Alkane Reactions
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• Alkanes have only single, covalent, nonpolar bonds
• Alkanes are relatively inert to most
chemical reagents
• Alkanes are also called paraffins (“little
affinity”)
1. Combustion Reaction
• also called destructive oxidation
• When used as a fuel, such as natural
gas; gasoline
• Products of combustion are always
carbon dioxide and water
• Ex.;
C3H8 + 5O2  3CO2 + 4H20
2. Substitution Reactions
• a hydrogen is removed and another
species replaces it
• a. Halogenation - a substitution of a
hydrogen with a halogen (either Cl or Br)
- this is sometimes called chlorination
or bromination
• Ex.; Chlorinate methane
CH4
Cl2
CH3Cl + HCl
heat or sunlight
• Halogenation does not occur in one step; it
involves a series of steps called the
Free Radical Chain Mechanism
Step 1: Chain Initiation – this step provides
the chlorine atoms that get this going.
Chlorine molecule absorbs energy and splits
into free radical chlorines. A free radical is
an atom or group possessing an odd (unpaired)
electron. Free radicals are extremely
reactive.
Cl:Cl
Heat or sunlight
2Cl·
• Step 2: Chain Propogation - this step is where
almost all the reactant is consumed and all the
products are formed. The chlorine free radical
attacks the hydrocarbon and removes a hydrogen,
leaving a methyl free radical. The methyl free
radical collides with a chlorine molecule and pulls
off a chlorine.
• Step 3: Chain Termination - if any two
radicals combine, chains are terminated.
There are three possibilities:
• Successive substitutions are possible.
You can dichlorinate, trichlorinate,
tetrachlorinate…until there are no
hydrogens left to be replaced.
• CH4 + Cl2/uv  CH3Cl + Cl2/uv 
CH2Cl2 + Cl2/uv  CHCl3 + Cl2/uv  CCl4
• Larger alkanes can produce side reactions
and by-products
• Ex.: Dichlorinate ethane
• CH3—CH3
Cl2
CH3—CH2Cl
uv
• CH3—CH2Cl
Cl2
uv
CH3-CHCl2 + CH2Cl-CH2Cl
dichloroethane
1,2-dichloroethane
• b. Nitration - substitution of an –NO2
group. Only mononitration occurs.
– Reagent is nitric acid (HNO3)
– Condition for reaction to proceed: 400°C
• In higher alkanes, breaking of carboncarbon bonds as well as carbon-hydrogen
bonds occurs
• Ex.; Nitrate methane
• Ex.; Nitrate propane
Synthesis of Alkanes
• 1. Wurtz Reaction – treatment of an
alkyl halide with metallic sodium
• Product is always symmetrical and has
twice as many carbons as the original
alkyl halide
• Ex.; React bromoethane with sodium
• Ex.; React 2-bromopropane with sodium
• 2. Reduction of an Alkyl Halide replacement of a halide with hydrogen
using zinc and acetic acid
• Ex.; React bromoethane with zinc &
acetic acid
• Ex.; React 2-bromopropane with zinc and
acetic acid
• 3. Hydrolysis of a Grignard Reagent - a
Grignard reagent is an organomagnesium
halide
• Grignard reagents are prepared from an
alkyle halide that reacts with magnesium in an
ether solution. When the Grignard is in the
presence of water it acts as a strong base.
• Ex.; React bromopropane with Mg/ether in
water.
• 4. Catalytic Hydrogenation - addition of
hydrogen across a double bond
• Uses either Pt or Pd as the catalyst
• Ex.; Hydrogenate ethene
• Ex.; Hydrogenate propene