4.1.3: Alkenes

Question 1

General formula (alkenes)

Answer 1

CnH2n

Question 2

What is an alkene?

Answer 2

An unsaturated hydrocarbon with a C=C double bond which comprises of 1 σ bond and 1 𝜋 bond.

Question 3

𝜋 bond

Answer 3

sideways overlap of adjacent p orbitals, above and below σ bonds in the plane.

Question 4

Which is stronger, a σ bond or a 𝜋 bond?

Answer 4

• A σ bond is stronger

* Because a 𝜋 bond is further from the nucleus and easier to break

Question 5

σ bond

Answer 5

Head-on overlap of atomic or hybrid orbitals directly between the bonding atoms. All single bonds are σ bonds.

Question 6

Electrophile

Answer 6

Electron pair acceptor

Question 7

Angle between two hydrogens bonded to a C in a C=C double bond

Answer 7

120⁰

Question 8

Angle between two hydrogens bonded to a C in a C–C single bond

Answer 8

109.5⁰

Question 9

Conditions for hydrogenation

Answer 9

• 150⁰C (423K)

* Nickel catalyst

Question 10

Hydrogenation reagents

Answer 10

• Hydrogen gas

* Alkene

Question 11

What type of reaction is hydrogenation?

Answer 11

Addition reaction: adds across the double bond

Question 12

Polymerisation (mechanism)

Answer 12

Addition polymerisation

Question 13

Conditions for polymerisation

Answer 13

• Heat (200⁰C)
• Pressure (2000 atmospheres)
• Presence of O₂ initiator

Question 14

What is the shape around each of the carbon atoms in a C=C double bond?

Answer 14

Trigonal planar.

Question 15

Describe rotation around the double bond

Answer 15

• Rotation is restricted because the 𝜋 bond locks the two carbon atoms in position, preventing them from rotating.
• Contrast to in alkanes, where rotation is possible around every atom.

Question 16

Conditions for E/Z isomerism

Answer 16

• a C=C double bond

* 2 different groups attached to each carbon in the C=C

Question 17

How do you determine the “sides” of the double bond?

Answer 17

Draw the split in the same direction that the bond lines go.

Question 18

What does the ‘E/Z’ in ‘E/Z isomerism’ mean?

Answer 18

• E = Entgegen: the two higher priority groups are on (diagonally) opposite sides of the double bond.
• Z = Zusammen: the two higher priority groups are on the same side of the double bond.

Question 19

What determines priority in E/Z isomerism?

Answer 19

• Mᵣ = priority
• Higher Mᵣ = higher priority
• Work along until the atoms have different Mᵣs. The one with the highest Mᵣ wins priority.

Question 20

What are the Cahn-Ingold-Prelog rules?

Answer 20

• If the groups of higher priority are on the same side of the double bond, the compound is a Z isomer.
• If the groups of higher priority are diagonally placed across the double bond, the compound is an E isomer.

Question 21

What is cis-trans isomerism?

Answer 21

A case of E/Z isomerism where one of the groups attached to each carbon of the C=C double bond is a hydrogen.

Question 22

Conditions for halogenation of alkenes

Answer 22

Room temperature

Question 23

Describe the bromine water test

Answer 23

• Test for the presence of a double bond
• Bromine water = orange
• If double bond present, turns colourless

Question 24

Alkenes can be hydrated to form

Answer 24

alcohols

Question 25

Conditions for hydration of alkenes

Answer 25

• H₃PO₄ (phosphoric acid catalyst)

Question 26

Hydration (reagents)

Answer 26

* Alkene * Steam * Phosphoric acid catalyst

Question 27

What is Markownikoff's rule?

Answer 27

• When a hydrogen halide reacts with an unsymmetrical alkene the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen (and the smaller number of carbon) atoms.

Question 28

Why is Markownikoff's rule correct?

Answer 28

* Where the H from the hydrogen halide is added depends on the stability of the carbocation * Due to the inductive effect, alkyl R groups push e- density onto the carbocation, making it more stable * The more alkyl groups, the more stable the carbocation; tertiary carbocations are the most stable, primary carbocations the least stable * Less stable = more attractive to H, so H will be added to the carbon atom with the least alkyl groups