Unit 6: Alkyl Halides and Nucleophilic Substitution
One stereocenter, two fates: SN2 inverts it like an umbrella catching the wind, SN1 flattens it into a carbocation and erases it entirely.
8 topics · 31 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.
The SN2 Mechanism and Backside Attack
- How it worksHow does a nucleophile's backside attack push a stereocenter through an inversion, like an umbrella flipping in the wind?
- Worked exampleFind the rate change for an SN2 reaction when the nucleophile concentration is doubled and the substrate stays fixed.
- Common mistakeWhy doesn't an SN2 reaction ever pass through a separate, detectable intermediate between reactant and product?
- Going furtherWhat happens to an SN2 reaction's rate when the substrate carbon gets more crowded with bulky groups?
Your turn1 of 2
An SN2 reaction follows the rate law rate = k[Nucleophile][Substrate]. If the nucleophile's concentration is multiplied by 3.94 and the substrate's concentration is multiplied by 3.77 at the same time, by what overall factor does the reaction rate change?The SN1 Mechanism and Carbocation Formation
- How it worksHow does the leaving group depart on its own first, leaving a flat carbocation before the nucleophile ever attacks?
- Worked exampleFind the rate law for an SN1 reaction and predict what happens when only the nucleophile concentration is doubled.
- Common mistakeWhy doesn't doubling the nucleophile concentration speed up an SN1 reaction the way it would an SN2 reaction?
- Going furtherWhat happens to an SN1 reaction's rate when the solvent switches from a nonpolar one to a polar protic one?
Your turn1 of 2
An SN1 reaction follows the rate law rate = k[Substrate], independent of nucleophile concentration. If the substrate's concentration is multiplied by 4.63 while the nucleophile's concentration is separately multiplied by 4.42 (which has no kinetic effect), by what overall factor does the reaction rate change?Stereochemistry of SN2: Inversion of Configuration
- How it worksHow does the nucleophile pushing in from the back flip the other three groups inside out like an umbrella in the wind?
- Worked exampleFind the configuration of the product when (R)-2-bromobutane reacts with hydroxide in an SN2 reaction.
- Ties togetherHow does this single inversion event let a chemist use SN2 reactions to deliberately control a product's stereochemistry?
Your turn1 of 2
When (R)-2-bromobutane reacts with hydroxide in an SN2 reaction, what is the configuration of the 2-butanol product?Stereochemistry of SN1: Racemization
- How it worksHow does a flat carbocation intermediate let the nucleophile attack from either face with equal ease?
- Worked exampleFind the expected optical purity of the product when pure (S)-3-bromo-3-methylhexane undergoes a full SN1 reaction.
- Common mistakeWhy doesn't an SN1 reaction on a single enantiomer give back that same single enantiomer as its product?
- Ties togetherHow does watching SN2 invert a stereocenter while SN1 racemizes it prove the two mechanisms can't share one pathway?
Your turn1 of 2
In an SN1 reaction on a single pure enantiomer, incomplete planarization of the carbocation from ion pairing with the departing leaving group means the nucleophile attacks from the face giving inversion 81 percent of the time and from the face giving retention the rest of the time. What is the resulting product's enantiomeric excess?Carbocation Stability and Rearrangements
- How it worksHow does a hydride or methyl group migrate over to turn a less stable carbocation into a more stable one mid-reaction?
- Worked exampleFind the rearranged product when 3-bromo-2-methylbutane ionizes and a hydride shift gives a more stable carbocation.
- Common mistakeWhy doesn't every SN1 or E1 reaction give the simple, unrearranged product the starting material seems to promise?
- Going furtherWhat happens to a reaction's product mix when a secondary carbocation can rearrange into a nearby tertiary one?
Your turn1 of 2
3-bromo-2-methylbutane ionizes to form a secondary carbocation at C3. A hydride shifts from the adjacent C2 to C3. What is the result of this shift?Effect of Substrate Structure on SN1 vs SN2
- How it worksHow does steric crowding around a tertiary carbon block the backside approach an SN2 reaction requires?
- How it worksHow does that same tertiary carbon's stable carbocation make an SN1 pathway easy once the leaving group departs?
- Worked exampleFind which mechanism, SN1 or SN2, dominates when tert-butyl bromide reacts with water.
- Common mistakeWhy doesn't a primary substrate ever go through SN1, even though a primary carbocation would relieve steric strain?
- Ties togetherHow does substrate structure alone let you predict SN1 versus SN2 before even looking at the nucleophile or solvent?
Your turn1 of 2
Tert-butyl bromide reacts with water, a weak nucleophile. Which mechanism dominates, SN1 or SN2?Nucleophile Strength and Solvent Effects
- How it worksHow does a polar protic solvent cage a nucleophile in hydrogen bonds and weaken its attack in an SN2 reaction?
- Worked exampleFind which solvent, polar protic ethanol or polar aprotic DMSO, speeds up an SN2 reaction with iodide the most.
- Common mistakeWhy isn't a bigger, bulkier nucleophile automatically a weaker one?
- Going furtherWhat happens to a nucleophile's strength moving down a column of the periodic table, from fluoride to iodide?
Your turn1 of 2
Which solvent speeds up an SN2 reaction with iodide the most: polar protic ethanol, or polar aprotic DMSO?Leaving Group Ability
- How it worksHow does a leaving group's ability to stabilize the negative charge it carries away decide how easily it departs?
- Worked exampleFind which leaving group departs fastest among fluoride, chloride, bromide, and iodide on the same substrate.
- Going furtherWhat happens to a poor leaving group like hydroxide once it gets protonated or converted into a tosylate first?
Your turn1 of 2
Among fluoride, chloride, bromide, and iodide as leaving groups on the same substrate, which one departs fastest?