Drawing Organic Compounds

Representing Organic Compounds

Lewis structures, condensed formulas, and molecular formulas are three ways of representing compounds that are used extensively in chemistry. In organic chemistry, bond-line formulas are the most commonly used structures because they allow for all the information of a Lewis structure or any other structural formula but in a simpler, easier-to-draw manner.

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.
Every carbon and hydrogen atom and any other atoms are shown explicitly in the Lewis structures of ethanol and dimethyl ether.
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.
A condensed formula is an easier way to write a compound's formula, but it does not show the bonds. A Lewis structure shows the bonds between elements but can be time-consuming to draw.
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 CH{\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.
Bond-line formulas do not show carbon and hydrogen unless they are part of a functional group. In ethanol, hydrogen is part of a functional group, so it is displayed in its bond-line formula. Dimethyl ether does not have hydrogen and carbon as part of a functional group, so they are omitted from the bond-line formula.

Examples of a Condensed Formula, Lewis Structure, and Bond-Line Formula

A condensed formula is converted into a Lewis structure by including all bond-lines between atoms. A Lewis structure is converted to a bond-line formula by removing the bond lines between carbon and hydrogen and removing the carbon and hydrogen labels except where necessary. Only when carbon or hydrogen is part of a functional group will that carbon or hydrogen be shown explicitly in the 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.