Alkenes

• reactions of alkenes

  1. adding H2
  2. adding a halogen
  3. adding hydrogen halide
  4. adding steam
• what happens in all the addition reactions of alkenes?

  • pi bond breaks
  • makes a saturated compound
  • the added molecules breaks and is added across the C=C bond
  • one product made
• Hydrogenation

  alkene + H2 -> Alkane

  conditions: Nickel (catalyst)
• Why is the product of hydrogenation solid?

  1. the molecules are packed together more effectively than alkenes
  2. there is more surface contact between molecules, so London forces are stronger
• Halogenation of alkenes

  alkene + halogen -> dihaloalkane
• How to test for unsaturation in alkenes

  bromination— bromine water which is brown/orange when added to a compound with a C=C bond, it will decolorise
• The addition of hydrogen halides

  Alkene + hydrogen halide -> haloalkane
• the hydration of alkenes

  alkene + water -> alcohol
• what are the conditions for the hydration of alkenes?

  • phosphoric acid (steam in the presence of phosphoric acid)
  • 300 degrees celcius
• What happens if the alkene molecule isn’t symmetrical and the added molecule is not symmetrical?

  Two products are formed which are structural Isomers of each other
• what is electrophilic addition mechanism?

  1. shows in series of steps, how the addition reactions take place
  2. explains why asymmetrical alkenes form structural isomers
• what is an electrophile?


  An atom or group of atoms that is attracted to an electron rich centre where it accepts a pair of electrons.

  An electrophile is usually a positive ion or a molecule with a partial positive charge (delta positive)
• What does a double C=C bond represent?

  a region with high electron density/electron rich centre and will attract electrophiles
• When an electrophile is approaching an alkene, it is attracted to the high electron density area of the pi bond.
• The pi bond breaks in heterolytic fission, both pi electrons go to one carbon atom and this C atoms form a covalent bond with an electrophile.
• The bond in the electrophile breaks in a heterolytic fission, at the same time.
• When the bond in the electrophile is broken, a negative ion is formed, as the leaving atom has 2 electrons, the intermediate is called a carbonation ( the carbon atom has a positive charge).
• The negative ion will be attracted to the positive carbon and will donate the pair of electrons to form a covalent bond.
• markownikoff’s rule

  the hydrogen from hydrogen halide or H2O attaches itself to the carbon atom in the C=C with the greater number of hydrogen atoms.
• carbonation

  primary, secondary and tertiary
• why are the major and minor products formed?

  due to the stability of the intermediate carbonations, more stable the carbonation, the more formed
• how do you explain major and minor products?

  classify the carbonation, primary, secondary etc, then say which one is more stable, more stable, more formed
• heterolytic fission is when a molecule splits into two molecules
• homolytic fission


  the dissociation of a given molecule wherein one electron is retained by each of the original fragments of the molecule.
• pi bond - covalent bond formed when ​2 orbitals overlap sideways, the area above and below a sigma bond
• what is a sigma bond?

   A type of covalent bond which forms when atomic orbitals overlap head-on. Sigma (σ) bonds can rotate freely.
• Pi bonds can only be formed after a sigma bond is formed
• stereoisomerism in alkenes

  the pi bond stops the C=C atoms from rotating around the double bond which leads to stereoisomerism
• stereoisomers 


  Same molecular and structural formulae but a different arrangement of atoms in a space
• E/Z stereoisomersm

  1. A C=C bond present - restricted rotation about the C=C bond; the groups attached to each C in C=C are fixed in position
  2. Different atoms or group of atoms attached to EACH carbon atom in C=C
• Z - together (on same side)
• E - across (on opposite sides)
• cis/trans stereoisomerism

  1. one of the attached groups on each carbon in C=C must by hydrogen
  2. a C=C bond present, restricted rotation
  3. different atoms or groups of atoms attach to EACH carbon atom in C=C
• The cahn ingold prolog nomenclature (CIP) rules:

  If the groups of higher priority are placed across the C=C, the compound is the E isomer.

  If the groups of higher priority are placed on the same side of the C=C, the compound is the Z isomer.

  higher priority = higher atomic number
• Alkenes are unsaturated hydrocarbons that contain at least one C=C bond made up of a π bond and a σ bond
• The general formula of alkenes is CnH2n
• A π bond is formed when electrons in the adjacent p orbitals overlap above and below the carbon atoms, after a σ bond is formed
• The π bond restricts the rotation of carbon atoms
• The angle and shape of a double bond is trigonal planar with 120°
• Alkenes are more reactive than alkanes due to the high electron density of the double bond and the fact that the pi-bond is slightly easier to break