Chapter 2: Alkanes

and Cycloalkanes

Chem 30A

Lecture 8

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)

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.

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

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

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)

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

Heat of Combustion

Strain in cycloalkane rings determined by

heats of combustion

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

Petroleum

• Fractional distillation of petroleum.

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)

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

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