Overview
This lecture covers how to combine the four key arrow-pushing patterns in organic chemistry mechanisms, illustrating correct and incorrect electron movement for reaction mechanisms.
Review of Arrow-Pushing Patterns
- Four main patterns: proton transfer, loss of a leaving group, carbocation rearrangement, and nucleophilic attack.
- Multiple arrow patterns can occur in one reaction step.
Example: Combining Patterns in a Reaction
- Step 1: Proton transfer uses two arrows, indicating proton movement to a reactant.
- Step 2: Loss of a leaving group is shown by a single arrow with a negative sign over the reaction arrow (e.g., –H₂O).
- Step 3: Carbocation rearrangement involves shifts (methyl or hydride) to form a more stable carbocation.
- Step 4: Nucleophilic attack and loss of leaving group can occur simultaneously, depicted by two arrows (one for attack, one for group departure).
Mechanistic Arrow-Pushing: Correct and Incorrect Approaches
- Arrows should start at electron pairs or bonds and point toward electrophilic centers or leaving groups.
- Do not start arrows at inappropriate atoms or bonds—incorrect arrows may appear in multiple-choice questions.
- Electron movement should never violate the octet rule; carbon cannot expand its octet beyond eight electrons.
- Only adjacent atoms (not distant ones) should be involved in electron shifts.
Key Terms & Definitions
- Proton transfer — movement of a proton (H⁺) from one species to another.
- Leaving group — atom or group that departs with a pair of electrons in a reaction.
- Carbocation rearrangement — shift of a group to stabilize a positive carbon center.
- Nucleophilic attack — nucleophile donates electrons to an electrophilic atom, forming a bond.
- Octet rule — atoms (especially carbon) must not have more than eight electrons in their valence shell in organic mechanisms.
Action Items / Next Steps
- Practice drawing combined arrow-pushing mechanisms for given reactions.
- Review the four basic arrow-pushing patterns and their correct usage.
- Prepare for further discussion on mechanism patterns in the next lecture.