Alkenes

• Are alkanes or alkanes more reactive and why?

  Alkenes are more reactive than alkanes and this is because of the C=C
• What happens to pi and sigma bonds when alkenes react

  - The pi bond breaks as it has a lower bond enthalpy than the sigma bond

  - The sigma bond remains intact

  - This is because the pi bond is weaker than the sigma bond
• Do C-C have a higher bond enthalpy or C=C?

  C=C because it has both pi AND sigma, not just sigma
• What are addition reactions?

  In an addition reaction, two reactant combine together to form on product
• What happens in the addition reactions of alkanes?

  C=C opens its legs and molecule added

  goes from being UNSATURATED ALKENE to SATURATED ALKANE
• What are the four types of addition reactions of alkenes?

  - Hydrogenation - Addition of hydrogen
• What is hydrogenation?


  Addition of hydrogen to alkane
• What is addition of hydrogen to alkane called?

  Addition of hydrogen
• What is reacted and made in hydrogenation and what are the conditions needed?

  Alkene + Hydrogen -> Alkane

  
  

  - Ni catalyst needed

  - Heat is needed (150 degrees celcius)
• What is dihalogenation?


  Addition of halogens (X2)
• What is addition of halogens to alkenes??

  Dihalogenation
• What is reacted and made in dihalogenation and what are the conditions needed? How do they react?

  Alkene + DIATOMIC Halogen -> Dihalogenalkane

  
  

  CONDITIONS:

  - Room temperature and pressure

  
  

  They react RAPIDLY at RTP
• Do alkenes require UV light to react with halogens?

  No - Only alkanes require UV light to react with halogens
• IF BROMINE (Br2) reacts with alkene, what happens?

  - A colour change occurs!!

  - Orange bromine is DECOLOURISED - indicating it has reacted with C=C

  - colour change confirms the presence of a C=C double bond and can be used as a test for alkenes
• What can be used as a test for alkenes and why?

  BROMINE (Br2) in dihalogenation:

  Because..

  - A colour change occurs!!

  - Orange bromine is DECOLOURISED - indicating it has reacted with C=C

  - colour change confirms the presence of a C=C double bond and can be used as a test for alkenes
• What would happen if you added bromine to an alkene and alkane

  BROMINE + ALKENE

  Goes from Orange to colourless

  
  

  BROMINE + ALKANE

  Remains colourless
• Explain how you would carry out simple test-tube tests to distinguish cyclohexane and cyclohexene

  - Add Br2 to a sample of each + shake

  - Cyclohexane = orange - stays the same - therefore no reaction

  - Cyclohexene = orange to colourless -> indicating presence of C=C as dihalogenisation has occured
• What is the addition of steam to an alkene called?

  Hydration
• What is hydration of an alkene?

  the addition of steam to an alkene
• what are the conditions needed for hydration?

  - the water must be in the form of steam

  - Phosphoric ACID catalyst (H3PO4)

  - HIGH TEMP (300 degrees)

  - pressure (60atm)
• what is made in the hydration of an alkene?

  alcohol
• What are the reactants and products of hydration?

  Alkene + water (GAS) --> Alcohol
• What can Addition of hydrogen halides or water to UNSYMMETRICAL ALKENES lead to?

  
  

  With the example of hydrogen bromide added to prop-1-ene


  Addition of hydrogen halides or water to unsymmetrical alkenes results in a mixture of isomeric products.

  
  

  EG:

  Hydrogen bromide with propene can lead to:

  2-bromopropane OR 1-bromopropane
• What is electrophilic addition?

  Electrophilic addition is the addition of an electrophile to an alkene double bond, C=C;
• Which halogen commonly goes through electrophilic addition?

  bromine
• where is the highest region of electron density in an alkene and why?

  The C=C in alkenes is a region of high electron density due to the electrons in the double bond.
• What is an electrophile?


  An electron pair acceptor
• what attacks the C=C of an alkene in electrophilic addition?

  The C=C attracts and is attacked by 'electron-liking' species called electrophiles.
• what is a CARBOCATION?

  A carbocation is an organic ion containing a positively charged carbon atom.
• What is heterolytic bond fission?

  Heterolytic bond fission:

  one of the bonded atoms takes BOTH of the shared pair of electrons when the bond breaks -> forming ions

  
  

  (Think of sharing cookie in half and one person getting al the chocolate chips and other none - therefore uneven)
• Explain the mechanism of HBr added to Ethene

  - HBr is polar (delta negative + delta positive) because Br is more electronegative than H

  
  

  • The electron pair in the Pi bond of the C=C bond is attracted to the H delta+ causing the Pi bond to break

  
  

  • The H delta+ end of the HBr is the electrophile as it accepts the

  electron pair from the Pi bond

  
  

  • A new bond forms between one of the C atoms and the H atom.

  
  

  • The H―Br bond breaks by heterolytic fission, with the Br taking the electron

  pair

  
  

  • An intermediate carbocation is formed, along with a bromide ion, Br-

  
  

  - In the second stage, the positively charged carbocation intermediate is

  immediately attacked by the negative bromide ion to form the product
• Explain the mechanism of Br2 added to Ethene

  - electrons in Pi bond repels electrons in Br2 -> POLARISING Br2 into: (Br Delta + and Br Delta -)

  - The Br delta + end behaves as the electrophile in the reaction
• Which end of the polarised Br2 acts as an electrophile?

  Br delta +
• state and explain the type of bond fission associated with the breaking of the Br-Br bond

  HETEROLYIRC BOND FISSION:

  Because the covalent bond of Br-Br breaks and both of the shared pair of electrons go to one bonded atom only
• Why are tertiary carbocations the most stable?

  each alkyl group can donate

  electrons towards the positive charge so the positive charge is spread out making the ion more stable.
• Why are primary carbocations the least stable?

  They have the fewest alkl groups, so are less able to stabilise the positive charge
• When do major and minor products form?

  Major and minor products form when an unsymmetrical alkene reacts.

  The major product will form from the most stable carbocation intermediate, with the most alkyl groups attached.
• How are major products formed?

  Major products are formed from a more stable carbocation
• how is the stability of a carbocation deterimened?

  Whether it is a:

  Primary (weakest) carbocation

  Secondary carbocation

  Tertiary carbocation `
• what is an alkyl group?


  An alkene with a missing hydrogen on a branched compound.