structural formula

(noun)

A formula that indicates not only the number of atoms, but also their arrangement in space.

Related Terms

Examples of structural formula in the following topics:

  • A Structure Formula Relationship

    • Recall that the molecular formula of a hydrocarbon (CnHm) provides information about the number of rings and/or double bonds that must be present in its structural formula.
    • In the formula shown below a triple bond is counted as two double bonds.
    • This molecular formula analysis may be extended beyond hydrocarbons by a few simple corrections.
    • All halogens present in the molecular formula must be replaced by hydrogen.
    • Each nitrogen in the formula must be replaced by a CH moiety.

  • Molecular Formulas

    • Although a molecular formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula.
    • Molecular formulas are more limiting than chemical names and structural formulas.
    • Because of this, one molecular formula can describe a number of different chemical structures.
    • A structural formula is used to indicate not only the number of atoms, but also their arrangement in space.
    • Identify the molecular formula of a compound given either its name or structural formula.

  • Empirical Formulas

    • These notations, which include empirical, molecular, and structural formulas, use the chemical symbols for the elements along with numeric values to describe atomic composition.
    • The molecular formula for a compound is equal to, or a whole-number multiple of, its empirical formula.
    • An empirical formula (like a molecular formula) lacks any structural information about the positioning or bonding of atoms in a molecule.
    • It can therefore describe a number of different structures, or isomers, with varying physical properties.
    • However, one structural representation for butane is CH3CH2CH2CH3, while isobutane can be described using the structural formula (CH3)3CH.

  • Analysis of Molecular Formulas

    • Although structural formulas are essential to the unique description of organic compounds, it is interesting and instructive to evaluate the information that may be obtained from a molecular formula alone.
    • The number of hydrogen atoms in stable compounds of carbon, hydrogen & oxygen reflects the number of double bonds and rings in their structural formulas.
    • Consider a hydrocarbon with a molecular structure consisting of a simple chain of four carbon atoms, CH3CH2CH2CH3.
    • From the above discussion and examples it should be clear that the molecular formula of a hydrocarbon (CnHm) provides information about the number of rings and/or double bonds that must be present in its structural formula.
    • Each nitrogen in the formula must be replaced by a CH moiety.

  • Percent Composition of Compounds

    • The atomic composition of chemical compounds can be described using a variety of notations including molecular, empirical, and structural formulas.
    • For example, butane has a molecular formula of C4H10.
    • Mass percents can be determined experimentally via elemental analysis, and these values can be used to calculate the empirical formula of unknown compounds.
    • However, this information is insufficient to determine the molecular formula without additional information on the compound's molecular weight.
    • Translate between a molecular formula of a compound and its percent composition by mass

  • Isomers

    • It is necessary to draw structural formulas for organic compounds because in most cases a molecular formula does not uniquely represent a single compound.
    • There are seven constitutional isomers of C4H10O, and structural formulas for these are drawn in the following table.These formulas represent all known and possible C4H10O compounds, and display a common structural feature.
    • When discussing structural formulas, it is often useful to distinguish different groups of carbon atoms by their structural characteristics.
    • Our ability to draw structural formulas for molecules is remarkable.
    • All the isomers for a given formula are not shown, and each structurally unique carbon is designated by color only once in each structure.

  • Resonance

    • Kekulé structural formulas are essential tools for understanding organic chemistry.
    • However, the structures of some compounds and ions cannot be represented by a single formula.
    • For clarity the two ambiguous bonds to oxygen are given different colors in these formulas.
    • Since experimental evidence indicates that this molecule is bent (bond angle 120º) and has equal length sulfur : oxygen bonds (1.432 Å), a single formula is inadequate, and the actual structure resembles an average of the two formulas.
    • Therefore the middle formula represents a more reasonable and stable structure than the one on the right.

  • Polyatomic Molecules

    • A pure substance is composed of molecules with the same average geometrical structure.
    • A molecule's chemical formula and structure are the two important factors that determine its properties, particularly reactivity.
    • A compound's empirical formula is the simplest integer ratio of its constitutional chemical elements.
    • The molecular formula characterizes different molecules by reflecting their exact number of compositional atoms.
    • The empirical formula is often the same as the molecular formula, but not always; for example, the molecule acetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH.

  • Alkenes & Alkynes

    • The molecular formulas of these unsaturated hydrocarbons reflect the multiple bonding of the functional groups:
    • As noted earlier in the Analysis of Molecular Formulas section, the molecular formula of a hydrocarbon provides information about the possible structural types it may represent.
    • For example, consider compounds having the formula C5H8.
    • The formula of the five-carbon alkane pentane is C5H12 so the difference in hydrogen content is 4.

  • Fischer Projection Formulas

    • Since this is not the usual way in which we have viewed such structures, the following diagram shows how a stereogenic carbon positioned in the common two-bonds-in-a-plane orientation ( x–C–y define the reference plane ) is rotated into the Fischer projection orientation (the far right formula).
    • When writing Fischer projection formulas it is important to remember these conventions.
    • The structure must not be flipped over or rotated by 90º.
    • Fischer formulas for these isomers, which Fischer designated as the "D"-family, are shown in the diagram.
    • The aldopentose structures drawn above are all diastereomers.