acid catalyst
• carbocations are classified as either primary, secondary or tertiary
• the classification is based on the number of carbon atoms directly attached to the carbon with the positive charge
• alkyl groups have a tendency to release electrons which help to stabilise the positive charge of carbocations. This is called positive inductive effect
• Tertiary carbocations are the most stable as they have the most alkyl groups atached
• Stability of Cations?

  3->2->1
• The major product is produced in the largest amount
• the minor product is produced in the smallest amount