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UIN: RESONANCE
At the end of this activity, I will be able to…
- determine whether a given molecule or functional group is stabilized through resonance;
- use the curved arrow formalism to depict the relationship between resonance structures; and
- draw reasonable resonance forms of a given molecule, including formal charge and non-bonding electrons. WARM-UP review skeletal structures and formal charge ❖ Add formal charges directly to each structure. All non-bonding electrons are provided. ❖ Provide the molecular formula of each structure. By convention, molecular formulas should be written carbon (C), hydrogen (H), then all other elements in alphabetical order of their chemical symbols, e.g., C 2 H 5 NO 2. All formal charges are provided but no non-bonding electrons are shown.
UIN: RESONANCE
WORKOUT
identify, draw, and interpret valid resonance forms Resonance structures (or forms) are used when a single structure cannot accurately capture the bonding and electron distribution of a molecule. By representing delocalized electrons across multiple atoms, resonance helps us explain the added stability observed in many molecules, often referred to as resonance stabilization. This stabilization arises because the electron density is spread out over a larger area, lowering the overall energy of the molecule. Resonance structures are simply different depictions of the same molecule. They are not distinct, isolable forms, nor are they “snapshots” of the molecule frozen in time. Think of them like the color green: It’s not blue, it’s not yellow, it’s a mix of the two. In the same way, the true structure of a resonance-stabilized molecule lies somewhere between the individual forms we draw. A proper depiction of resonance includes both the individual structures and the curved arrows that show the relationship between the two structures. The arrows aren’t optional—they’re what connect the forms and explain how each one contributes to the overall picture. Notice that curved arrows are only drawn on the structure to the left of the resonance arrow; because we read from left to right, curved arrows are used describe how the structure on the left relates to the one on the right. This approach to mastering resonance was adapted from Organic Chemistry as a Second Language by David Klein. To identify molecules that may have resonance forms, look for an unbroken chain of (at least two) atoms, each of which has a π bond, non-bonding electrons, and/or a positive formal charge. (We’ll start with lone-pair electrons for now, but the same rules apply to radicals , unpaired non-bonding electrons.) EXERCISE 1. Each of the molecules below has at least one valid resonance form. Circle the unbroken chain of atoms that meets the criteria above.
UIN: RESONANCE
EXERCISE 2. One resonance form and arrow(s) are shown for each of the following molecules. Draw the resonance form described by the arrows. An example is provided below. Don’t forget to account for formal charges! ❖ Some functional groups are stabilized by resonance—a characteristic that plays a major role in its reactivity. Circle the functional groups below that have at least one valid resonance form. Note: The squiggle at a chain end simply means that the rest of the molecule doesn’t matter in our current analysis. carboxylic acid hydroxyl amine ketone iminium amide ester imine carboxylate ether
UIN: RESONANCE
In a previous worksheet, we introduced “R” as a placeholder for a variable group. It can be explicitly defined (e.g., R = CH 3 , CH 2 CH 3 ) or left undefined to indicate that a variety of groups could fit in that position. The squiggle line is more like a “zoom in”—it signals that we’re only interested in the part of the molecule that’s drawn out; everything else can be ignored. EXERCISE 3. Provide one new resonance structure for each of the following compounds. For full points, you must include all lone-pair electrons, all formal charges, and all curved arrows depicting the relationship between the original structure and your new structure.
UIN: RESONANCE
Identify the hybridization (top box) and geometry (bottom box) of the indicated atoms.
