Hydrocarbons

Hydrocarbons are organic compounds with the general formula CxHy. As a rule of thumb, many carbon atoms in a hydrocarbon molecule give higher boiling and melting points. For example, buthane C4H10 is a gas at room temperature where as the compound octane C8H18 is a liquid. All of them are flammable and often used as fuels in various applications. For example buthane is found in LPG, liquefied petroleum gas and octane is the major hydrocarbon in gasoline. Hydrocarbons are also common for fabrication of plastics, in many other types of chemical industries and as solvents.Hydrocarbons are divided into four main groups:

  1. Saturated, alkanes; only single bonds
  2. Unsaturated, alkenes or alkynes; double or triple bonds
  3. Aromatic, arenes; based upon benzene or other aromatic molecules
  4. Cyclic, cycloalkanes; ringformed hydrocarbons

Propane, C3H8, is a typical alkane with three carbon atoms. Ethene, C2H4, is the smallest alkene, with a C=C double bond

Two variants of drawings of the most typical arene benzene, C6H6. The example to the right is more common, showing that all electrons are equally shared between six carbon atoms in a flat ring.

Cyclohexane C6H12, a cycloalkane with general formula CnH2n. The molecule is not flat since the flexible carbon atoms are moving up or down.

 

IR spectrum of hydrocarbons

Regardless of the molecule formula or exact composition of a hydrocarbon mixture, the hydrocarbons have many C - H bonds which stretching modes strongly absorbs IR radiation at ~2900-1, or 3.4 µm. The C–C stretchings from alkane chains absorb in weak broad bands between 800 and 1300 cm−1 corresponding to 12.5 - 7.7 µm. Besides, there are strong peaks from –CH3 or –CH2 groups at 1300-1500 cm-1 (7.7 – 6.7 µm) due to C-H bending. Typical alkane peaks are shown below for hexane.

 

IR spectrum of hexane, C6H14 from a tuturial web page by Chris Schaller: http://employees.csbsju.edu/cschaller/Principles%20Chem/ structure%20determination/Hydrocarbon%20IR.htm.

A double bond in an alkene gives C=C stretching at 1650 cm -1 (6.1 µm). A sharp vibration band near 1500 cm-1 (6.7 µm) indicates presence of a benzene ring in an arene. Some common hydrocarbon bonds and their corresponding IR-absorption peaks are presented in this table, where the active bond is marked: