3.3.4 - ALKENES

Question 1

• what is an alkene
• how is it formed

Answer 1

• unsaturated hydrocarbon, C=C bond
• general formula; CnH2n
• double bond formed from
  1 sigma bond (s orbital overlap)
  1 pi bond (p orbital overlap)

Question 2

1. what is electrophilic addition
2. what determines which product is “major”

Answer 2

1. When a + or δ+ species is attracted to an area of high electron density and “adds” itself to the molecule
2. Major product is formed through the most stable carbocation, as it provides a greater inductive effect

Question 3

1. What is a carbocation
2. Describe the types of carbocations that can form during electrophilic addition
3. How does this influence the abundance of products

Answer 3

1. When the C=C bond breaks, one Carbon will gain a + charge, forming a carbocatION
2. Primary - C+ is bonded to 1 other carbon
   Secondary - C+ is bonded to 2 other carbons
   Tertiary - C+ is bonded to 3 other carbons
3. The more C’s a C+ is bonded to, the more stable it is, meaning it forms the major product, due to a greater inductive effect

Question 4

Outline the halogenoalkane electrophilic addition mechanism (Ex. Br2)

Answer 4

1. Electron dense C=C repels electron pair in Br—Br bond, pushing the e-‘s to the second Br atom
2. Induces a temporary dipole in the Br—Br bond
3. Br—Br splits through heterolytic fission, 1 Br takes the electron pair
4. Br δ+ bonds to the C atom
5. Br- atom bonds to the C+, forming a halogenoalkane

Question 5

Describe and explain how the test for alkenes work

Answer 5

Bromine water turns orange ➡️ colourless
1. Bromine water added to sample (giving orange colour)
2. Bromine atoms “consumed” through electrophilic addition
3. Orange colour removed, making solution go colourless

Question 6

Outline the sulfuric acid electrophilic additio mechanism
- H2SO4

Answer 6

1. δ+ H in H2SO4 attracted to electron dense C=C bond, which breaks, forming a carbocation
2. Electrons from the H — O bond in H2SO4 move to the O atom
3. Lone pair of e’s move to the carbocation, bonding the HSO4- molecule to the organic molecule

Question 7

Outline the water electrophilic addition mechanism
- H2O, strong acid catalyst

Answer 7

1. H+ (acid catalyst) attracted to C=C bond, which breaks
2. Broken C=C bond forms a carbocation
3. Lone pair on H2O bonds to carbocation
4. O atom has 3 bonds, so must gain electrons to stabilise positive charge
5. Electrons from O — H bond move to the O atom
6. An alcohol is formed and a H+ ion, which can catalyse the reaction again