Core concepts
In this tutorial, you will learn about hydrocarbons, their structures, and the organic chemistry reactions that relate to them.
What is a Hydrocarbon?
A hydrocarbon is a type of chemical compound that contains only carbon and hydrogen atoms. Despite this simplicity, hydrocarbons are an incredibly diverse group of compounds.
As mentioned above, hydrocarbons are organic compounds that consist entirely of hydrogen and carbon atoms. They are the simplest and most common type of organic compound, and are the building blocks of more complex organic molecules such as carbohydrates, proteins, and fats.
There are three types of hydrocarbons, each with unique and shared characteristics. These are alkanes, alkenes, and alkynes. Alkanes are hydrocarbons with only single bonds between the carbon atoms, while alkenes have at least one double bond between the carbon atoms. Both alkanes and alkenes can exist as either straight-chain or branched-chain molecules, and can have varying numbers of carbon atoms.
Hydrocarbons are important in many industrial and commercial applications. For example, alkanes are commonly used as fuels, while alkenes are used in the production of plastics and other synthetic materials. Hydrocarbons are also found in natural gas, petroleum, and other fossil fuels, which are extracted and refined for use as energy sources.
In addition to their practical uses, hydrocarbons are also important in many scientific fields, including chemistry, biology, and environmental science. For example, the study of hydrocarbons is essential for understanding the chemical reactions involved in the formation and degradation of organic materials, and for developing methods for cleaning up hydrocarbon pollutants in the environment.
Naming Hydrocarbons
All three types of hydrocarbons have the same naming scheme. The prefix of a hydrocarbon’s name comes from the largest number of carbon atoms it has in a single chain. The second part of the name depends on what type of bonds are in the compound. The name has an -ane suffix if there are only single bonds. If there is a double bond, the name has an -ene suffix. With a triple bond, the name has an -yne suffix.
Prefixes
The following are some of the common hydrocarbon prefixes:
| Number of Carbons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Prefix | Meth | Eth | Prop | But | Pent | Hex | Hept | Oct |
Branches in Hydrocarbons
A shorter hydrocarbon chain coming attached to a longer one is called an alkyl group, branch, or substituents. When naming the hydrocarbon, branches are numbered by which carbon on the principal chain it is attached to and take the prefix that corresponds to its number of carbons.
If the same substituent appears multiple times, each of its locations on the principal chain receives a number, and another prefix is added that indicates the number of times it appears.
Stereochemistry in Hydrocarbons Molecules
R and S Configuration
Determining stereochemistry is an important part of classifying and naming hydrocarbons. When a carbon has four different groups attached to it, it is called a stereocenter. Stereocenters receive an R or S label depending on how the groups around them are positioned. The first step in determining R or S configuration is labeling groups in order of priority. For hydrocarbons, groups on the stereocenter receive a number from 1 to 4 based on their size. Larger groups receive lower numbers, and hydrogen always receives a 1.
The lowest priority group should be on a dashed bond, facing backward. Then, a circular arrow should be drawn around the stereocenter starting at group 1 and ending at group 3. If the arrow goes clockwise, the stereocenter has an R configuration. If it goes counterclockwise, the stereocenter has an S configuration. The R or S goes in parentheses before the rest of the compound’s name.
If two groups have the same number of carbons, but one of the groups has a double or triple bond, the group with more bonds receives priority.
Alkene Stereochemistry
Alkenes also have stereochemical considerations in naming. Internal alkenes can either be in Z or E configuration. To determine whether an alkene is E or Z, it is helpful to imagine a line extending in the direction of the alkene bond. When the priority groups are on the same side of this line, the alkene is Z. When the priority groups are on opposite sides of the line, the alkene is E.
In an alkene, the priority groups are the two groups that make up the principal chain.
Cyclic Hydrocarbons
Cyclic hydrocarbons receive the prefix “cyclo” before the rest of their name.
Examples of Hydrocarbon Molecules and Names
Hydrocarbon Properties
Despite the vast number of hydrocarbons, they all share many properties. Because they lack polarity, the only intermolecular forces that hydrocarbons experience are London Dispersion Forces.
Additionally, the boiling and melting points of hydrocarbons increase with the number of carbons. Branching decreases the freezing and boiling point by reducing packing efficiency. This refers to how tightly packed together compounds within an area are. 2,3-dimethylhexane and octane have the same number of carbon and hydrogen atoms, but octane is unbranched and therefore has a higher boiling point.
All hydrocarbons are flammable. They perform combustion reactions with oxygen to produce carbon dioxide, water vapor, and heat.
Methane, the smallest hydrocarbon, is a greenhouse gas. This means that when large quantities of it collect in the atmosphere, they trap in heat energy from the sun.
Uses of Hydrocarbons
Hydrocarbons of all types perform a diverse array of functions both in industrial and everyday use. Many of these uses are either a part or a direct outgrowth of, the fossil fuel industry.
Fuels
Vehicles with internal combustion engines, such as cars, trucks, and planes, use hydrocarbons as fuel. These fuels are not usually a single compound, but a mixture of hydrocarbons of similar sizes. Larger vehicles tend to use larger fuels.
Hydrocarbons with chain lengths of 18 to 50 carbons are used as motor oil, which lubricates the engine and allows for it to run smoothly.
Plastics
Not all plastics are hydrocarbons, but many common ones are. Polystyrene, commonly known as styrofoam, consists of styrene polymerized into long chains. The polymerization of propylene and polyethylene also yields plastics used in packaging.
Solvents
In labs around the world, chemists use hydrocarbons as non-polar solvents. Hexanes and toluene are two common solvents in organic laboratories. Chemists of the past often used benzene as a solvent, though it is now considered too carcinogenic for use in such large quantities.
Reactions
Alkanes
Alkanes participate in the fewest types of reactions. When exposed to oxygen, alkanes can combust to produce CO2, water vapor, and heat. Additionally, alkanes can undergo halogenation reactions. These involve the replacement of a hydrogen atom with a halogen. Alkanes can also be broken down and attached with the help of metal catalysts.
Alkenes
Alkenes have more capabilities when it comes to reacting. Halide molecules and hydrogen halides can add across a double bond to form an alkyl halide. Similarly, a hydrogen molecule can hydrate the double bond with the help of a metal catalyst.
Adding water or an alcohol in the presence of acid will add, respectively, an -OH group or an ether to the alkene.
Usually, these additions follow Markovnikov’s rule. This means that the hydrogen will add to the less substituted side of the alkene. Using Borane and an ethereal solvent followed by base, water, and hydrogen peroxide allows an -OH group to add to the less substituted side instead of Hydrogen.
Ozone followed by a reducing agent breaks the alkene apart entirely at the double bond. If the alkene was terminal, this creates two aldehydes. If the alkene was interior, this creates two ketones. When the Ozone is followed by an oxidizing agent, the result is two acids.
Alkynes
Alkynes react similarly to alkenes. The primary difference between the two is that where addition to an alkene creates an alkane, addition to an alkyne creates an alkene.