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Chemical Reactions of Alkanes - Coggle Diagram
Chemical Reactions of Alkanes
Combustion reactions of the alkanes
burn in oxygen in exothermic reactions
used extensively as fuels
propane C3H8 (home cooking systems)
butane C4H10 (home cooking systems)
octane C8H18 (component of petrol/gasoline)
The amount of oxygen available determines the type of combustion that alkanes undergo
Complete combustion
burning in an excess of oxygen – produces carbon dioxide and water
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g)
Incomplete combustion
if oxygen supply is limited: the products are either carbon monoxide (CO) and water, or solid carbon and water
Carbon monoxide is a highly poisonous gas (no taste, smell or colour). It binds strongly to haemoglobin in the blood, which leads to suffocation and eventually death if it is inhaled in sufficiently high concentrations.
characterised by the appearance of a yellow or orange flame
2C3H8 (g) + 7O2 (g) → 6CO (g) + 8H2O (l)
C3H8 (g) + 2O2 (g) → 3C (s) + 4H2O
Free-radical substitution reactions (reactions with halogens such as chlorine (Cl2) or bromine (Br2)
The products of these reactions are a halogenoalkane and a hydrogen halide
methane reacts with chlorine in the presence of ultraviolet (UV) radiation to form chloromethane and hydrogen chloride
known as photochemical reactions, because they only take place in the presence of ultraviolet (UV) radiation
conditions: UV radiation
CH4 (g) + Cl2 (g) → CH3Cl (l) + HCl (g)
Photochemical Reactions
A photon of ultraviolet (UV) radiation has sufficient energy to break the bond between two chlorine atoms in a molecule
The individual chlorine atoms have unpaired electrons, which make them very reactive species known as radicals, or free-radicals
When formed, these radicals initiate a chain reaction which results in the formation of a halogenoalkane
Reaction takes place in three steps: initiation, propagation and termination
In the initiation step, the bond between the atoms in the halogen molecule (Cl2 or Br2) is broken by UV radiation. This
homolytic bond fission
results in the formation of free-radicals (represented by a dot after the symbol).
Homolytic bond fission occurs when a bond breaks evenly, with each atom taking one electron from the bond. This results in the formation of radicals, or free-radicals, highly reactive species with unpaired electrons.
In the propagation step, a chlorine free-radical reacts with a molecule of methane, producing a methyl radical (•CH3) and hydrogen chloride (HCl). In the next step, the methyl radical reacts with a chlorine molecule to produce chloromethane (CH3Cl) and another chlorine radical (Cl•). This free-radical goes on to take part in further reactions and in this way the reaction is propagated: it is a chain reaction. The idea of this being a chain reaction is confirmed by the experimental finding that 10000 molecules of chloromethane (on average) are produced for each ultraviolet photon absorbed.
This final step in the reaction is the termination step. In this step, two free-radicals recombine with each other. There are three possible termination steps, shown in Figure 3. These reactions result in free radicals being removed from the reaction mixture. The possible products of this reaction are molecular chlorine (Cl2), ethane (C2H6) and chloromethane (CH3Cl).
Alkanes
saturated hydrocarbons: contain carbon–carbon single bonds and carbon–hydrogen single bonds
make very good fuels, but relatively unreactive
carbon–hydrogen bonds are considered to be non-polar (or weakly polar) bonds, because of the small difference in electronegativity of 0.4 units
carbon–carbon and carbon–hydrogen bonds are relatively strong covalent bonds: alkanes are kinetically stable unless supplied with enough activation energy for the reaction
