Alkenes

Question 1

General formula of alkenes

Answer 1

CₙH₂ₙ

Question 2

Examples of unsaturates hydrocarbons

Answer 2

• Alkenes
• Cycloalkenes

Question 3

Bonding in alkenes
(Sigma and pi)

Answer 3

Sigma bonds:
* Sigma bonds form when two s-orbitals overlap horizontally to give a single covalent bond
* there is a strong electrostatic attraction between the nuclei and shared electron pair due to high electron density between nuclei
* this means the sigma bond has a high bond enthalpy (it is the strongest type of covalent bond)

Pi bonds:
* formed by parallel overlap of p-orbitals
* p-orbitals are dumbell shaped and when they merge, they form two oblong shapes (top and bottom)
* pi bonds are weaker than sigma bonds as the electron density is spread out above and below the nuclei, meaning pi bonds have lower bond enthalpy

Question 4

Electrophile definition

Answer 4

An electrophile is a species that accepts a pair of electrons forming a new covalent bond

(often positively charged)

Question 5

Addition reaction of alkenes with hydrogen (hydrogenation)

Answer 5

C₂H₄ + H₂ → C₂H₆
reagent and conditions: Nickel catalyst, 150°C

• This process is called hydrogenation
• The sigma bond in the alkene breaks, allowing two hydrogen atoms to add across the double bond, forming a saturated alkene

Industrial application:
Used to make margarine by hydrogenating unsaturated vegetable oils, to become more solid

Question 6

Addition reaction of alkenes with halogens

Answer 6

C₂H₄ + Br₂ → C₂H₄Br₂
* Electropilic addition reaction (MECHANISM)
* Halogen molecule is polarised by electron-rich double carbon bond
* Pi bond breaks, halogen atoms add accross it

This can also be used as a test for alkenes:
Bromine water decolorises (orange –> colourless) in the presence of a C=C bond

Question 7

Addition reaction of alkenes with hydrogen halides

Answer 7

C₂H₄ + HBr → C₂H₅Br

Reagents and conditions: HBr(g), room temp

• Electropilic addition reaction (MECHANISM)
• The hydrogen attaches to the carbon with more hydrogens already, and the halide adds to the other carbon
• the product is a halogenoalkane

Question 8

Addition reaction of alkenes with steam

Answer 8

C₂H₄ + H₂O → C₂H₅OH
Reagent and conditions: H₂SO₄/H₃PO₄ catalyst, high pressure/temperature

• Called the hydration of alkenes, also an addition reaction
• produces an alcohol

Question 9

Addition reaction of alkenes with potassium manganate (VII)

Answer 9

CH₂=CH₂ + H₂O + [O] → HO–CH₂–CH₂–OH
Reagents and conditions: Dilute acidified KMnO₄ with dilute H₂SO₄ or NaOH, shake together at room temp

• Oxidises the alkene to a diol (compound with two -OH groups)
• Purple KMnO₄ is decolourised as MnO₄⁻ is reduced to Mn²⁺

Question 10

What does the heterolytic fission of covalent bonds form?

Answer 10

Heterolytic fission of covalent bonds form ions

Question 11

Differences between homolytic and heterolytic fission

Answer 11

Homolytic:
* Covalent bond breaks evenly, each atom gets one electron from the bond
* produces: two free radicals
* ex. free radical substitution

Heterolytic:
* Covalent bond breaks unevenly, one atom gets both bonding electrons
* produces: a cation and an anion
* ex. electrophilic addition

Question 12

Electrophilic addition mechanism of alkenes and halogens

Answer 12

1. Double C=C becomes a single C-C bond, arrow to delta+ halogen atom
2. Bond breaks between halogen atoms, arrow to delta- halogen atom
   v
3. Halogen atom bonded with a carbon atom
4. Other halogen atom (delta- and lone pair) arrow to delta+ carbon
   v
5. Halogen atom bonds to the carbon and complete product

Question 13

Electrophilic addition mechanism of alkenes and hydrogen halides

Answer 13

1. Double C=C becomes a single C-C bond, arrow to delta+ hydrogen
2. Bond breaks between halogen and hydrogen, arrow to delta- halogen atom
   v
3. Hydrogen bonded with a carbon atom
4. halogen atom (delta- and lone pair) arrow to delta+ carbon
   v
5. Halogen atom bonds to the carbon and complete product

Question 14

What is the test for C=C double bond?

Answer 14

Bromine water decolorises (orange –> colourless) in the presence of a C=C bond

Question 15

How do alkenes form polymers?

Answer 15

Alkenes form polymers through addition polymerisation

Question 16

What can be done with waste polymers?

Answer 16

• Recycling
• Incineration - to release energy
• Used as feedstock for cracking

Question 17

How can the problems caused by polymer disposal be limited?

Answer 17

• Developing biodegradable polymers
• Removing toxic waste gases caused by incineration of plastics