The structural formula of a chemical compound is a graphical representation of the molecular structure, showing how the atoms are arranged. The chemical bonding within the molecule is also shown, either explicitly or implicitly. There are three common representations used in publications: text, Lewis type and line-angle formula. Also several other formats are used, as in chemical databases, such as SMILES, InChI and CML.
Unlike chemical formulae or chemical names, structural formulas provide a representation of the molecular structure. Chemists nearly always describe a chemical reaction or synthesis using structural formulae rather than chemical names, because the structural formulas allow the chemist to visualize the molecules and the changes that occur.
Many chemical compounds exist in different isomeric forms, which have different structures but the same overall chemical formula. A structural formula indicates the arrangements of atoms in a way that a chemical formula cannot.
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Text formulas
In early organic chemistry publications, where use of graphics was severely limited, a text-based system arose to describe organic structures in a line of text. Although this system tends to break down with complex cyclic compounds, it remains a convenient way to represent simple structures
- CH3–CH2–OH or CH3CH2OH
Lewis structures
Lewis structures are flat graphical formulas that show the atom connectivity, but lack information about the three-dimensional structure of molecules. This notation is mostly used for small linear molecules. A single line represents a single bond or single electron pair. Two and three lines represent double and triple bonds, respectively. Alternatively, dots (•) may be used to represent single electrons. Occasionally, this is referred to as a Lewis Dot Structure.
Three-dimensional structures
Several methods exist to picture the the three-dimensional arrangement of atoms in a molecule.
Fischer projection
The Fischer projection is mostly used for linear monosaccharides. The vertical backbone is assumed to form a bridge-like structure on the paper plane with the substituents sticking up:
Newman projection
The Newman projection and the Sawhorse projection are used to depict the stereochemistry at two connected carbon atoms:
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Newman projection |
Sawhorse projection |
Haworth projection
The Haworth projection is used for cyclic sugars:
Skeletal formulas
Main article: Skeletal formulaSkeletal formulas are the standard notation for more complex organic molecules. Carbon (C) atoms are represented by the vertices (corners) and termini of line segments that are not marked with an atomic symbol. Each carbon atom is in turn assumed to bear enough hydrogen atoms to give the carbon atom four bonds. Hydrogen atoms on atoms other than carbon must be written explicitly.
Chirality in skeletal formulas is indicated by the Natta projection method. Solid or dashed wedged bonds represent bonds pointing above-the-plane or below-the-plane of the paper, respectively.
See also
External links
References
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Categories: Chemical formulas
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