3.4 Alkenes

Question 1

What is the functional group in alkenes

Answer 1

A carbon-carbon double bond (they are unsaturated hydrocarbons).

Question 2

How does the double carbon-carbon bond allow for electrophilic addition to occur

Answer 2

• The double bond is an area of high electron density
• This consists of a normal covalent bond (σ - sigma bond) and a π bond.
• This makes it susceptible to attack from electrophiles

Question 3

How do covalent bonds form (in terms of orbitals)

Answer 3

• Each atom has an atomic orbital containing a single unpaired electron
• A covalent bond forms when the atomic orbitals overlap to form a combined orbital containing 2 electrons ( a molecular orbital)

Question 4

How are sigma bonds formed

Answer 4

• From end to end overlap of atomic orbital
• The electron density is symmetrical about a line joining the nuclei of the atoms forming the bond
• The pair of electrons is found between the nuclei of the 2 atoms
• The electrostatic attraction between the electrons and nuclei bonds the atoms to each other

Question 5

How are pi bonds formed

Answer 5

• From the sideways overlap of adjacent p orbitals
• The 2 lobes that make up the pi bond lie above and below the plane of the sigma bond
• This maximises overlap of the p orbitals
• The 2 clouds of electrons in a pi bond represent one bond containing 2 electrons

Question 6

Why does isomerism happen in alkenes

Answer 6

• The groups attached to the C=C remain fixed in their position
• This is because free rotation of the bonds about the C=C bond is not possible due to the presence of a pi bond

Question 7

What is an electrophile

Answer 7

A species that accepts electrons.

Question 8

What are 3 examples of electrophiles

Answer 8

• A halogen (e.g Br2)
• A hydrogen halide (e.g. HBr)
• Sulfuric acid (H2SO4)

Question 9

Describe the electrophilic addition of hydrogen bromide

Answer 9

• HBr is polar as Br is more electronegative than H
• The delta positive H accepts a pair of electrons from the C=C bond, forming a Br- ion
• This results in a carbocation intermediate which reacts with the Br- ion

Question 10

Describe the electrophilic addition of bromine

Answer 10

• Bromine is non-polar, but because the C=C bond is an area of high electron density, it induces a dipole in the Br2 molecule
• It then acts in the same way as HBr

Question 11

What are the 3 types of carbocation (from least to most stable)

Answer 11

• Primary (least stable)
• Secondary
• Tertiary (most stable)

Question 12

Why is a secondary carbocation more stable than a primary carbocation

Answer 12

• Inductive effect
• Electrons bonded to the surrounding carbon atoms push electrons to the positively charged carbon atom
• This slightly decreases the strength of the positive charge, making it more stable

Question 13

What is a major and minor product

Answer 13

• The major product is formed from the most stable carbocation intermediate
• E.g. if the most stable carbocation intermediate is secondary, then the major product will form from the secondary carbocation intermediate and the minor product will form from the primary carbocation intermediate

Question 14

Describe the test for alkenes

Answer 14

Adding bromine water to a solution containing an unsaturated molecule will cause it to turn from orange to colourless.

Question 15

How are addition polymers formed

Answer 15

From alkenes where the double bond breaks to form a repeating unit.

Question 16

What happens when you change the reaction conditions

Answer 16

You can produce plastics with different properties.

Question 17

What kind of polymers are produced from high temperatures and pressures

Answer 17

Branched chain polymers with weak intermolecular forces.

Question 18

What kind of polymers are produced from low temperatures and pressures

Answer 18

Unbranched/ straight chain polymers with strong intermolecular forces.