College · CHEM 220
Organic Reactions & Mechanisms (CHEM 220) — study guide & practice questions
Substitution, elimination and addition, alcohol/ether/alkyne chemistry, and multistep synthesis.
Study guide
Structure & Stereochemistry Foundations
- Hybridization (sp³/sp²/sp) sets bond angles and geometry; a carbon with four different groups is a stereocenter.
- R/S labels come from CIP priority rules; diastereomers differ from enantiomers by not being mirror images, and meso compounds are achiral despite having stereocenters.
Nucleophiles, Electrophiles & Reaction Formalism
- Curved arrows track electron pairs moving from a source to a sink; good leaving groups are weak bases (conjugate bases of strong acids).
- Protonating a poor leaving group like OH⁻ converts it into a good one (H₂O) before a mechanism can proceed.
The SN2 Mechanism in Depth
- One concerted step, backside attack, inversion of configuration (Walden inversion); rate = k[substrate][nucleophile].
- Favored by methyl/primary substrates (steric access) and strong nucleophiles in polar aprotic solvents.
The SN1 Mechanism in Depth
- Two steps through a carbocation intermediate; rate = k[substrate] only; the planar cation gives racemization and can rearrange.
- Favored by tertiary (most stable cation) substrates in polar protic solvents.
E2 Elimination in Depth
- One concerted step with a strong base; requires anti-periplanar geometry; usually gives the Zaitsev (more substituted) alkene unless the base is bulky (Hofmann).
- Stereospecific: a given starting stereoisomer predictably yields a specific alkene geometry.
E1 Elimination & Substitution vs. Elimination
- E1 goes through the same carbocation as SN1, so it competes with SN1 under the same conditions, with no strict geometric requirement.
- Substrate class, nucleophile/base strength and bulk, solvent, and temperature together decide which of the four mechanisms wins.
Electrophilic Addition to Alkenes
- HBr without peroxides adds Markovnikov (H to the carbon with more H's) via the more stable carbocation; X₂ addition goes through a halonium ion, giving anti stereochemistry.
- Acid-catalyzed hydration is Markovnikov-selective through the same carbocation logic as HX addition.
Hydroboration-Oxidation & Radical Addition
- Hydroboration–oxidation gives anti-Markovnikov, syn addition with no carbocation rearrangement; oxidation retains configuration.
- HBr with peroxides adds anti-Markovnikov via a radical mechanism — the one hydrogen halide where this pathway applies.
Course modules
8 foundation modules free to read here; 1 advanced module with Pro. Every module ends with a quiz in the app.
1. Structure & Stereochemistry Foundations Free
The 3D structure and stereochemical vocabulary every mechanism argument depends on.
Read the 4 lessons →2. Nucleophiles, Electrophiles & Reaction Formalism Free
The vocabulary and arrow-pushing notation used to describe every mechanism that follows.
Read the 4 lessons →3. The SN2 Mechanism in Depth Free
The concerted, backside-attack substitution pathway: kinetics, sterics, stereochemistry, and solvent.
Read the 4 lessons →4. The SN1 Mechanism in Depth Free
The stepwise, carbocation-based substitution pathway: rate, stability, racemization, and rearrangement.
Read the 4 lessons →5. E2 Elimination in Depth Free
The concerted elimination pathway: geometry requirements, product selectivity, and stereospecificity.
Read the 4 lessons →6. E1 Elimination & Substitution vs. Elimination Free
The stepwise elimination pathway, and how to predict which of the four mechanisms actually wins.
Read the 4 lessons →7. Electrophilic Addition to Alkenes Free
How alkenes react with HX, halogens, and water — and why the regiochemistry and stereochemistry differ.
Read the 4 lessons →8. Hydroboration-Oxidation & Radical Addition Free
The two ways chemists flip the regiochemistry of alkene addition: a concerted mechanism and a radical one.
Read the 4 lessons →9. Alcohols, Ethers, Epoxides & Alkynes Pro · Orgo I · Functional Groups
Turning alcohols into leaving groups and carbonyls, the Williamson ether synthesis and epoxide opening, alkyne chemistry, and planning multistep syntheses.
Sample practice questions
From module 1, Structure & Stereochemistry Foundations. Pick your answers, then check them in the app, with an explanation for every question.
1. What is true about enantiomers' physical properties, other than the direction they rotate polarized light?
2. In CIP priority rules, what determines which group has higher priority?
Sample flashcards
4 of the cards in this course's deck. Tap a card to flip it.
SubstitutionDescribe the SN2 mechanism, its kinetics and its stereochemical outcome.
One concerted step with backside attack; rate = k[substrate][nucleophile] (second order); inversion of configuration. Favored by methyl/primary substrates, strong nucleophiles and polar aprotic solvents.
SubstitutionDescribe the SN1 mechanism, its kinetics and its stereochemical outcome.
Two steps: the leaving group departs to give a carbocation, then the nucleophile attacks. Rate = k[substrate] (first order); racemization. Favored by tertiary substrates and polar protic solvents.
EliminationWhat are the requirements and outcome of an E2 elimination?
One concerted step with a strong base; the H and the leaving group must be anti-periplanar. Usually gives the more substituted (Zaitsev) alkene, but a bulky base favors the less substituted (Hofmann) product.
EliminationState Zaitsev's rule.
In an elimination, the major product is the more substituted (more stable) alkene.
The full course is in LectureMate's Course Library: every module quiz with explanations, the complete flashcard deck with spaced repetition, and a 6-step strategy for Organic Reactions & Mechanisms questions. Foundation modules are free; advanced modules need Pro.