Alkenes Flashcards
Describe alkenes.
Alkenes are unsaturated hydrocarbons containing C-C double bonds as the functional group in their structures.
Describe the hybridisation of carbon atoms in C-C double bonds.
Each carbon atom in the C-C double bond is sp2 hybridised and bonded to other atoms in a trigonal planar manner with a bond angle of 120 degrees.
Two of the three sp2 hybrid orbitals of the carbon atom overlap head on with the 1s orbitals of two hydrogen atoms to form two C-H sigma bonds. The remaining sp2 hybrid orbitals overlaps head on with the sp2 hybrid orbital of the neighbouring carbon atom to form a C-C sigma bond. Unhybridised 2p orbital overlaps side on with the 2p orbital of neighbouring carbon atom to form a pi bonds which lies above and below the plane containing atoms.
Compare the strength of a C-C single bond and a C-C double bond.
More energy is required to break a C-C double bond as it is stronger due to the stronger electrostatic force of attraction for the more electrons shared between the two carbon atoms. However, the energy required to break a C-C double bond is less than twice the energy required to break a C-C single bond as the pi bond is weaker than the sigma bond because of the less effective side-on overlap of orbitals. + the C-C double bond is shorter as the two carbon atoms in ethene are closer together to allow the formation of a pi bond via side on overlap of two unhybridised 2p orbitals.
Why does cis-trans isomerism exist in alkenes?
It is due to the presence of restricted rotation about the C-C double bond due to the presence of a pi bond, also each carbon atom in the C-C double bond is joined to two different groups
When is cis-trans isomerism possible in cycloalkenes?
It is only possible in cycloalkanes with a ring size larger than seven carbon atoms.
Why are trans isomers more stable than their corresponding cis isomers?
This is because the cis isomers experience greater steric strain due to the inter-electronic repulsion between the electron clouds.
Describe and state the reagents and conditions needed for the elimination of H2O from alcohol to form alkenes.
Alcohols with H and OH on adjacent carbon atoms can be dehydrated to yield alkenes, this is an elimination reaction where H2O is eliminated.
Reagents and conditions:
1. excess concentrated sulfuric acid under heat
2. solid alumina Al2O3 under heat (industrial)
3. concentrated phosphoric acid H3PO4 under heat (industrial)
Describe and state the reagents and conditions needed for the elimination of HX.
Alkyl halides with H and X on adjacent carbon atoms are dehydrohalogenated to yield alkenes, this is an elimination reaction where HX is eliminated.
Reagent and condition: KOH or NaOh in ethanol under heat
State the formula for the combustion of alkenes.
CxHx + (x + y/4)O2 -> xCO2 + y/2H2O
or
CnH2n + 3n/2O2 -> nCO2 + nH2O
State the reagents and conditions for the reduction/catalytic addition of hydrogen to alkenes to give alkanes.
H2(g), nickel as a catalyst under heat
Why are alkenes electophilic?
Even though alkenes are generally non-polar they are highly reactive as compared to the alkanes because of the pi electron cloud between the doubly bonded carbon atoms, these loosely held pi electrons will attract electrophiles or even induce dipoles, creating electrophilic sites in some nearby neutral molecules.
Why do alkenes undergo addition reactions?
Alkenes are unsaturated hence they undergo addition reactions. During the reaction, the weaker pi bond is broken instead of the sigma bond. In place, two strong sigma bonds are formed in the saturated product. The shape of the C atom in C-C double bond now changes from trigonal planar to tetrahedral.
State the reagent and conditions of the electrophilic addition of hydrogen halides.
gaseous HCl o HBr or HI at room temperature
State the reagent and conditions of the electrophilic addition of halogens with CCl4 as the inert solvent and any observations.
Cl2 in CCl4 or Br2 in CCl4, at room temperature.
The orange-red of Br2 in CCl4 is rapidly decolourised.
State the reagent and conditions of the electrophilic addition of halogens with H2O as the nucleophilic solvent and any observations.
Aqueous Cl2 or Br2 at room temperature.
The orange aqueous Br2 is rapidly decolourised.
State the reagent and conditions of the electrophilic addition of water to alkenes to produce alcohol.
Cold concentrated H2SO4, warm with water.
or
Steam,H3PO4(catalyst) high temperature and pressure
first one is for laboratory and second is industrial
What types of oxidation can alkenes undergo?
- Cleavage of the pi bond of the C-C double bond (mild oxidation)
- Cleavage of the C-C double (vigorous oxidation)
State the reagents and conditions for mild oxidation of alkenes and any observations.
Reagents and conditions: KMnO4(aq), NaOH(aq), under cold/ KMnO4(aq), H2SO4(aq), under cold
Observations: Purple KMnO4 solution is decolourised
State the reagents and conditions of oxidative cleavage in an acidic medium and any observations.
Reagents and conditions: KMnO4(aq), H2SO4(aq) under heat
Observations: Purple KMnO4 solution is decolourised
State the reagents and conditions of oxidative cleavage in an alkaline medium and any observations.
Reagents and conditions: KMnO4(aq), NaOH(aq) under heat
Observations: Decolourisation of purple KMnO4 and formation of brown precipitate (MnO2)
What are the chemical tests for alkenes?
- Br2 in CCl4 or water in room temperature, he reddish brown or orange bromine water turns colourless.
- KMnO4(aq), H2SO4(aq) in cold/heat, the purple KMnO4 solution is decolourised and a brown solid is observed
- KMnO4(aq), H2SO4(aq) in cold/heat, the purple KMnO4 solution is decolourised
