14 Alkenes

• Name the compounds

  A) But-1-ene

  B) But-2-ene

  
• The carbon carbon double covalent bond consists of one sigma (σ) bond and one pi (π) bond
• Pi bonds are exposed and have a high electron density, so they are vulnerable to attack by species which are attracted to electrons e.g. electrophiles
• Alkenes are unsaturated hydrocarbons with the general formula:

  $CnH2n$

  and they contain a carbon carbon double bond in their structure
• The arrangement of bonds around the carbon carbon double bond is planar and has the bond angle 120 degrees
• Fill in the blanks

  A) sigma

  B) pi

  C) sigma

  
• Stereoisomers have the same structural formulae but a different arrangement of atoms in space
• E-Z isomers exist due to restricted rotation about the carbon carbon double bond
• E-Z stereoisomers arise when:

  • There is restricted rotation around the carbon carbon double bond
  • There are two different atoms or groups of atoms attached to both ends of the double bond
• Name the compounds:

  A) Z-but-2-ene

  B) E-but-2-ene

  
• But-1-ene is a structural isomer of but-2-ene but does not show E-Z isomerism
• How to name E-Z isomers

  k

  1. Determine the priority groups on both sides of the double bond - The atom with the larger atomic number is classified as the priority atom
  2. If the priority atom is on the same side of the double, it is labelled Z
  3. If the priority atom is on the opposite side of the double bond, it is labelled E
• An electrophile is an electron pair acceptor
• An addition reaction is a reaction where two molecules react to produce one
• The double bonds in alkenes are areas with high electron density, so this attracts electrophiles and the alkenes undergo addition reactions
• Reaction of bromine with alkenes:

  • Change in functional group - alkene -> dihalogenoalkane
  • Reagent - bromine
  • Conditions - room temperature (not in UV light)
  • Mechanism - electrophilic addition
  • Type of reagent - electrophile
• electrophilic addition of an ethene with bromine:

  A) 1,2-dibromoethane

  
• In the reaction of bromine with alkenes, as the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in the bromine bromine bond, which induces a dipole. Br2 becomes polar and electrophilic (Brδ+).
• Outline the mechanism for the reaction of bromine with ethene:

  This is the answer

  
• Reaction of hydrogen bromide with alkenes:

  • Change in functional group - alkene -> halogenoalkane
  • Reagent - HCl or HBr
  • Conditions - room temperature
  • Mechanism - electrophilic addition
  • Type of reagent - electrophile
• Reaction of hydrogen bromide with alkenes:

  A) but-2-ene

  B) 2-bromobutane

  
• HBr is a polar molecule because bromine is more electronegative than hydrogen so the slightly polar positive hydrogen is attracted to the electron-rich pi bond
• Outline the mechanism for the reaction of hydrogen bromide with alkenes

  This is the answer

  
• Markownikoff's rule states that in most cases, bromine will be added to the carbon with the fewest hydrogens attached to it
• If the alkene is unsymmetrical, the addition of hydrogen bromide can lead to two isomeric products
• In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate
• Tertiary carbocations are the most stable because the methyl groups on the sides of the positive carbon are electron releasing and so they reduce the charge on the ion, therefore stabilising it
• Tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations
• Reaction of sulphuric acid with alkenes STAGE 1:

  • Change in functional group - alkene -> alkyl hydrogensulfate
  • Reagent - concentrated sulfuric acid
  • Conditions - room temperature
  • Mechanism - electrophilic addition
  • Type of reagent - electrophile
• Sulfuric acid is best written as H-OSO2OH
• Reaction of sulfuric acid with alkenes STAGE 2:

  • Change in functional group - alkyl hydrogensulfate -> alcohol
  • Reagent - water
  • Conditions - warm mixture
  • Type of reaction - hydrolysis
• Mechanism of the reaction of sulfuric acid with alkenes:

  A) electrophilic addition

  B) hydrolysis

  C) H2SO4

  D) OSO2OH

  
• Reaction of sulfuric acid with alkenes STAGE 1 Equation:

  A) ethene

  B) sulfuric acid

  C) ethyl hydrogensulfate

  
• Reaction of sulfuric acid with alkenes STAGE 2 Equation:

  A) ethyl hydrogensulfate

  B) water

  C) ethanol

  D) sulfuric acid

  
• Hydrolysis is a reaction where a molecule is split by the addition of water
• In the reaction of sulfuric acid with alkenes, the overall role of sulfuric acid is that of a catalyst because it is regenerated
• Industrially, alkenes are converted to alcohols in one step rather than two with sulfuric acid, as they react with water in the presence of an acid catalyst.
• Hydration is a reaction where water is added to a molecule
• Hydration of an alkene:

  A) ethene

  B) steam

  C) ethanol

  
• Essential conditions for the hydration of alkenes industrially:

  1. High temperature - 300 to 600 degrees celsius
  2. High pressure - 70 atmospheres
  3. Catalyst - concentrated phosphoric acid (H3PO4)