2.13 Sources of Alkanes and Cycloalkanes

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"Cracking"

	Cracking
		converts high molecular weight hydrocarbons to more useful, low molecular weight ones
	Reforming
		increases branching of hydrocarbon chains branched hydrocarbons have better burning characteristics for automobile engines

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Boiling Points of Alkanes

	governed by strength of intermolecular attractive forces
	alkanes are nonpolar, so dipole-dipole and dipole-induced dipole forces are absent
	only forces of intermolecular attraction are induced dipole-induced dipole forces

Induced dipole-Induced dipole attractive forces

	two nonpolar molecules
	center of positive charge and center of negative charge coincide in each

Induced dipole-Induced dipole attractive forces

	movement of electrons creates an instantaneous dipole in one molecule (left)

Induced dipole-Induced dipole attractive forces

	temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right)

Induced dipole-Induced dipole attractive forces

	temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right)

Induced dipole-Induced dipole attractive forces

	the result is a small attractive force between the two molecules

Induced dipole-Induced dipole attractive forces

	the result is a small attractive force between the two molecules

Boiling Points

	increase with increasing number of carbons
	more atoms, more electrons, more opportunities for induced dipole-induced dipole forces
	decrease with chain branching
	branched molecules are more compact with smaller surface area�fewer points of contact with other molecules

Boiling Points

	increase with increasing number of carbons
	more atoms, more electrons, more opportunities for induced dipole-induced dipole forces

Boiling Points

	decrease with chain branching
	branched molecules are more compact with smaller surface area�fewer points of contact with other molecules

"All alkanes burn in air..."

	All alkanes burn in air to give carbon dioxide and water.

Heats of Combustion

	increase with increasing number of carbons
	more moles of O2 consumed, more moles of CO2 and H2O formed

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Heats of Combustion

	increase with increasing number of carbons
	more moles of O2 consumed, more moles of CO2 and H2O formed
	decrease with chain branching
	branched molecules are more stable (have less potential energy) than their unbranched isomers

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Important Point

	Isomers can differ in respect to their stability.
	Equivalent statement:
		Isomers differ in respect to their potential energy.
	Differences in potential energy can be measured by comparing heats of combustion.

Figure 2.5

"Oxidation of carbon corresponds to..."

	Oxidation of carbon corresponds to an increase in the number of bonds between carbon and oxygen and/or a decrease in the number of carbon-hydrogen bonds.

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"But most compounds contain several..."

	But most compounds contain several (or many) carbons, and these can be in different oxidation states.
	Working from the molecular formula gives the average oxidation state.

"Fortunately,"

	Fortunately, we rarely need to calculate the oxidation state of individual carbons in a molecule .
	We often have to decide whether a process is an oxidation or a reduction.

"Oxidation of carbon occurs when..."

	Oxidation of carbon occurs when a bond between carbon and an atom which is less electronegative than carbon is replaced by a bond to an atom that is more electronegative than carbon.  The reverse process is reduction.

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