The Simplest Alkanes. Physical Properties 2/16/2012. Butanes are still gases. bp -160 C bp -89 C bp -42 C. CH 3 CH 2 CH 2 CH 2 CH 3 n-pentane.

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1 The Simplest Alkanes Butanes are still gases Methane (CH 4 ) Ethane (C 2 H 6 ) Propane (C 3 H 8 ) n-butane CH 2 CH 2 Isobutane ( ) 3 CH bp -160 C bp -89 C bp -42 C bp -0.4 C bp C Branched isomer has lower boiling point! CH 2 CH 2 CH 2 n-pentane Liquids Physical Properties Boiling points and melting points increase as size of alkane increases CH 2 CH 2 CH 2 CH 2 n-hexane CH 2 CH 2 CH 2 CH 2 CH 2 n-heptane Increased branching lowers an alkane s boiling point 1

2 Branching Lowers Boiling Points Octane: bp 125 C 2-Methylheptane: bp 118 C 2,2,3,3-Tetramethylbutane: bp 107 C SOURCES OF HYDROCARBONS branched molecules are more compact with smaller surface area fewer points of contact with other molecules NATURAL SOURCES OF HYDROCARBONS NATURAL GAS 90-95% methane (C1-C4) 5-10% ethane small amounts of propane, butane and 2-methylpropane CRUDE PETROLEUM IS DISTILLED AT A REFINERY Gasoline, diesel fuel, and aviation fuel are mixtures of hydrocarbons. Gases Distillation of Crude Petroleum below 20 o C3-C4 LPG PETROLEUM Thick viscous mixture of hundreds of hydrocarbons (mostly C5-C20). Formed from the decomposition of ancient marine plants and animals. Naphtha Kerosene Fuel Oil o o o C5-C12 C9-C15 C15-C18 Gasoline Diesel Fuel COAL COAL TAR Mixture of carbon and solid, high molecular weight hydrocarbons (>C20). Many aromatic and heteroaromatic compounds. Lubricating Oil Asphalt above 350 o residues 2

3 GASOLINE Complex mixture of C 5 to C 12 hydrocarbons Branched better than linear hydrocarbons to prevent engine knock. HYDROGENATION catalyst C C + H H C C H H catalyst = Pt, Pd, Ni (CH 2 ) 5 C CH 2 CH heptane 2,2,4-trimethylpentane (isooctane) octane rating = 0 octane rating = 100 toluene octane rating = 120 benzene octane rating = 106 Hydrogenation is covered in more detail in Chapter 7 - we ll cover it there. Hydrogenation is included in this chapter (briefly) because it is a method of making ALKANES from alkenes and alkynes by adding hydrogens. + H 2 Pd It basically wipes out double and triple bonds. the standard - against which others are compared H 3 C Pt + H 2 CH 2 CH 2 H H OTHER PREPARATION METHODS Metal/Acid reduction COOH, HCl n-c 16 H 33 -I + Zn n-c 16 H 100 o C, 25 hrs 34 Grignard Reagents: + HI Mg H 2 O C CH 2 Cl C CH 2 MgCl C dry ether Corey-House Synthesis ether CuX R -X R-X + 2Li R-Li R 2 CuLi R-R ether ether Alkyl halide Lithium dialkylcopper regent alkane REACTIONS OF HYDROCARBONS Alkanes are quite unreactive toward most reagents. They have no functional group. OXIDATION HALOGENATION 3

4 OXIDATION (COMBUSTION) All hydrocarbons burn to form carbon dioxide, water and heat. The heat can be used to heat homes or run engines. EXAMPLES OF OXIDATION (COMBUSTION) example CH 4 methane natural gas CH O 2 CO H 2 O + Heat C n H 2n+2 + O 2 n CO 2 + (2n+2)/2 H 2 O + Heat NOTE Each carbon gives a CO 2 molecule, and every two hydrogens give a water molecule. C n n CO 2 H 2n+2 2n+2 2 H 2 O example C 3 H 8 propane liquefied propane CH O 2 3 CO H 2 O + Heat HALOGENATION Halogenation replaces one or more hydrogens in a molecule with a halogen most commonly a chlorine. It converts a relatively inert hydrocarbon molecule to one that has a reactive functional group a site where chemical reactions can take place. Halogenation of Alkanes light R H + Cl Cl R Cl + H Cl examples chlorination CH 4 + Cl 2 Cl + HCl free-radical substitution reaction CH 2 + Cl 2 CH + HCl Cl CH 4 + Cl 2 Cl + HCl takes place at a refinery or a chemical plant - not easy to do in the lab + CH 2 CH 2 -Cl 4

5 unpaired Radicals, also called free radicals have at least one unpaired electron in the valence shell. A. : They are often formed by a homolytic cleavage of an unstable bond. WHAT IS RADICAL? A weak bond A 2 A. C X C + X carbon radical Extremely reactive! THE REACTION MUST BE INITIATED It does not occur in the dark. Exposure to ultraviolet light (sunlight) will start the reaction. An electric spark or heat will also start the reaction. Once reaction starts, it is exothermic and continues almost explosively. The first step is thought to be the dissociation of chlorine : hν : Cl-Cl : 2. chlorine Cl : or Δ atoms (radicals) diatomic molecule ABSTRACTION OF HYDROGEN BY A CHLORINE FREE RADICAL (ATOM) C H +. Cl : C. + H-Cl : : Cl-Cl : unpaired electron = free radical HYDROGEN ABSTRACTION Chlorine takes the hydrogen and one of its electrons R-Cl R E P E A T I N G S T E P S MECHANISM OF CHLORINATION OF METHANE 1. Initiation : Cl Cl : light dissociation 2. Chain Propagation second step. CH3 + 2 : Cl. first step hydrogen abstraction H + : Cl. chlorine scission : Cl Cl : a free radical CHAIN REACTION H Cl : +. CH3 methyl radical cycle repeats CH : 3 Cl + : Cl. 5

6 3. Termination Steps 2 : Cl. CH CH3 CH. 3 + : Cl. recombinations : Cl Cl : Cl THE PREVIOUS EXAMPLES GIVEN ASSUMED MONOCHLORINATION ( one chlorine added ) BUT the reaction can repeat itself COMMON NAMES CH 4 + Cl 2 Cl + HCl methyl chloride Cl + Cl 2 CH 2 Cl 2 + HCl methylene chloride These steps stop the chain Termination could be a combination of these steps or it could be determined by which species are still present at the end of the reaction. CH 2 Cl 2 + Cl 2 CHCl 3 + HCl chloroform CHCl 3 + Cl 2 CCl 4 + HCl carbon tetrachloride fully chlorinated product ( perchloro ) Chapter 2 #9 Chapter 3 #7 #8 #9 #10 #11. hint: one initiation and two propagation steps #12 6