Sources of alkanes and cycloalkanes. Crude oil презентация

2.13 Sources of Alkanes and Cycloalkanes

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

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 to give carbon dioxide and water. 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

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

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

But most compounds contain several (or many) carbons, and these can be in different oxidation states. 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, we rarely need to calculate the oxidation state of individual carbons in a molecule . 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 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. 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.