08 mechanism steps

Arrow convention for polar steps

Curved arrow (full head) from electron source (lone pair or bond) to sink

Arrow convention for radicals

Fishhook (half-headed) arrows for single electrons

Hydrohalogenation step 1

Pi bond attacks H of H-X; X- leaves; carbocation forms

Hydrohalogenation step 2

Check for 1,2-hydride/methyl shift to a more stable carbocation

Hydrohalogenation step 3

X- attacks the carbocation to form the alkyl halide

HBr/ROOR initiation

RO-OR homolysis (heat/light) to 2 RO•; RO• + HBr gives ROH + Br•

HBr/ROOR propagation 1

Br• adds to the less substituted carbon, forming the more stable radical

HBr/ROOR propagation 2

Carbon radical abstracts H from HBr, giving the product and a new Br•

HBr/ROOR termination

Any two radicals couple (Br•+Br•, R•+Br•, R•+R•)

Halogenation step 1

Pi bond attacks X2, forming a bridged halonium ion plus X-

Halogenation step 2

X- attacks the halonium from the back side (anti addition)

Halohydrin step 2

H2O attacks the halonium at the more substituted carbon

Halohydrin step 3

Water deprotonates the oxonium to give the halohydrin

Haloether formation

Same as halohydrin but ROH is the nucleophile; solvent deprotonates

Intramolecular ether formation (e.g., pent-4-en-1-ol + Br2)

The tethered OH attacks the halonium to make a cyclic ether (5-membered ring favored)

Acid-catalyzed alkene hydration step 1

Pi bond protonated by H3O+; carbocation forms (rearrange if needed)

Acid-catalyzed alkene hydration step 2

H2O attacks the carbocation, forming an oxonium ion

Acid-catalyzed alkene hydration step 3

Water deprotonates the oxonium to give the alcohol

Alkyne hydration (HgSO4) step 1

Alkyne attacks Hg2+ to form a mercury-bridged vinyl intermediate

Alkyne hydration step 2

H2O attacks the more substituted carbon, then deprotonation

Alkyne hydration step 3

Proton replaces Hg, giving the enol

Alkyne hydration step 4

Enol tautomerizes to the ketone

Acid-catalyzed enol tautomerization step 1

C=C pi bond attacks H3O+ at the beta carbon, giving an oxocarbenium ion (C=O+-H)

Acid-catalyzed enol tautomerization step 2

Water removes the O-H proton to give C=O

Base-catalyzed tautomerization

Base deprotonates O-H to give the enolate, then the alpha carbon is protonated

Na/NH3 reduction step 1

Na donates one e-, forming the radical anion

Na/NH3 reduction step 2

NH3 protonates, forming the vinyl radical

Na/NH3 reduction step 3

Second e- gives the vinyl anion (trans geometry favored)

Na/NH3 reduction step 4

NH3 protonates the vinyl anion, giving the trans alkene

Radical polymerization initiation

Peroxide homolysis to RO•; RO• adds to the CH2 end of the monomer, forming a carbon radical

Radical polymerization propagation

The growing radical adds to another monomer; repeat to extend the chain

Radical polymerization termination

Two chains couple, or one abstracts H from another (disproportionation)

Which carbon does the initiator radical attack in styrene or vinyl chloride?

The unsubstituted CH2, forming the more stable radical

Free-radical halogenation initiation

X2 homolysis with heat or light to 2 X•

Free-radical halogenation propagation

X• abstracts H from R-H giving R•; R• abstracts X from X2 giving R-X and X•

Hydroboration mechanism summary

Concerted syn addition of B-H across C=C; then oxidation with H2O2/NaOH replaces B with OH with retention

Epoxidation mechanism summary

Concerted butterfly transition state; peracid transfers O to the alkene

Common mechanism-drawing errors

Missing lone pairs, missing formal charges, arrows starting at atoms instead of electrons, skipping intermediates