There are multiple ways to represent or draw compounds in organic chemistry. The simplest way that we may represent a compound is to use its molecular formula, which is a formula indicating the number of atoms of each type of element in a molecule. For example, ethanol (drinking alcohol), a simple organic molecule, can be represented by its molecular formula, C2H6O. Understanding organic chemistry relies on understanding the structure and reactivity of organic molecules. Unfortunately, molecular formulas do not give any details on the structure of a compound.
A better way of representing organic molecules is to use the Lewis structure instead of the molecular formula. A Lewis structure is a model that represents covalent bonds and nonbonding electrons with symbols, dots, and lines. Symbols are the one- or two-letter abbreviations from the periodic table, such as H for hydrogen and O for oxygen. Dots are the way to represent lone electron pairs. For carbon, only carbanions, which are carbon ions with a negative charge, will have one set of lone electron pairs. For oxygen, divalent oxygen will have two sets of lone electron pairs, and trivalent oxygen will have one set of lone electron pairs. For nitrogen, divalent nitrogen will have one set of lone electron pairs, trivalent nitrogen will have one set of lone electron pairs, and tetravalent nitrogen will have no lone electron pairs. Lines are used to represent the single, double, and triple covalent bonds between atoms.
For example, ethanol and dimethyl ether are both constitutional isomers of each other with the molecular formula C2H6O. Constitutional isomers are compounds with the same molecular formula but different connectivity. In Lewis structures, every single atom and bond is explicitly shown, along with all lone pairs in the molecule. It is important to note that Lewis structures do not imply geometry. Lewis structures often use vertical and horizontal dashes to represent bonds. By their nature, they are drawn at 90o instead of the correct 3-D tetrahedral angles of 109.5o. A tetrahedral atom is a central atom bonded to four other atoms with no lone electron pairs and bond angles of 109.5o.Examples of a Condensed Formula, Lewis Structure, and Bond-Line Formula
Summary of Different Types of Formulas
Type of Formula | Hydrogen | Carbon | Covalent Bonds | Lone Electron Pairs |
---|---|---|---|---|
Molecular formula | Always shown | Always shown | Never shown | Never shown |
Condensed formula | Always shown | Always shown | Never shown | Never shown |
Lewis structure | Always shown | Always shown | Always shown | Always shown |
Bond-line formula | Shown only in functional groups | Almost never shown | Always shown | Usually shown |
Condensed formulas show all hydrogen and carbon but not bonds or lone pairs. Lewis structures show all hydrogen, carbon, bonds, and lone pairs. Bond-line formulas always shows functional groups and bonds and sometimes lone pairs.