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Chapter 7
Organic Chemistry- Exam 3- Fall 2026
| Question | Answer |
|---|---|
| Why do alkyl halides undergo substitution and elimination? | The C–X bond is polarized: the α-carbon is partially positive, and the halide can leave |
| What is an alkyl halide? | A compound in which an alkyl group is bonded to a halogen: F, Cl, Br, or I |
| What is the α-carbon? | The carbon directly bonded to the halogen; substitution occurs at this carbon. |
| What is a β-carbon? | A carbon adjacent to the α-carbon; a β-hydrogen can be removed during elimination |
| What makes a good leaving group? | A stable species after departure; good leaving groups are generally conjugate bases of strong acids. |
| What are the two broad substitution mechanisms? | SN2 is concerted; SN1 is stepwise and forms a carbocation. |
| What happens in a concerted SN2 reaction? | The nucleophile forms a bond as the leaving group departs, in one step. |
| What happens in a stepwise SN1 reaction? | The leaving group departs first, forming a carbocation; the nucleophile attacks afterward. |
| What is the stereochemical outcome of SN2? | Inversion of configuration at the reacting carbon due to backside attack. |
| Why does an SN2 nucleophile attack from the back side? | Front-side approach is hindered by the leaving group and its bonding electron density. |
| Which alkyl halides are generally best for SN2? | Methyl and primary substrates; secondary can react more slowly, while tertiary is too hindered. |
| Why do tertiary alkyl halides not undergo SN2 effectively? | Steric hindrance blocks the nucleophile's backside approach. |
| How does branching near the α-carbon affect SN2? | More branching at α or β carbons hinders backside attack and slows the reaction |
| What is the SN2 rate law? | Rate = k[alkyl halide][nucleophile]; both substrate and nucleophile affect rate. |
| What kind of nucleophile is generally needed for SN2? | A strong nucleophile. |
| What general features can make a good nucleophile? | Negative charge and polarizability often increase nucleophilicity; solvent also matters. |
| What solvent type favors SN2? | Polar aprotic solvents. |
| Why do polar protic solvents slow many anionic SN2 reactions? | Hydrogen bonding solvates the anion, making its electron pair less available to attack. |
| What is the halide nucleophilicity order in protic solvents? | I-> Br- > Cl-> F- |
| What is the halide nucleophilicity order in polar aprotic solvents? | F- > Cl- > Br- > I- |
| What is an E2 reaction? | A concerted elimination: a base removes a β-hydrogen as the leaving group departs and a C=C forms |
| What type of reagent favors E2? | A strong base |
| What is the E2 rate law? | Rate = k[substrate][base]; both substrate and base participate in the rate-determining event. |
| What bonds change in an E2 transition state? | The C–H and C–leaving-group bonds break while the C=C π bond forms |
| What geometric requirement does E2 have? | The β-hydrogen and leaving group must be coplanar, preferably anti-coplanar |
| Why is anti-coplanar E2 geometry preferred? | It can be reached in a staggered, lower-energy conformation |
| What does E2 regioselectivity mean? | More than one constitutional alkene product is possible, and one is favored |
| What is the Zaitsev product? | Usually the more substituted, more stable alkene |
| What is the Hofmann product? | Usually the less substituted alkene, favored by a bulky base |
| How does base size affect E2 product distribution? | Bulky bases favor Hofmann; smaller, unhindered bases usually favor Zaitsev. |
| What is E2 stereoselectivity? | One stereoisomer forms preferentially over another |
| What is E2 stereospecificity? | The substrate's geometry determines which alkene stereoisomer can form |
| Why might only one E/Z alkene form in E2? | Only a β-hydrogen anti-coplanar to the leaving group can be eliminated in the required geometr |
| What is required for E2 on a cyclohexane ring? | A trans-diaxial arrangement: the leaving group and β-hydrogen must both be axial and anti |
| What are SN1 and E1? | Unimolecular substitution and elimination reactions that proceed through a carbocation |
| What is the rate-determining step in SN1 and E1? | Ionization: leaving-group departure to form the carbocation |
| What are the rate laws for SN1 and E1? | Rate = k[substrate]; the nucleophile or base is not in the rate law |
| What solvent favors SN1 and E1? | Polar protic solvent, which stabilizes ions and carbocation intermediates |
| What substrate features favor SN1/E1? | Stable carbocations: typically tertiary, allylic, or benzylic substrates |
| Why can allylic and benzylic carbocations be stable? | Their positive charge is resonance-stabilized |
| What can happen to a carbocation before SN1/E1 product formation? | A 1,2-hydride or 1,2-methide shift may rearrange it to a more stable carbocation |
| What is solvolysis? | A reaction in which the solvent acts as the nucleophile, such as ethanol in an SN1 reaction |
| What happens after a neutral alcohol attacks in SN1? | A proton transfer occurs to give the neutral substitution product |
| What is the major alkene trend in E1? | The most stable alkene, generally the more substituted one, is the major product |
| Is E1 stereospecific? | No. A carbocation can lose different β-hydrogens, giving possible alkene stereoisomers; the more stable is favored |
| How does SN1 stereochemistry differ from SN2? | SN1 attack can occur from either face of a planar carbocation; SN2 gives inversion by backside attack. |
| What are OMs, OTs, and OTf? | Mesylate, tosylate, and triflate—excellent sulfonate leaving groups |
| How does acid make an alcohol's OH group a better leaving group? | Protonation turns –OH into H2O, which can depart as a stable neutral molecule |
| According to the slides, what happens when alcohols react with HBr? | 1° alcohols react by SN2; 2° and 3° alcohols react by SN1. |
| According to the slides, what happens with concentrated H2SO4 and heat? | 2° and 3° alcohols undergo E1; the slides list E2 for 1° alcohols |
| What is retrosynthetic analysis? | Working backward from a target molecule to choose starting materials and reactions |
| What is a common way to make an ether in synthesis? | An SN2 reaction between a suitable alkyl halide and an alkoxide |
| What three steps help predict alkyl-halide products? | Classify the reagent; analyze substrate structure; apply regioselectivity and stereochemistry rules. |
| What does a strong nucleophile and strong base potentially promote? | SN2 and E2; substrate structure and steric hindrance help determine which dominates. |
| What should be checked when predicting SN1 products? | Draw the carbocation, check for rearrangement, then add the nucleophile; consider both configurations if chiral. |
| What should be checked when predicting E2 products? | Identify eligible β-hydrogens, check anti-coplanar geometry, then compare alkene regio- and stereoisomers |
Created by:
KristenR2025