Alkanes IB Chemistry Topic 10.2

10.2 Alkanes Asmt. Stmts 10.2.1 Explain the low reactivity of alkanes in terms of bond enthalpies and bond polarity. 10.2.2 Describe, using equations, the complete and incomplete combustion of alkanes. 10.2.3 Describe, using equations, the reactions of methane and ethane with chlorine and bromine. 10.2.4 Explain the reactions of methane and ethane with chlorine and bromine in terms of a free-radical mechanism.

10.2.1 Explain the low reactivity of alkanes in terms of bond enthalpies and bond polarity.

10.2.1 Because of the relatively strong C-C and C-H bonds and because they have low polarity, alkanes tend to be quite unreactive. They only readily undergo combustion reactions with oxygen and substitution reactions with halogens in ultraviolet light.

10.2.2 Describe, using equations, the complete and incomplete combustion of alkanes.

Reactions of Alkanes: Complete Combustion Alkanes burn in an excess supply of oxygen to form carbon dioxide and water: CH4 (g) + 2 O2 (g) → CO2 (g) + H2O (l) ∆H = -890 kJ mol-1 Liquid alkanes (such as octane) must be vaporized before they will burn: C8H18 (g) + 2 O2 (g) → CO2 (g) + H2O (l) ∆H = -5510 kJ mol-1

Reactions of Alkanes: Incomplete Combustion If the oxygen supply is limited, the gas carbon monoxide is formed: C8H18 (l) + 9O2 (g) → C (s) + CO (g) + CO2 (g) + H2O (l) Problems associated with C: The carbon is released as soot. Problems associated with CO: Carbon monoxide is a poisonous gas, which is one reason why complete combustion is preferred to incomplete combustion. Gas fires and boilers must be serviced regularly to ensure they do not produce carbon monoxide.

10.2.3 & 10.2.4 Describe, using equations, the complete and incomplete combustion of alkanes. Explain the reactions of methane and ethane with chlorine and bromine in terms of a free-radical mechanism.

Reactions of Alkanes: with Halogens Alkanes do not react with halogens in the dark at room temperature, but will react in the presence of sunlight: C6H14 (g) + Br2 (g) → C6H13Br (l) + HBr (g)

Reactions of Alkanes: with Halogens Energy absorbed from light allows homolytic fission: each resulting atom receives one unpaired electron, known as free radicals Cl-Cl → Cl• + Cl•

Free Radical Substitutions Many organic molecules undergo substitution reactions. In a substitution reaction one atom or group of atoms is removed from a molecule and replaced with a different atom or group. Example: Cl2 + CH4  CH3Cl + HCl 11

Three Basic Steps in a Free Radical Mechanism Chain initiation The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals. Cl2  2Cl. Chain propagation reactions These are the reactions which keep the chain going. CH4  +  Cl.  CH3.  +  HCl CH3.  +  Cl2  CH3Cl  +  Cl Chain termination reactions These are reactions which remove free radicals from the system without replacing them by new ones. 2 Cl.  Cl2 CH3. + Cl.  CH3Cl CH3. + CH3.  CH3CH3 12

Free Radical Mechanism-The Initiation Step The ultraviolet light is a source of energy that causes the chlorine molecule to break apart into 2 chlorine atoms, each of which has an unpaired electron The energies in UV are exactly right to break the bonds in chlorine molecules to produce chlorine atoms. 13

Homolytic Fission Free radicals are formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is homolytic fission. 14

Free Radical Propagation The productive collision happens if a chlorine radical hits a methane molecule. The chlorine radical removes a hydrogen atom from the methane. That hydrogen atom only needs to bring one electron with it to form a new bond to the chlorine, and so one electron is left behind on the carbon atom. A new free radical is formed - this time a methyl radical, CH3 . 15

Free Radical Propagation II If a methyl radical collides with a chlorine molecule the following occurs: CH3.  +  Cl2  CH3Cl  +  Cl. The methyl radical takes one of the chlorine atoms to form chloromethane In the process generates another chlorine free radical. This new chlorine radical can now go through the whole sequence again, It will produce yet another chlorine radical - and so on and so on. 16

Termination Steps The free radical propagation does not go on forever. If two free radicals collide the reaction is terminated. 2Cl.  Cl2 CH3.  +  Cl .  CH3Cl CH3 .  +  CH3.  CH3CH3 17

Reactions of Alkanes: with Halogens Substitution of an alkane with a halogen has 3 steps: Initiation Propagation Termination See pg. 266 in the green book for an example of these steps Electronegativity values!!!

Exercise Write the steps in the free radical mechanism for the reaction of chlorine with methyl benzene. The overall reaction is shown below. The methyl group is the part of methyl benzene that undergoes attack. Initiation Propagation Termination 19

Solution Cl2 ----- 2Cl• Termination Cl• + Cl•  Cl2 Initiation uv Propagation Termination Cl• + Cl•  Cl2 20