Alkane Reactions

t Alkanes have only single, covalent, non- polar 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.; C 3 H 8 + 5O 2  3CO 2 + 4H 2 0

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 CH 4 Cl 2 CH 3 Cl + 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. Heat or sunlight Cl : Cl 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. CH 4 + Cl 2 /uv  CH 3 Cl + Cl 2 /uv  CH 2 Cl 2 + Cl 2 /uv  CHCl 3 + Cl 2 /uv  CCl 4

Larger alkanes can produce side reactions and by-products Ex.: Dichlorinate ethane CH 3 —CH 3 Cl 2 CH 3 —CH 2 Cl uv CH 3 —CH 2 Cl CH 3 -CHCl 2 + CH 2 Cl-CH 2 Cl uv dichloroethane 1,2-dichloroethane Cl 2

b. Nitration - substitution of an –NO 2 group. Only mononitration occurs. – Reagent is nitric acid (HNO 3 ) – Condition for reaction to proceed: 400 ° C In higher alkanes, breaking of carbon- carbon 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