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:;g244;none;1;g244;;;block;;;;no;1;139392n;242442n;;;;;formula244;;;left;stack;;;;;CE5In 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:
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.
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:;g244C;none;1;g244C;;;block;;;;no;1;4224n;124947n;;;;;formula244c;;;left;stack;;;;;CE5