Alkanes
Introduction
Alkanes are hydrocarbons with only single bonds between the carbon atoms. They do not contain a reactive functional group, so they are pretty unreactive chemically. They do burn well, though. I’m very glad the alkane, propane, burns well so I can cook my burgers and brats on my gas grill.
Since most organic compounds have a framework built out of alkanes, we need to have a good understanding of them. Let’s look at some of the simplest straight-chain alkanes to start.
Normal alkanes
These straight-chain alkanes can be thought of as a string of –CH2– groups, called methylene groups, with two more H atoms—one on each end—to make the two –CH3 groups, called methyl groups.
The general formula for an alkane with n carbon atoms is (CH2)n+2H, or maybe more simply, CnH2n+2.
Let’s look at ethane to see that it works. Ethane’s formula is C2H6 which fits the formula with n=2. Compare that to ethene and ethyne.
The formula is reduced by two hydrogen atoms for every π-bond a compound has. For one π-bond in a double bond as in ethene, there are two fewer hydrogen atoms than ethane. For two π-bonds in a triple bond, like in ethyne, there are four fewer hydrogen atoms. Alkanes are saturated. They are saturated with all of the hydrogen atoms the carbon atoms can possibly take. They are all single bonds. Unsaturated compounds contain π-bonds, double or triple bonds.
1. Are the following compounds saturated or unsaturated?
Answers
1.
