The acetate ion, C2H3O2-, has both oxygens bonded to the same carbon. Draw the Lewis structure and all resonance forms. H3C-C=O | O- Label the hybridization around each carbon: Pick one resonance structure and label the hybridization of each oxygen. How many pi bonds are present? Which atom carries the formal negative charge?
The Correct Answer and Explanation is:
Answers:
a) Lewis structure and resonance forms:
The acetate ion (C2H3O2−)(\text{C}_2\text{H}_3\text{O}_2^-)(C2H3O2−) has the following two resonance structures:
Structure 1:
mathematicaCopyEdit H H
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H - C - C (=O) - O⁻
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O
Structure 2:
mathematicaCopyEdit H H
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H - C - C (-O⁻) = O
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O
Each structure shows a double bond between carbon and one oxygen, and a single bond between carbon and the negatively charged oxygen. The negative charge is delocalized between the two oxygen atoms through resonance.
b) Hybridization around each carbon:
- Methyl carbon (CH₃): sp³ hybridized (single bonds to three hydrogens and one carbon).
- Carbon in the carboxyl group (COO⁻): sp² hybridized (one double bond and two single bonds in a trigonal planar arrangement).
c) Hybridization of each oxygen (using one resonance structure):
- Double bonded oxygen: sp² hybridized (one sigma bond, one pi bond, and two lone pairs).
- Single bonded oxygen (with the negative charge): sp³ hybridized (one sigma bond and three lone pairs).
d) Number of sigma and pi bonds:
Using one resonance structure:
- Sigma (σ) bonds: 7 total
- 3 C–H
- 1 C–C
- 1 C=O (only one sigma in a double bond)
- 1 C–O (single bond)
- 1 C–O (in COO⁻)
- Pi (π) bonds: 1 (from one C=O double bond)
e) Atom carrying the formal negative charge:
- The single bonded oxygen (in each resonance structure) carries the formal negative charge. The charge is delocalized through resonance between both oxygens, meaning neither oxygen permanently holds the charge.
Explanation
The acetate ion is a polyatomic anion formed from acetic acid by removing one proton from the hydroxyl group. Its molecular formula is C₂H₃O₂⁻. In its structure, both oxygen atoms are bonded to the same carbon, forming the carboxylate group.
To construct the Lewis structure, begin by placing the carbon-carbon bond. The methyl carbon (CH₃) is single bonded to the central carbon, which is part of the carboxyl group. This central carbon forms one double bond with one oxygen and one single bond with another oxygen. The single bonded oxygen holds a negative formal charge. Two resonance forms exist because the position of the double bond can shift between the two oxygen atoms. These resonance forms stabilize the acetate ion by delocalizing the negative charge over two electronegative oxygen atoms.
The hybridization of atoms helps explain molecular geometry and electron distribution. The methyl carbon is sp³ hybridized because it forms four single bonds. The carboxyl carbon is sp² hybridized due to three regions of electron density around it (one double bond and two single bonds). In one resonance form, the double bonded oxygen is sp² hybridized (involved in a sigma and a pi bond), while the single bonded oxygen is sp³ hybridized, having three lone pairs and one sigma bond.
The molecule has seven sigma bonds and one pi bond in any given resonance form. The pi bond accounts for the carbon-oxygen double bond. The single bonded oxygen carries the formal negative charge, although resonance allows for that charge to be spread across both oxygens. This delocalization contributes to the overall stability of the acetate ion.
