Alkenes Flashcards
(17 cards)
General formula of alkenes
CₙH₂ₙ
Examples of unsaturates hydrocarbons
- Alkenes
- Cycloalkenes
Bonding in alkenes
(Sigma and pi)
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
Electrophile definition
An electrophile is a species that accepts a pair of electrons forming a new covalent bond
(often positively charged)
Addition reaction of alkenes with hydrogen (hydrogenation)
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
Addition reaction of alkenes with halogens
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
Addition reaction of alkenes with hydrogen halides
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
Addition reaction of alkenes with steam
C₂H₄ + H₂O → C₂H₅OH
Reagent and conditions: H₂SO₄/H₃PO₄ catalyst, high pressure/temperature
- Called the hydration of alkenes, also an addition reaction
- produces an alcohol
Addition reaction of alkenes with potassium manganate (VII)
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⁺
What does the heterolytic fission of covalent bonds form?
Heterolytic fission of covalent bonds form ions
Differences between homolytic and heterolytic fission
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
Electrophilic addition mechanism of alkenes and halogens
- Double C=C becomes a single C-C bond, arrow to delta+ halogen atom
- Bond breaks between halogen atoms, arrow to delta- halogen atom
v - Halogen atom bonded with a carbon atom
- Other halogen atom (delta- and lone pair) arrow to delta+ carbon
v - Halogen atom bonds to the carbon and complete product
Electrophilic addition mechanism of alkenes and hydrogen halides
- Double C=C becomes a single C-C bond, arrow to delta+ hydrogen
- Bond breaks between halogen and hydrogen, arrow to delta- halogen atom
v - Hydrogen bonded with a carbon atom
- halogen atom (delta- and lone pair) arrow to delta+ carbon
v - Halogen atom bonds to the carbon and complete product
What is the test for C=C double bond?
Bromine water decolorises (orange –> colourless) in the presence of a C=C bond
How do alkenes form polymers?
Alkenes form polymers through addition polymerisation
What can be done with waste polymers?
- Recycling
- Incineration - to release energy
- Used as feedstock for cracking
How can the problems caused by polymer disposal be limited?
- Developing biodegradable polymers
- Removing toxic waste gases caused by incineration of plastics
