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Unit 3
Elements of Organic Chemistry

3-4. HYDROCARBONS

The simplest organic compounds are the hydrocarbons, which are composed solely of carbon and hydrogen. Since there are only two elements involved, one might expect there would be only a few different compounds. However, carbon does bond to itself and form long chains. So there are many, many different hydrocarbons. They can be classed in two general groups, aliphatic and aromatic. These compounds are the starting point for all organic compounds.

Aliphatic Hydrocarbons. Aliphatic hydrocarbons consist of straight or branched chains of carbon atoms with the other valence electrons involved in bonds with hydrogen. Examples are:

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We can subdivide aliphatic hydrocarbons into two groups based on the types of carbon-carbon bonds the compounds contain.

  1. Saturated aliphatic hydrocarbons. Saturated aliphatic hydrocarbons are hydrocarbons in which all of the carbon-carbon bonds are single bonds. These compounds are also referred to as alkanes, as mentioned for single bonds earlier. We often refer to the alkanes as the methane series. Methane is the simplest hydrocarbon with the formula CH4. All other alkanes are formed by adding CH2s to the formula (table 3-2). In this series, the names from C5 to C10 all begin with the Greek prefix for the number (e.g., penta- for five) and end in -ane from "alkane." The two low-molecular weight alkanes are gases. Alkanes are not very reactive chemically and are insoluble in water. About the most important reaction they undergo is that they burn to form carbon dioxide and water (combustion reaction). Some typical saturated compounds you might encounter are:
NAME FORMULA BOILING POINT ( C)
Methane CH4 -161.5
Ethane C2H6 - 88.3
Propane C3H8 - 44.5
Butane C4H10 - .5
Pentane C5H12 + 36.2
Hexane C6H14  
Heptane C7H16  
Octane C8H18 +125.8
Nonane C9H20  
Decane C10H22 +174.0

Table 3-2. Common alkanes.

(2) Unsaturated aliphatic hydrocarbons. The second type of aliphatic hydrocarbon is unsaturated hydrocarbons. These are hydrocarbons, which contain at least one double or triple bond (that is, they are alkenes or alkynes). An example of an alkene is ethene, the simplest alkene, which consists of two double-bonded carbon atoms and four hydrogen atoms.

CH2 = CH2

Note that the name is similar to the saturated compound ethane. The -ene ending comes from the word alkene and denotes that it contains a double bond. Similarly, if there were a triple bond between the two carbon atoms, the name would be ethyne with the -yne ending denoting the triple bond (from alkyne). The physical properties of alkenes and alkynes are similar to the properties of alkanes of similar molecular weights. Chemically, the word unsaturated implies that these compounds can form additional bonds. This is the case, for alkenes and alkynes are much more reactive and undergo many reactions not possible with alkanes.

b. Aromatic Hydrocarbons. The second major group of the hydrocarbons is the aromatic hydrocarbons, which are hydrocarbons that contain a benzene ring as part of their structure. Benzene has the formula C6H6 and consists of six carbon atoms in a ring with three alternating double bonds. The benzene ring is represented with the following symbols:

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  1. Benzene is completely insoluble in water, it is a volatile liquid at room temperature, and it is fairly unreactive. The properties of other aromatics are reflective of benzene but vary according to the substituents added to the ring in place of one of the hydrogen atoms.
  2. The term "aromatic" has its origin in the fact that certain aromatic substances (for example: oil of bitter almonds, vanilla, and oil of wintergreen) contain the benzene ring. The possession of an odor is not characteristic, however, of all aromatic substances.
  3. Aromatic hydrocarbons are the starting point for many medicinally important compounds, as the following examples indicate.

You will notice in the compounds above that they are not pure aromatic hydrocarbons because they contain elements other than carbon and hydrogen. These additional elements are the basis for the classification of substituted organic compounds and are called functional groups. The important functional groups will be considered in the following paragraphs.