3.4 Alkanes

Alkanes are saturated hydrocarbons with only single bonds. Examples include methane CH₄, ethane CH₃CH₃, propane CH₃CH₂CH₃, and so on.

Octane is the 8-carbon straight chain alkane, but it is not the octane that's desirable in gasoline. That's isooctane, or (CH₃)₃CCH₂CH(CH₃)₂. Iso- is short for isomer, meaning "made up of the same parts"; it contains the same atoms but in a different sequence. Since it's easier to say "octane" than "isooctane" with its weird "oh-ah" vowel juxtaposition, petroleum industry executives decided to go with the flashier name at the expense of accuracy. Technically, "iso-" most often means the least branched possible isomer, so "isooctane" should be (CH₃)₂CH(CH₂)₄CH₃, but the precedent has already been set.

Saturated rings also exist and are prefixed with cyclo-, e.g. cyclohexane:

It should be noted that cyclohexane has a flexible ring, and it displays two well defined shapes, or conformers, that it can freely convert back and forth between: the chair form, and the boat form.

Chair form

Boat form

The chair form is the more stable configuration. It has three of its hydrogen atoms pointing "up" perpendicular to the ring, three pointing "down", and the remaining six pointing outward. The "up" and "down" hydrogens are called polar hydrogens, and the others are called equatorial. Later we will see why that distinction matters.

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