Reaction Mechanisms Flashcards

Concerted, bimolecular (rate = k[sub][nuc]). Backside attack causes Walden inversion. Favored by strong nucleophiles, polar aprotic solvents, and methyl/1° substrates.

Two-step, unimolecular (rate = k[sub]). Carbocation intermediate forms, then nucleophile attacks, giving racemization. Favored by 3° substrates, weak nucleophiles, polar protic solvents.

They dissolve salts but don't hydrogen-bond to the nucleophile, leaving it unsolvated and more reactive.

They stabilize the carbocation intermediate and leaving group anion through hydrogen bonding, lowering the ionization activation energy.

Ionization of the substrate to form a carbocation and a leaving group (unimolecular bond cleavage without nucleophilic assistance).

Concerted, bimolecular (rate = k[sub][base]). Requires antiperiplanar geometry (180° dihedral). Favored by strong, bulky bases like tert-butoxide.

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.

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).

The major elimination product is the more substituted (more stable) alkene. Applies to most E1 and E2 reactions with small bases.

When a bulky base (e.g., KOtBu) is used. Steric hindrance forces removal of the less hindered proton, giving the less substituted alkene.

Pi electrons attack H of HBr, forming the more stable carbocation (Markovnikov). Br⁻ then attacks the carbocation to give the alkyl bromide.

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.

In hydroboration-oxidation (OH to less substituted carbon) and radical HBr addition with peroxides (Br to less substituted carbon).

Initiation: heat/light homolyzes X₂ into radicals. Propagation: X· abstracts H to form carbon radical, which reacts with X₂. Termination: two radicals combine.

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.

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.

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.

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.

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.

It activates Br₂ by coordinating with it to generate Br⁺ character, making it electrophilic enough to attack the aromatic ring.

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).

[4+2] cycloaddition: a conjugated diene and a dienophile form a six-membered ring. Concerted, stereospecific (syn addition), and regioselective.

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).

The Grignard (R–MgX) adds to the carbonyl via nucleophilic addition. Acid workup gives a tertiary alcohol (secondary from aldehydes, primary from formaldehyde).

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.