Synthesizing Long-Chain Hydrocarbon Fuels
Hydrocarbon Basics
The Simplest Carbon Compounds
Organic chemistry is the study of carbon compounds. At its heart are the simplest of these: hydrocarbons. As the name suggests, they are molecules made up of only hydrogen and carbon atoms. Everything from the natural gas that heats your home to the plastic in your water bottle is derived from these fundamental molecules.
Hydrocarbons are compounds containing carbon and hydrogen only
Carbon's unique ability to form stable, long chains and rings with itself allows for a vast number of different hydrocarbon structures. We can group these compounds into a few main families based on the types of bonds they contain.
Meet the Families
The four main types of hydrocarbons are alkanes, alkenes, alkynes, and aromatic compounds. The first three are distinguished by the kind of bonds between their carbon atoms.
Alkanes contain only single bonds between carbon atoms. They are called saturated hydrocarbons because each carbon atom is bonded to the maximum possible number of hydrogen atoms.
Alkenes have at least one carbon-carbon double bond.
Alkynes have at least one carbon-carbon triple bond.
Alkenes and alkynes are unsaturated because the double or triple bonds mean they have fewer hydrogen atoms than a corresponding alkane.
Then there are the aromatic hydrocarbons, a special class of cyclic (ring-shaped) compounds. The most common example is benzene (), which has a six-carbon ring with a unique bonding structure that makes it very stable.
Drawing the Molecules
Chemists have several ways to draw molecules. An expanded structural formula shows every single atom and bond. It's thorough but can get cluttered for large molecules.
Expanded Butane (C₄H₁₀):
H H H H
| | | |
H-C - C - C - C-H
| | | |
H H H H
A condensed structural formula groups the hydrogen atoms with the carbon atom they're bonded to. It's a cleaner look that still provides all the necessary information.
Condensed Butane:
CH₃-CH₂-CH₂-CH₃ or CH₃(CH₂)₂CH₃
The most common method is the skeletal structure (or line-angle formula). This simple format is the fastest to draw. In it:
- Carbon atoms are not shown. They are assumed to be at each vertex (corner) and at the ends of the chain.
- Hydrogen atoms bonded to carbons are not shown. It's assumed each carbon has enough hydrogens to form four total bonds.
- Atoms other than carbon and hydrogen are shown.
What's in a Name?
To avoid confusion, chemists created a standardized naming system called the IUPAC (International Union of Pure and Applied Chemistry) nomenclature. For simple alkanes, the name has two parts: a prefix that indicates the number of carbon atoms and the suffix -ane.
| Prefix | # of Carbons |
|---|---|
| Meth- | 1 |
| Eth- | 2 |
| Prop- | 3 |
| But- | 4 |
| Pent- | 5 |
| Hex- | 6 |
So, a one-carbon alkane is methane (), a two-carbon one is ethane (), and so on. For alkenes, the suffix is -ene, and for alkynes, it's -yne. The naming system gets more complex for branched molecules, but this basic structure is the foundation.
Physical Properties
The structure of a hydrocarbon determines its physical properties. Because carbon-hydrogen bonds are nonpolar, hydrocarbons are generally nonpolar molecules. This has a few key consequences.
Solubility: They don't dissolve in water, which is polar. You've seen this in action—oil (a mix of hydrocarbons) and water don't mix.
Boiling/Melting Points: The forces holding hydrocarbon molecules together are relatively weak (van der Waals forces). As the carbon chain gets longer, these forces increase, which means more energy is needed to pull the molecules apart. Therefore, boiling and melting points increase with molecular size. Methane is a gas at room temperature, while octane () is a liquid, and paraffin wax (around ) is a solid.
What two elements are hydrocarbons exclusively composed of?
Which type of hydrocarbon is characterized by containing at least one carbon-carbon triple bond?
With these basics of structure, naming, and properties, you're ready to explore the more complex world of organic chemistry.