Structural Formulas

In many cases, especially with organic compounds, even the molecular formula does not provide enough information to identify a compound, so that structural formulas are needed. For example, both ethanol (ethyl alcohol) and dimethyl ether have the molecular formula C₂H₆O (see isomer). Their structural formulas are: In these formulas each line represents a single covalent chemical bond. A double bond is represented by a double line and a triple bond by a triple line. In ethene (ethylene), C₂H₄, the carbon atoms are joined by a double bond. The structural formula of ethene is: (In many representations of structural formulas, the angles of the lines indicating bonds do not necessarily have meaning.) In ethyne (acetylene), C₂H₂, the carbon atoms are joined by a triple bond. The structural formula of ethyne is:

Semistructural Formulas

Structural formulas are often simplified so that they can be written on a single line; the simplified formulas are often called semistructural formulas. The semistructural formula for ethanol is CH₃CH₂OH, or more simply C₂H₅OH. In such a semistructural formula the OH is written explicitly to indicate that the oxygen has a hydrogen bonded to it. The C₂H₅ indicates that the two carbon atoms are bonded to one another. The semistructural formula for dimethyl ether may be written CH₃OCH₃. Here the O is placed between the two carbon atoms to show that the carbons are bonded to the oxygen. A carbon often has three hydrogens bonded to it, and the H₃ is written after the C. In some cases the H₃ is written before the C for clarity; thus the formula for dimethyl ether might be written H₃COCH₃.

Electron Dot Diagrams

Dots are used in a type of formula called the electron dot diagram, where each pair of dots represents a pair of shared electrons in a covalent bond. The diagrams for ethane (CH₃CH₃), ethene, and ethyne are:

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2007, Columbia University Press. All rights reserved.

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