When organic chemists think about how molecules undergo change during a reaction, they commonly find it helpful to write down a scheme that includes Lewis structures. Write the Lewis structure for acetic acid, CH₃COOH, which is found in vinegar.

The formula of acetic acid suggests that the molecule consists of a CH₃— group and a —COOH group. In the —COOH group, both O atoms are attached to the same C atom, and one of them is bonded to the final H atom. The two C atoms are bonded to each other.

ANTICIPATE You should anticipate that the CH₃— group, by analogy with methane, will consist of a C atom joined to three H atoms by single bonds.

PLAN Apply the procedure in Toolbox 2B.1.

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Toolbox 2B.1 HOW TO WRITE THE LEWIS STRUCTURE OF A POLYATOMIC SPECIES CONCEPTUAL BASIS A valid Lewis structure accommodates all the valence electrons to give each atom, if possible, an octet (or duplet). PROCEDURE Step 1 Count the number of valence electrons on each atom; for ions, adjust the number of electrons to account for the charge. Divide the total number of valence electrons in the molecule by 2 to obtain the number of electron pairs. Step 2 Write down the most likely arrangements of atoms by using common patterns and the clues indicated in the text. Step 3 Place one electron pair between each pair of bonded atoms. Step 4 Complete the octet (or duplet, in the case of H) of each atom by placing any remaining electron pairs around the atoms. If there are not enough electron pairs, form multiple bonds in place of one or more single bonds. Step 5 Represent each bonding electron pair by a line. To check on the validity of a Lewis structure, verify that each atom has an octet or (for hydrogen) a duplet. As explained in Topic 2C, a common exception to this rule arises when the central atom is an atom of an element in Period 3 or later. Such an atom can accommodate more than eight electrons in its valence shell. Consequently, the lowest energy Lewis struc- ture may be one in which the central atom has more than eight electrons. This procedure is illustrated in Examples 2B.1 and 2B.2.