Alkenes - Organic (6)

Question 1

What are sigma and pi bonds? (2)

Answer 1

1. Sigma bonds are covalent bonds formed when electron orbitals overlap axially (end-on)
2. Pi bonds are covalent bonds formed when parallel electron orbitals overlap sideways

Question 2

Draw the mechanism for electrophilic addition of alkenes and halogens/hydrogen halides (3)

Answer 2

See red Pearson textbook p191

Question 3

Define electrophile (1)

Answer 3

1. An electrophile is a chemical species that is attracted to a region of high electron density

Question 4

Describe the addition reactions of alkenes with:

1. Hydrogen (4)
2. Halogen (1)
3. Halogen (aq) (1)
4. Hydrogen halide (1)
5. Steam (4)
6. Potassium Manganate (VII) (2)

State conditions where necessary

Answer 4

1. Alkene (CH₂CH₂) + H₂ → CH₃CH₃
   
   • Ni catalyst
   • 150°C
   • 5 atm
2. Alkene (CH₂CH₂) + Halogen (Cl₂) → CH₂Cl-CH₂Cl
3. Alkene (CH₂CH₂) + Halogen (aq) [Cl₂(aq)] → CH₂Cl-CH₂OH
4. Alkene + hydrogen halide → halogenoalkanes
5. Alkene + steam → alcohol
   
   • phosphoric acid (catalyst)
   • 300 degrees C
   • 60 atm
6. Alkene + Potassium Manganate (VII) [KMnO₄] → diol
   
   • Acid conditions

Question 5

Draw the mechanism for electrophilic addition reactions between unsymmetrical Alkene and hydrogen halide e.g. Propene + HBr (5)

Label the major and minor products and explain why this happens according to carbocations. (2)

Answer 5

1. For mechanism see red Pearson textbook p192
2. Secondary carbocation (attached to two other carbon atoms) is more stable than primary carbocation
3. Therefore, stays in solution longer resulting in an increased chance for collisions to occur forming the major product