Consider the acetic acid molecule: H3C-COOH. Draw the Lewis Dot structure. What is the electron pair geometry around each non-hydrogen atom? A. C B. C C. O D. O Pairs, single bond:
The Correct Answer and Explanation is:
Here’s the correct answer for each atom’s electron pair geometry:
- A. Carbon (Ca): Tetrahedral
- B. Carbon (Cb): Trigonal planar
- C. Oxygen (Oa, double bonded): Trigonal planar
- D. Oxygen (Ob, single bonded): Tetrahedral
Now for the explanation:
Acetic acid, H₃C–COOH, consists of a methyl group (CH₃) connected to a carboxylic acid group (COOH). To predict the electron pair geometry, we use the Valence Shell Electron Pair Repulsion (VSEPR) theory, which considers both bonding and lone pairs around each atom.
The first carbon atom, Ca, is part of the methyl group. It forms four single bonds: three with hydrogen atoms and one with the adjacent carbon (Cb). This gives Ca four regions of electron density arranged as far apart as possible, resulting in a tetrahedral geometry.
The second carbon, Cb, is bonded to Ca with a single bond, to a double-bonded oxygen (Oa), and to a single-bonded hydroxyl oxygen (Ob). These three bonding regions—regardless of whether a bond is single or double—create a trigonal planar arrangement.
The oxygen labeled Oa is double bonded to Cb and holds two lone pairs. With one bond and two lone pairs, it also has three regions of electron density. This corresponds to a trigonal planar geometry around Oa.
The other oxygen atom, Ob, is single bonded to both Cb and a hydrogen atom, and it has two lone pairs. This makes four regions of electron density, leading to a tetrahedral electron pair geometry. However, because two positions are occupied by lone pairs, the visible molecular shape is bent.
These spatial arrangements explain molecular shape and predict how the molecule will interact in physical and chemical environments, especially through dipole moments and hydrogen bonding. Understanding these geometries is essential for grasping reactivity and polarity in organic compounds like acetic acid.
