Unit 5: An Overview of Mechanism and Reactivity
The handful of moves that run every mechanism in the course: what a curved arrow really shows, and why some products win on speed while others win on stability.
5 topics · 18 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Curved Arrows: What They Actually Show
- How it worksHow does a curved arrow's tail mark where electrons start and its head mark where they end up?
- Common mistakeWhy doesn't a curved arrow ever show an atom physically moving from one place to another?
- Common mistakeWhy does a curved arrow always start on electrons, a lone pair or a bond, and never on a bare atom?
Your turn1 of 2
A curved arrow in a mechanism is drawn from a lone pair on a nucleophile to the carbon it attacks. What does this arrow actually represent?Nucleophiles and Electrophiles
- How it worksHow does a nucleophile's lone pair attack an electrophile's empty or weakly held orbital to form a new bond?
- Worked exampleFind the nucleophile and the electrophile in the reaction between hydroxide and bromomethane, CH3Br.
- Common mistakeWhy isn't a strong base always a strong nucleophile?
- Going furtherWhat happens to a species' nucleophilicity when steric bulk crowds the atom that's supposed to attack?
Your turn1 of 2
In the reaction between hydroxide, OH-, and bromomethane, CH3Br, which species is the nucleophile and which is the electrophile?Thermodynamics vs Kinetics of a Reaction
- How it worksHow does a lower activation energy make a pathway kinetically favored even when its product isn't the most stable?
- Worked exampleFind which product wins at low temperature when the kinetic product sits 2 kcal/mol above the thermodynamic one.
- Common mistakeWhy isn't the most stable product the one that always forms in a reaction?
- Going furtherWhat happens to a product ratio when the kinetic product that formed first gets heated after the fact?
Your turn1 of 2
A reaction can form a kinetic product that sits 2 kcal/mol higher in energy than a thermodynamic product, but the kinetic pathway has a lower activation energy. Which product dominates at low temperature, where the reaction cannot reverse?Reaction Coordinate Diagrams and Transition States
- How it worksHow does a reaction coordinate diagram turn a multi-step mechanism into one continuous curve of rising and falling energy?
- Worked exampleFind the overall energy change for a reaction where the intermediate sits 5 kcal/mol above the start and the product sits 10 below it.
- Common mistakeWhy isn't the peak of a reaction coordinate diagram a real, isolable molecule?
- Ties togetherHow does the same reaction-coordinate diagram drawn here reappear to explain every mechanism later in the course?
Your turn1 of 2
In a multi-step reaction, an intermediate sits 11 kcal/mol above the starting material, and the final product sits 10 kcal/mol below the starting material. What is the overall energy change, delta G, for the reaction from start to product, in kcal/mol?Hammond Postulate and Reactive Intermediates
- How it worksHow does the Hammond postulate let a stable carbocation's structure predict an unstable transition state's shape?
- Worked exampleFind whether the transition state of a highly endothermic step resembles the starting material or the product more.
- Going furtherWhat happens to a reaction's rate when a more stable intermediate, like a tertiary carbocation, becomes available?
Your turn1 of 2
A reaction step is highly endothermic, so its transition state sits much closer in energy to the product than to the starting material. According to the Hammond postulate, does this transition state more closely resemble the starting material or the product structurally?