1. Alkanes and Alkenes
Topic 10.2 part 1

2. Alkanes have low reactivity bond enthalpies are relatively strong
348 kJ mol-1 to break a C-C bond
412 kJ mol-1 to break a C-H bond
low polarity
only readily undergo combustion reactions with oxygen (very flammable) and substitution reactions with halogens in UV light

3. Reactions of Alkanes: Combustion
hydrocarbons (only contain C & H)
complete combustion
alkanes burn in an excess supply of oxygen to form carbon dioxide and water:
example:
2 C2H6 (g) + 7 O2 (g) → 4 CO2 (g) H2O (l)
exothermic (-∆H)
incomplete combustion
if oxygen supply is limited, the gas carbon monoxide, carbon and water is formed
2 C2H18 (l) + 5 O2 (g) → 4 CO (g) + 6H2O (l)
2 C2H18 (l) + 3 O2 (g) → 4 C (s) + 6H2O (l)
(notice left over carbon (black soot) and dangerous CO)

5. Reactions of Alkanes (methane and ethane) with Halogens (Cl and Br)
alkanes do not react with halogens in the dark at room temperature, but will react in the presence of sunlight (UV)
a substitution reaction will occur where some or all of the hydrogens will be replaced with a halogen
C2H6 (g) + Br2 (g) → C2H5Br (l) + HBr (g)
Cl2 + CH4  CH3Cl + HCl

6. this happens by a process know as free radical substitution that happens in 3 steps
Initiation
Propagation
Termination

7. Cl2  Cl* + Cl* CH4 + Cl*  CH3* + HCl Cl* + Cl*  Cl2 initiation
initiated by UV light breaking a chlorine molecule into
two free radicals by a process called homolytical
fission (* = unpaired electron)
Cl2  Cl* + Cl*
propagation
keeps the chain going (free radical in reactants and products)
CH4  +  Cl*  CH3*  +  HCl
CH3*  +  Cl2  CH3Cl  +  Cl*
termination
this removes free radicals (*) from the system without replacing them by new ones
Cl* + Cl*  Cl2
CH3* + Cl *  CH3Cl
CH3* + CH3*  CH3CH3
each resulting atom receives one unpaired electron, known as free radicals that have lots of energy

8. each resulting atom receives one unpaired electron, known as free radicals

10. Reactions of Alkenes the general mechanism
alkenes react (more reactive than alkenes) with many substances to form a new substance
Alkene bonds are not twice as strong as alkanes and usually break to create a single bonds. The energy to break only the pi bond is less than to break a sigma bond
The double bond has a lot of electrons and can attract electrophiles (things that love electrons)

11. Reactions of Alkenes the general mechanism
catalysts, acids or other substances may be required to complete the reaction (addition reactions not substitution reactions):
C2H4 + XY → CH2XCH2Y
process occurs by breaking the double bond.

12. Reactions of Alkenes with hydrogen (Hydrogenation)
alkenes react with hydrogen gas to create an alkane, using nickel as a catalyst at 150ºC:
C2H4 + H2 → CH3CH3

13. Reactions of Alkenes: with halogens
The hydrogenation of alkenes is used to produce margarine. Vegetable oil is partial hydrogenated (some C=C are converted to C-C bonds). This process creates trans fats. These are very bad thus margarine has been banned in many countries.

14. Reactions of Alkenes: with halogens
alkenes react readily with chlorine or bromine to create a dihalogenated alkane
C2H4 + Cl2 → CH2Cl CH2Cl
1,2-dichloroethane

15. Reactions of Alkenes with hydrogen halides
alkenes react readily with hydrogen halides to create a mono-halogenated alkane
C2H4 + HBr → CH3CH2Br

16. Reactions of Alkenes with water
alkenes do not react readily with water
if phosphoric (V) acid is used as a catalyst, an alcohol will be created (catalytic hydration, high temp & very high pressure of 6-7 MPa)
remember that H2O can be dissociated into H+ and OH-
C2H4 + H2O → CH3CH2OH
H3PO4

19. Distinguish between alkanes and alkenes using bromine water
bromine water (a red liquid) tests for unsaturated hydrocarbons (alkenes)
alkanes → stay redish/orange
no reaction
alkenes → turn clear / colourless
because of reaction with unsaturated hydrocarbon

20. Comparing alkenes and benzene reactions
Benzene does not react like an alkene even though they both have C=C.
Benzene does not undergo addition reactions and will not decolorize Br2.
This is because addition reactions would make benzene less stable by ruining the resonance.
Benzene will undergo substitution reactions like alkanes.

21. Reactions of Alkenes: Polymerization
naming polymers
put “poly-” in front of the name of the monomer
there are 3 polymerization mechanisms that you need to be familiar with:
polyethene
polychloroethene
Polypropene
ONLY C=C reacts in polymerization

22. Polyethene monomer: ethene CH2=CH2
undergoes addition reactions with itself to make a chain at high temperatures and pressures with a catalyst present.
The general formula: where n is the number of monomers
n CH2=CH2  [-CH2-CH2-]n
Ex. 4 CH2=CH2  [-CH2-CH2-]4

23. Polychloroethene each chloroethene contains 1 chlorine
therefore when the chloroethene
molecules polymerize, every
other carbon will bond to 1 chlorine
this is PVC

25. Polypropene
+ =

26. Teflon non-stick pans