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10.2: Organic Structural Representations: Molecular Formulas

  • Page ID
    237747
  • Learning Objectives
    • Define molecular formula.
    • List the order in which elemental symbols should be written in a molecular formula.
    • Write the molecular formula of an organic molecule.
    • Identify the compositional characteristics of a molecule that can be determined by analyzing a molecular formula.

    Chemists use several different notations, including chemical formulas and chemical names, to represent compounds and molecules.  Furthermore, because, as stated in the previous section of this chapter, organic substances are covalently-bonded molecules, their structures can also be visually-represented using two-dimensional Lewis structures and three-dimensional Valence Shell Electron Pair Repulsion, or VSEPR, pictures.  However, since organic molecules can contain thousands of individual carbon atoms, generating these types of images, which can become prohibitively-large in size, is unreasonably time-consuming.  As a result, organic chemists have developed two additional drawing conventions, called condensed structural notation and bond-line notation, to visually-represent organic molecules using smaller and, therefore, more manageable, pictures. 

    The first type of organic structural representation that will be discussed in this chapter is a molecular formula, which is a modified version of a covalent chemical formula.  Recall that a chemical formula is an abbreviation that indicates both the types and quantities of elements that are present in a molecule.  Because a formula is, by definition, a symbolic representation, elemental symbols are used to identify which elements are contained in the corresponding substance.  As stated previously, an organic molecule must be comprised of carbon, C, and usually contains hydrogen, H.  Therefore, the symbols of these elements are written first and second, respectively, in a molecular formula.  Since nitrogen, N, oxygen, O, phosphorus, P, sulfur, S, and the halogens, X, are less prevalent in organic molecules, relative to carbon and hydrogen, the elemental symbols of these eight non-metal heteroatoms are written in the final position, or positions, in a molecular formula.  If multiple heteroatoms are present in a single organic molecule, their corresponding elemental symbols should be listed in alphabetical order.

    Whole-number subscripts are used to indicate the quantity of each element that is present in a given molecule.  The subscripts of covalent molecules must not be reduced to the lowest-common ratio of whole numbers, even if it is mathematically-possible to do so, as dividing these subscripts would cause their values to be inconsistent with the quantity of elements that are contained in the corresponding molecule.  Furthermore, values of "1" are usually implicitly-understood in chemistry and, therefore, should not be written as subscripts in a chemical formula.

    For example, write the molecular formula of an organic molecule that contains four hydrogens, two carbons, and two chlorines.

    Because an organic molecule must be comprised of carbon, C, and usually contains hydrogen, H, these elemental symbols are written first and second, respectively, in the molecular formula that is being developed, and the symbol for chlorine, Cl, which is classified as a halogen and, therefore, as a heteroatom, is written last.  Because two carbons, four hydrogens, and two chlorines are present in the corresponding molecule, subscripts of "2," "4," and "2," respectively, are written on the elemental symbols that are indicated above.  Therefore, C2H4Cl2 is the chemically-correct molecular formula of an organic molecule that contains four hydrogens, two carbons, and two chlorines.  These subscripts should not be divided by 2, even though it is mathematically-possible to do so, as the resultant formula, CH2Cl, would not be consistent with the given description of the organic molecule.

    Finally, while a molecular formula indicates both the types and quantities of elements that are present in a molecule, this organic structural representation does not provide any information about the connectivity of those elements.  Furthermore, neither the number, type, or location of the bonds that are present in the corresponding structure, nor the quantity or location of lone pairs that are found in the molecule, can be determined by analyzing a molecular formula.  Therefore, in order to obtain more information about the compositional characteristics of an organic chemical, the structure of that substance must be represented in a visual format.

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