Organic Chemistry Basics: A High School Guide to Hydrocarbons

Organic chemistry is often introduced as the chemistry of carbon compounds. Carbon is unusually versatile because each carbon atom can form four covalent bonds, including bonds to other carbon atoms. This allows carbon to build chains, branches, rings, and three-dimensional structures with many different properties.

What makes hydrocarbons special?

Hydrocarbons contain only carbon and hydrogen. They are the simplest organic compounds and provide a useful starting point for learning how structure affects properties. Methane, CH₄, is the smallest alkane. Larger hydrocarbons can be found in fuels, waxes, and many materials made from petroleum.

Carbon can connect to another carbon with a single, double, or triple bond. A single bond is a sigma bond formed by sharing one pair of electrons. A double bond contains one sigma bond and one pi bond, while a triple bond contains one sigma bond and two pi bonds. At high school level, the most important consequence is that multiple bonds change the shape and reactivity of a molecule.

Alkanes, alkenes, and alkynes

Alkanes contain only single carbon-carbon bonds. They are called saturated hydrocarbons because each carbon has as many hydrogen atoms as possible. In an open chain, their general formula is CₙH₂ₙ₊₂. Examples include methane, ethane, and propane.

Alkenes contain at least one carbon-carbon double bond. An open-chain alkene with one double bond follows the general formula CₙH₂ₙ. Ethene, C₂H₄, is the simplest example. Alkynes contain at least one carbon-carbon triple bond and, for one open-chain triple bond, follow CₙH₂ₙ₋₂. Ethyne, C₂H₂, is also called acetylene.

The double and triple bonds make alkenes and alkynes generally more reactive than alkanes. A common reaction of an alkene is addition, in which atoms add across the double bond. This chemistry is important in making polymers such as polyethylene.

A first look at organic naming

The name of a simple hydrocarbon gives clues about its structure. The beginning of the name tells you the number of carbon atoms: meth- for one, eth- for two, prop- for three, but- for four, and pent- for five. The ending tells you the bond type: -ane for single bonds, -ene for a double bond, and -yne for a triple bond.

For a basic chain, choose the longest continuous carbon chain, identify the type of bonds, and number the chain so that a multiple bond receives the lowest possible number. A four-carbon chain with a double bond beginning at carbon 1 is but-1-ene. Learning to read names alongside structural formulas is more powerful than memorizing either list by itself.

What are isomers?

Isomers have the same molecular formula but different arrangements of atoms. Butane and 2-methylpropane both have the formula C₄H₁₀, but one is a straight chain and the other is branched. Because shape affects intermolecular forces and how molecules fit together, isomers can have different boiling points and other physical properties.

Structure and properties

As hydrocarbon chains become longer, their boiling points generally increase because larger molecules have stronger London dispersion forces. Branching often lowers the boiling point because a branched molecule has a smaller surface area for neighboring molecules to contact. These patterns show why a structural formula is not just a drawing; it helps predict behavior.

The main idea to remember

Hydrocarbons are a foundation for organic chemistry. Carbon’s four bonds allow a huge variety of structures, while single, double, and triple bonds lead to the families called alkanes, alkenes, and alkynes. Once you can connect a molecule’s name, formula, shape, and bond type, organic chemistry becomes a logical language rather than a list of unfamiliar words.