13 - Alkenes

Question 1

What are alkenes?

Answer 1

• unsaturated hydrocarbons

Question 2

What do the C=C bonds in alkene consist of?

Answer 2

• a sigma bond and a Pi bond
• this prevents the rotation around the bond (making them different from alkanes)

Question 3

What is a Pi bond?

Answer 3

• it is the sideways overlap of two “p” orbitals
• which results in a high electron density above and below a line directly in between two bonding atoms

Question 4

(E.g.) What is the bonding in ethene?

Answer 4

• it has a C=C double bond which consists of a sigma and a Pi bond
• the C-C sigma bond lies in the same plane as the C-H sigma bonds
• the p-orbitals sit at 90 degrees to the plane (overlap to form a Pi bond)
• which results in. a high electron density above and below the plane of the molecule

Question 5

What is the shape around the carbon atom?

Answer 5

• trigonal planar
• 120 degrees
  
  • there are 3 regions of electron density around each of the carbons
  • those regions repel each other as far apart as possible
  • all of those atoms are in the same plane

Question 6

What are stereoisomers?

Answer 6

• compounds with the same structural formula but with a different arrangement in space

Question 7

What is E/Z isomerism?

Answer 7

• example of stereoisomerism (only occurs when there’s a C=C double bond)
• this causes the groups attached to each carbon to be fixed relative to each other
  
  • E = ‘zig zag’ shape
  • Z = ‘same side’

Question 8

What is cis-trans isomerism?

Answer 8

• special case of E/Z isomerism
• cis =Z isomer
• trans = E isomer

Question 9

What is the Cahn-Ingold-Prelog rule?

Answer 9

• higher priority groups on the same side = Z isomer
• higher priority groups diagonally placed = E isomer

Question 10

What is the reactivity of alkenes?

Answer 10

• much more reactive than alkanes due to the Pi bond
• the Pi electrons are more exposed than the sigma electrons (so it readily breaks and alkenes undergo addition reactions)
• the Pi bond has a lower bond enthalpy than the sigma bond (broken more easily)

Question 11

What are the addition reactions of alkenes?

Answer 11

• hydrogenation: (alkanes)
  
  • H2 + nickel catalyst + 423K
• halogenation: (dihaloalkanes)
  
  • halogens (Cl2 or Br2) + room temp.
  • testing for unsaturation = add bromine water, orange colour disappears
• hydrohalogenation: (haloalkanes)
  
  • gaseous hydrogen halides + room temp.
• hydration: (alcohols)
  
  • H2O (steam) + (sulfuric/phosphoric) acid catalyst

Question 12

What is an electrophile?

Answer 12

• an electron pair acceptor
• it is attracted to an electron-rich centre
• usually a positive ion/molecule containing atom with partial positive charge

Question 13

What is electrophilic addition?

Answer 13

• mechanism for addition reactions of alkenes (to form saturated compounds)
  
  • e.g. HBr + but-2-ene
  • polar molecule as Br (-) is more electronegative than H (+)
  • electron pair in Pi bond is attracted to the partially positive H atom (breaks double bond)
  • bond forms between H and C (from C=C)
  • heterolytic fission breaks H-Br (Br- and carbocation are formed)
  • Br- ion reacts with carbocation and forms addition product

Question 14

What is Markownikoff’s Rule?

Answer 14

• hydrogen:
  
  • attaches itself to the C with a greater no. of H atoms and smaller no. of C atoms

Question 15

What is the order of stability of carbocations?

Answer 15

• tertiary > secondary > primary
• the more alkyl groups attached to the carbocation, the more stable as they are electron releasing which reduces the charge on the ion
• charge is more spread out

Question 16

What is addition polymerisation?

Answer 16

• where addition polymers are formed from alkenes
• ** poly(alkenes) are unreactive, like alkanes, due to the strong C-C and C-H bonds **

Question 17

What are