What are the key features of an SN2 reaction?	Concerted, bimolecular (rate = k[sub][nuc]). Backside attack causes Walden inversion. Favored by strong nucleophiles, polar aprotic solvents, and methyl/1° substrates.
What are the key features of an SN1 reaction?	Two-step, unimolecular (rate = k[sub]). Carbocation intermediate forms, then nucleophile attacks, giving racemization. Favored by 3° substrates, weak nucleophiles, polar protic solvents.
Why do polar aprotic solvents favor SN2?	They dissolve salts but don't hydrogen-bond to the nucleophile, leaving it unsolvated and more reactive.
Why do polar protic solvents favor SN1?	They stabilize the carbocation intermediate and leaving group anion through hydrogen bonding, lowering the ionization activation energy.
What is the rate-determining step of SN1?	Ionization of the substrate to form a carbocation and a leaving group (unimolecular bond cleavage without nucleophilic assistance).
What are the key features of an E2 reaction?	Concerted, bimolecular (rate = k[sub][base]). Requires antiperiplanar geometry (180° dihedral). Favored by strong, bulky bases like tert-butoxide.
What are the key features of an E1 reaction?	Two-step, unimolecular (rate = k[sub]). Carbocation forms, then base removes a proton. Favored by 3° substrates, weak bases, polar protic solvents, and heat. Competes with SN1.
How do you predict SN1 vs. SN2 vs. E1 vs. E2?	Consider substrate (1° favors SN2/E2, 3° favors SN1/E1), nucleophile/base strength and size (strong small nuc = SN2, strong bulky base = E2, weak = SN1/E1), solvent (aprotic = SN2, protic = SN1/E1), and temperature (heat favors elimination).
What is Zaitsev's rule?	The major elimination product is the more substituted (more stable) alkene. Applies to most E1 and E2 reactions with small bases.
When does E2 give a Hofmann product?	When a bulky base (e.g., KOtBu) is used. Steric hindrance forces removal of the less hindered proton, giving the less substituted alkene.
What is the mechanism of HBr addition to an alkene?	Pi electrons attack H of HBr, forming the more stable carbocation (Markovnikov). Br⁻ then attacks the carbocation to give the alkyl bromide.
What is Markovnikov's rule?	In HX addition to an unsymmetrical alkene, H adds to the less substituted carbon and X to the more substituted, via the more stable carbocation.
When does anti-Markovnikov addition occur?	In hydroboration-oxidation (OH to less substituted carbon) and radical HBr addition with peroxides (Br to less substituted carbon).
What are the three stages of free radical halogenation?	Initiation: heat/light homolyzes X₂ into radicals. Propagation: X· abstracts H to form carbon radical, which reacts with X₂. Termination: two radicals combine.
Why is bromination more selective than chlorination?	Br· is less reactive with a late, product-like transition state (Hammond's postulate), so it strongly reflects radical stability. Cl· has an early transition state and is less discriminating.
What is nucleophilic addition to a carbonyl?	A nucleophile attacks the electrophilic carbonyl carbon, breaking the pi bond to form an alkoxide. Occurs with aldehydes/ketones (no leaving group). Common nucleophiles: hydride, Grignard, cyanide.
How does nucleophilic acyl substitution differ from nucleophilic addition?	Acyl substitution occurs at carbonyls with a leaving group (acid chlorides, anhydrides, esters, amides): nucleophile attacks, tetrahedral intermediate forms, then the leaving group departs. In addition (aldehydes/ketones), no leaving group exists so the tetrahedral product is retained.
What is the mechanism of the aldol reaction?	Base forms an enolate, which attacks another carbonyl (nucleophilic addition) to give a beta-hydroxy carbonyl. Heating causes dehydration to an alpha,beta-unsaturated carbonyl.
What is electrophilic aromatic substitution (EAS)?	An electrophile replaces H on an aromatic ring. Pi electrons attack the electrophile forming an arenium ion, then a proton is lost to restore aromaticity.
What role does a Lewis acid catalyst play in bromination of benzene?	It activates Br₂ by coordinating with it to generate Br⁺ character, making it electrophilic enough to attack the aromatic ring.
What is a carbocation rearrangement?	A 1,2-hydride or methyl shift converts a less stable carbocation to a more stable one (e.g., 2° to 3°). Occurs in any mechanism with a carbocation intermediate (SN1, E1), not in concerted ones (SN2, E2).
What is the Diels-Alder reaction?	[4+2] cycloaddition: a conjugated diene and a dienophile form a six-membered ring. Concerted, stereospecific (syn addition), and regioselective.
What is the difference between syn and anti addition?	Syn: both groups add to the same face (e.g., hydroboration, catalytic hydrogenation). Anti: groups add to opposite faces (e.g., halogenation via halonium ion, acid-catalyzed epoxide opening).
What product forms when a Grignard reagent reacts with a ketone?	The Grignard (R–MgX) adds to the carbonyl via nucleophilic addition. Acid workup gives a tertiary alcohol (secondary from aldehydes, primary from formaldehyde).
What does the acid catalyst do in Fischer esterification?	It protonates the carbonyl oxygen, making the carbon more electrophilic for alcohol attack. The reaction is reversible; excess alcohol or water removal drives it forward.