Representing Organic Compounds

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 90^o instead of the correct 3-D tetrahedral angles of 109.5^o. A tetrahedral atom is a central atom bonded to four other atoms with no lone electron pairs and bond angles of 109.5^o. Even with small structures, such as ethanol and dimethyl ether, it becomes cumbersome to draw out all of the carbons and hydrogens explicitly. Two other ways to draw organic compounds have been developed: condensed formulas and bond-line (or skeletal) formulas. A condensed formula is a version of the Lewis structure where most or all of the bond lines are omitted and common atoms are combined in a parenthesis, such as CH₃(CH₂)₄CH₃. The condensed formula for ethanol is CH₃CH₂OH, and the condensed formula for dimethyl ether is (CH₃)₂O. The disadvantage is that although the condensed formula implies connectivity, it is not explicitly shown. An ideal way to draw organic molecules is to have the detail of the Lewis structure with the time-saving benefit of the condensed formula. The bond-line formula is the best and, by far, the most common way to draw organic molecules. A bond-line formula (or skeletal formula) shows the functional group(s) and carbon skeleton of the molecule without explicitly showing carbon (C) or hydrogen (H) (except those that are part of the functional group). A functional group is a group of atoms with specific physical, chemical, and reactivity properties. A bond-line formula is a Lewis structure minus the ${\rm {C{-}H}}$ bonds and without labels for C and H atoms, but still including nonbonding electrons. It is important to note that in bond-line formulas, carbon atoms are almost never drawn and hydrogen atoms are only drawn when they are part of functional groups. All other atoms, such as oxygen (O), nitrogen (N), fluoride (F), and so on, are always explicitly drawn. In bond-line formulas, triple bonds must be drawn in a linear fashion.