Download - Chapter 2: Alkanes and Cycloalkanes
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Chapter 2: Alkanes
and Cycloalkanes
Chem 30A
Lecture 8
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Physical Properties of Alkanes
• Intermolecular forces of attraction– ion-ion (Na+ and Cl- in NaCl)– ion-dipole (ions solvated in aqueous solution)– dipole-dipole and hydrogen bonding– dispersion forces (electrostatic attraction between
temporary dipoles)
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Physical Properties
• Low-molecular-weight alkanes (methane to butane) are gases at room temperature.
• Higher molecular-weight alkanes (pentane, decane, gasoline, and kerosene) are liquids at room temperature.
• High-molecular-weight alkanes (paraffin wax) are semisolids or solids at room temperature.
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Physical Properties
• Constitutional isomers have different physical properties.
NameDensity (g/mL)
hexane2-methylpentane
3-methylpentane
2,3-dimethylbutane
2,2-dimethylbutane
68.7
60.3
63.358.0
49.7
-95
-154
-118-129
-98
0.659
0.653
0.6640.661
0.649
bp(°C)
mp(°C)
Hexane
2,2-Dimethylbutane
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Oxidation of Alkanes
• Oxidation is the basis for the use of alkanes as energy sources for heat and power.– heat of combustion:heat of combustion: The heat released when one
mole of a substance in its standard state is oxidized to carbon dioxide and water.
CH4 2O2
CH3CH2CH3 5O2
CO2
3CO2 4H2O
2H2O+ + -890.4 (-212.8)
+ + -2220 (-530.6)
Methane
Propane
ΔH0 ( )/kJ kcal mol
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Heat of Combustion
• Heat of combustion for constitutional isomers
Hydrocarbon
Octane
2-Methyl-heptane
2,2-Dimethyl-hexane
2,2,3,3-Tetramethyl-butane
ΔH0
[kJ (kcal)/mol]Structural formula
-5470.6 (-1307.5)
-5465.6 (-1306.3)
-5458.4 (1304.6)
-5451.8 (1303.0)
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Heats of Combustion
• For constitutional isomers [kJ (kcal)/mol]
-5470.6 (-1307.5)-5465.6 (-1306.3)-5458.4 (1304.6)-5451.8 (1303.0)
8CO2 +9H2O
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Heat of Combustion
Strain in cycloalkane rings determined by
heats of combustion
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Sources of Alkanes
• Natural gas – 90-95% methane
• Petroleum– gases (bp below 20°C)– naphthas, including gasoline (bp 20 - 200°C)– kerosene (bp 175 - 275°C)– fuel oil (bp 250 - 400°C)– lubricating oils (bp above 350°C)– asphalt (residue after distillation)
• Coal
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Petroleum
• Fractional distillation of petroleum.
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Gasoline
• Octane rating:Octane rating: The percent 2,2,4-trimethylpentane (isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties.
Heptane(octane rating 0)
2,2,4-Trimethylpentane(octane rating 100)
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Synthesis Gas
• A mixture of carbon monoxide and hydrogen in varying proportions which depend on how it is produced.
C + H2O heat CO + H2Coal
1+catalyst +
2CH4 2H2COO2
Methane
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Synthesis Gas
• Synthesis gas is a feedstock for the industrial production of methanol and acetic acid.
– It is likely that industrial routes to other organic chemicals from coal via methanol will also be developed.
CH3OH + COcatalyst
CH3COHO
Methanol Acetic acid
+CO 2H2 CH3OHcatalyst
Methanol