What is Hydrocarbon? - Definition, Formula & Compounds

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  • 0:00 Definition
  • 0:45 Aliphatic Hydrocarbons
  • 4:31 Arenes
  • 5:32 Lesson Summary
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Lesson Transcript
Instructor: Nicholas Gauthier
Hydrocarbons are organic compounds that are made of only hydrogen and carbon atoms. They are found in many places, including crude oil and natural gas. Learn the different forms of these simple, yet varied, organic compounds.

Hydrocarbons: Definition

A hydrocarbon is an organic compound made of nothing more than carbons and hydrogens. It is possible for double or triple bonds to form between carbon atoms and even for structures, such as rings, to form.

Saturated hydrocarbons have as many hydrogen atoms as possible attached to every carbon. For carbons on the end of a molecular chain, three can be attached. For carbons in the middle of a chain or a ring, two can be attached. For a carbon atom all by itself, four hydrogen atoms can be attached. Saturated hydrocarbons have only single bonds between adjacent carbon atoms.

Unsaturated hydrocarbons have double and/or triple bonds between some of the carbon atoms.

Examples of saturated and unsaturated hydrocarbons
Saturated and Unsaturated Hydrocarbons

Aliphatic Hydrocarbons

Aliphatic hydrocarbons are compounds of hydrogen and carbon that do not contain benzene rings. We'll discuss compounds with benzene rings later in this lesson. Aliphatic hydrocarbons tend to be flammable. There are several types of aliphatic hydrocarbons: alkanes, alkenes, alkynes and alkenynes.

Let's start with alkanes and cycloalkanes. Alkanes are linear, or branching, compounds that are made of varying numbers of carbon atoms that are all saturated with hydrogen atoms. The formula for an alkane is CnH(2n+2). This means that the number of hydrogen atoms equals twice the number of carbon atoms, plus two.

Methane is the simplest hydrocarbon possible. It is released as a gas from decomposing bodies and from the intestinal tracts of many animals. Its formula is CH4. A methyl group, with the formula CH3, has one free bond to join something else.

Methane, the simplest hydrocarbon, along with its group form
Methane and Methyl Group

Building on methane, some other alkanes are ethane (C2H6), propane (C3H8), butane (C4H10) and pentane (C5H12). Each adds one carbon to the chain. Note in the diagram that each carbon is saturated.

Various alkanes. Note that all carbons are saturated and that there are no double or triple bonds.

Cycloalkanes are a type of alkane that contain a carbon ring but not a benzene ring. These molecules still have only single bonds and thus, are saturated. Cycloalkanes are generally similar to their alkane, like cyclopropane or cyclobutane, but may have higher boiling and melting points.

Cyclobutane structure

Next, we'll discuss alkenes. Alkenes are like alkanes, but they have at least one double bond between carbon atoms. The formula for an alkene is a little trickier, since the number of double bonds may vary. An alkene with only one double bond has the formula CnH(2n). Each double bond means fewer hydrogens than a corresponding alkane. For each additional double bond, subtract two more hydrogens. The simplest alkene is ethene, with a formula of C2H4.

When naming an alkene, one has to take care to note which carbons have the double bonds. If there is only one double bond, then put the number of the carbon in front of the name. For example, if the second carbon in pentene has the double bond, then name it 2-pentene. If the third carbon has the double bond, name it 3-pentene.

However, if there are two double bonds, it would have a name like 2,4-pentadiene. If there were three double bonds in octene (8 carbon atoms), it would have a name like 1,3,5-octatriene. Four double bonds would end it '-tetraene,' as in 1,3,5,7-octatetraene.

Example alkenes

Last in our list of aliphatic hydrocarbons are alkynes. Alkynes have at least one triple bond between carbon atoms. Alkenynes have both double and triple bonds. The formula for an alkyne is variable as well. An alkyne with only one triple bond has the formula CnH(2n-2). Each triple bond means two fewer hydrogens than a corresponding alkane. For each extra double bond, subtract four more hydrogens. Alkynes follow similar naming rules to alkenes.

Example alkynes

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