chem231 substitution and elimination

methyl alkyl halide + any nuc + any base

SN2

primary alkyl halide

SN2

primary alkyl halide + good nuc + bulky base

E2

secondary alkyl halide + poor nuc

SN1

secondary alkyl halide + poor nuc + heat

E1

secondary alkyl halide + weak base + good nuc

SN2

secondary alkyl halide + strong base + good nuc

E2

tertiary alkyl halide + poor nuc

SN1

tertiary alkyl halide + poor nuc + heat

E1

tertiary alkyl halide + weak base + good nuc

SN1

tertiary alkyl halide + weak base + good nuc + heat

E1

tertiary alkyl halide + moderate base (pKa conj acid 1-10) + good nuc

E2

tertiary alkyl halide + strong base + good nuc

E2

good leaving groups

conjugate acid <= 0; Cl-, Br-, I-

methyl and primary carbocations will not do…

SN1 and E1 because carbocations are unstable

tertiary alkyl halides will not do…

SN2 because they are sterically hindered

good nucleophile

negatively charged nucleophile; electron-rich; usually an anion from a dissociated ionic bond; favors SN2

poor nucleophile

neutral nucleophile; usually in a covalent bond (doesn’t dissociate); favors SN1

weak base

pKa of conjugate acid not >= 11; favors E1

strong base

pKa of conjugate acid >= 11; favors E2

moderate bases

exception to rule; pKa of conjugate acid 1<pKa<10; CN-, N3-

zaitsev product

product that contains more substituted alkene (alkene attached to more C)

hofmann product

product that contains less substituted alkene (alkene attached to less C)

E2 special considerations

H and leaving group must be anti-periplanar

SN2 vs E1 or SN1 vs E2

choose bimolecular (__2)

SN1 vs E1

heat (Δ, reflux, 100°C) prefers E1; no heat prefers SN1

SN2 vs E2

methyl or primary alkyl halide is SN2; secondary or tertiary alkyl halide is E2; primary alkyl halide + bulky base is E2

bulky base E2

creates Hofmann product