Introduction to Alkanes Methane, Ethane, and Propane

Two molecular orbitals (MOs) are generated by combining two hydrogen Is atomic orbitals (AOs). The bonding MO is lower in energy than either of the AOs that combine to produce it. 

The antibonding MO is of higher energy than either AO. Each arrow indicates one electron, and the electron spins are opposite in sign. Both electrons of H2 occupy the bonding MO. 

For a molecule as simple as H2, it is hard to see much difference between the valence bond and molecular orbital methods. The most important differences appear in molecules with more than two atoms. In those cases, the valence bond method continues to view a molecule as a collection of bonds between connected atoms. The molecular orbital method, however, leads to a picture in which the same electron can be associated with many, or even all, of the atoms in a molecule. We ll have more to say about the similarities and differences in valence bond and molecular orbital theory as we continue to develop their principles, beginning with the simplest alkanes methane, ethane, and propane. 

Alkanes have the general molecular formula C H2 +2. The simplest one, methane (CH4), is also the most abundant. Large amounts are present in our atmosphere, in the ground, and in the oceans. Methane has been found on Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, on Halley s Comet, even in the atmosphere of a planet in a distant solar system. About 2-8% of the atmosphere of Titan, Saturn s largest moon, is methane. When it rains on Titan, it rains methane. 

Methane is the lowest boiling alkane, followed by ethane, then propane. 

It is generally true that as the number of carbon atoms increases, so does the boiling point. All the alkanes with four carbons or fewer are gases at room temperature and atmospheric pressure. With the highest boiling point of the three, propane is the easiest one to liquefy. We are all familiar with propane tanks. These are steel containers in which a propane-rich mixture of hydrocarbons called liquefied petroleum gas (LPG) is maintained in a liquid state under high pressure as a convenient clean-burning fuel. 

The structural features of methane, ethane, and propane are summarized in Eigure 2.7. All of the carbon atoms have four bonds, all of the bonds are single bonds, and the bond angles are close to tetrahedral. In the next section we ll see how to adapt the valence bond model to accommodate the observed structures. 

Structures of methane, ethane, and propane showing bond distances and bond angles. 

Ethane (C2H6 CH3CH3) and propane (C3H8 CH3CH2CH3) are second and third, respectively, to methane in many ways. Ethane is the alkane next to methane in structural simplicity, followed by propane. Ethane ( 10%) is the second and propane ( 5%) the third most abundant component of natural gas, which is 75% methane. The characteristic odor of natural gas we use for heating our homes and cooking comes from 

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Introduction to Alkanes Methane, Ethane, and Propane 57 sp Hybridization and Bonding in Methane 58... 

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