Worksheet 02 Key PDF

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Zoom Worksheet 2 – Curved Arrow Notation 1. Resonance and formal charges. Formal charge = (valence #) – (# of bonds) – (# of nonbonding electrons) a. Add formal charges to the products.

b. How can you calculate these formal charges without knowing the identities (and therefore valence #) of the generic atoms A, B, C, and D? Formal charge becomes more positive at tail of arrow Formal charge becomes more negative at head of arrow.

c. Why does the charge generally change by 1 unit if the arrow shows movement of 2 electrons? Atom gets “half credit” for electrons in bonds, “full credit” for nonbonding electrons (in lone pairs)

© Dr. MaryAnn Robak – Chem 3A Fall 2020 – Zoom Worksheet 2 – Curved Arrow Notation – page 1

2. Resonance: Common Mistakes. Only a few of structures below show a correct set of curved arrows to show generation of an alternate resonance structure. Each of the other structures has a mistake. a. Cross out any incorrect arrow, and explain why that arrow would not lead to a valid resonance structure. b. If the arrows are valid, draw the resulting resonance contributor.

© Dr. MaryAnn Robak – Chem 3A Fall 2020 – Zoom Worksheet 2 – Curved Arrow Notation – page 2

3. Minor vs. insignificant. a. Label each of the resonance contributors in the following set as major, minor, or insignificant. (Use all three labels. Two of them will have the same label as each other).

b. What ranking criterion is used to distinguish major from minor in this set? Explain. “Formal charge location(s) matching relative electronegativity is better” The “major” and “minor” contributors above all have filled octets, and all have the same number of formal charges as each other. Oxygen is more electronegative than carbon, so the contributors with negative charge on carbon are ranked minor compared to the one with negative charge on oxygen. c. What ranking criterion is used to distinguish minor from insignificant in this set? Explain. “filled octets is better” The insignificant resonance contributor has an unfilled octet on carbon, while the rest do not have unfilled octets. Note that “fewer formal charges is better” would lead to the same ranking, but octets is a more important criterion than fewer formal charges. d. Compare this example to the “Significant vs insignificant contributors” section in the lecture notes. Explain why the insignificant resonance contributor from part c above does not meet the stated goal. Scenario 2: Overall charged structure (sum of formal charges ≠ 0) Goal: Draw resonance contributors to show how that charge is distributed across multiple atoms - Comparing the insignificant contributor to the first contributor in the set, the formal negative on carbon is still in the same place and the amount of charge separation has increased, so this does not show how the net charge (-1) is distributed.

© Dr. MaryAnn Robak – Chem 3A Fall 2020 – Zoom Worksheet 2 – Curved Arrow Notation – page 3

4. Venlafaxine (Effexor) is an antidepressant medication. [adapted from Fa19 Chem 3A Quiz #3] a. Draw curved arrows and the resulting structure for a minor resonance contributor of Venlafaxine, with a positive formal charge on an oxygen.

b. State the hybridization of each indicated atom on the venlafaxine structure.

c. Add curved arrows and label the reactant roles in the reaction step below.

d. Draw a sketch of the orbital interaction that corresponds to the reaction step above. Include names of the interacting orbitals, shading, and node labels. Use an R group abbreviation for a large portion of the Venlafaxine structure (circle/label above to define the abbreviation).

© Dr. MaryAnn Robak – Chem 3A Fall 2020 – Zoom Worksheet 2 – Curved Arrow Notation – page 4

5. Super Glue Polymerization! Ethyl-2-cyanoacrylate is the active ingredient in Super Glue and Krazy Glue. It acts as an adhesive by rapidly polymerizing upon exposure to a small amount of a nucleophile such as water or hydroxide.

a. Add curved arrows and two important resonance contributors of the intermediate, showing how ethyl-2-cyanoacrylate can react with a nucleophile followed by an electrophile. (Hint: each resonance contributor should have full octets and should have a negative charge on an electronegative atom). Label one of the contributors “major”.

b. Add the missing intermediates (only the major resonance contributor of each one) and curved arrows to finish the mechanism for the reaction below. (Hint: start by adding atom numbering (1-8) to the product structure and starting material structures to correspond to the numbers provided on the last intermediate.)

© Dr. MaryAnn Robak – Chem 3A Fall 2020 – Zoom Worksheet 2 – Curved Arrow Notation – page 5...