Reaction Mechanisms

Interactive step-by-step mechanism visualizations with arrow pushing

OrgoChem I

Substitution, elimination, alkene reactions, and carbonyl basics

Alkenes

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃

Carbocation rearrangements possible

Anti addition, vicinal dihalide

EXAMPLE

CH₃CH=CH₂ + Br₂ → CH₃CHBrCH₂Br (anti)

Halonium ion intermediate

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Hydroboration-Oxidation

Alkenes

REAGENTS

BH₃/THF, then H₂O₂/OH⁻

RESULT

Anti-Markovnikov alcohol, syn addition

EXAMPLE

CH₃CH=CH₂ → CH₃CH₂CH₂OH (anti-Markovnikov)

Four-membered transition state, syn stereochemistry

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Hydrohalogenation with Peroxides

Alkenes

REAGENTS

HBr + ROOR (peroxides)

RESULT

Anti-Markovnikov addition

EXAMPLE

CH₃CH=CH₂ + HBr + ROOR → CH₃CH₂CH₂Br (anti-Markovnikov)

Radical mechanism, no rearrangements

Anti addition, OH to more substituted carbon

EXAMPLE

CH₃CH=CH₂ + Br₂ + H₂O → CH₃CH(OH)CH₂Br (anti)

Halonium ion intermediate, water as nucleophile

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Acid-Catalyzed Hydration

CH₃CH=CH₂ + H₂SO₄ + H₂O → CH₃CH(OH)CH₃ (Markovnikov)

Rearrangements possible

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Oxymercuration-Demercuration

Alkenes

REAGENTS

Hg(OAc)₂ + H₂O, then NaBH₄

RESULT

Markovnikov alcohol

EXAMPLE

CH₃CH=CH₂ → CH₃CH(OH)CH₃ (Markovnikov, no rearrangement)

No rearrangements

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Catalytic Hydrogenation

CH₃CH=CH₂ + H₂ → CH₃CH₂CH₃ (syn addition)

Syn addition of H₂, reduces alkene to alkane

Epoxide, syn relative to alkene

EXAMPLE

CH₃CH=CH₂ + mCPBA → Epoxide (syn)

Syn addition, forms three-membered epoxide ring

OsO₄ or cold dilute KMnO₄

RESULT

Syn diol

EXAMPLE

CH₃CH=CH₂ + OsO₄ → CH₃CH(OH)CH₂OH (syn diol)

Syn addition, forms 1,2-diol (vicinal diol)

Alkene cleavage to aldehydes/ketones

EXAMPLE

CH₃CH=CH₂ + O₃ → CH₃CHO + HCHO (cleavage)

Oxidative cleavage, breaks C=C bond

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Carbonyls

Grignard Addition to Carbonyl

Carbonyls

REAGENTS

RMgX + carbonyl, then H₃O⁺

RESULT

Alcohol (1° from aldehyde, 2° from ketone)

EXAMPLE

CH₃CHO + CH₃MgBr → CH₃CH(OH)CH₃ (2° alcohol)

Nucleophilic addition, irreversible, must be anhydrous

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Sodium Borohydride Reduction

Carbonyls

REAGENTS

NaBH₄ + carbonyl, then H₃O⁺

RESULT

Alcohol (1° from aldehyde, 2° from ketone)

EXAMPLE

CH₃COCH₃ + NaBH₄ → CH₃CH(OH)CH₃ (2° alcohol)

Mild reducing agent, works in protic solvents, selective for aldehydes/ketones

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Substitution & Elimination

SN2 Substitution

Substitution & Elimination

REAGENTS

Strong nucleophile, polar aprotic solvent

RESULT

Substitution with inversion

EXAMPLE

CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

One step, backside attack, primary/secondary best

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SN1 Substitution

Substitution & Elimination

REAGENTS

Weak nucleophile, polar protic solvent

RESULT

Substitution with racemization

EXAMPLE

(CH₃)₃CBr + H₂O → (CH₃)₃COH + HBr

Two steps, carbocation intermediate, tertiary best

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E2 Elimination

Substitution & Elimination

REAGENTS

Strong base, heat

RESULT

Alkene formation, anti-periplanar

EXAMPLE

CH₃CH₂CHBrCH₃ + OH⁻ → CH₃CH=CHCH₃ + H₂O + Br⁻

One step, concerted, Zaitsev product preferred

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E1 Elimination

Substitution & Elimination

REAGENTS

Weak base, heat, polar protic solvent

RESULT

Alkene formation, Zaitsev product

EXAMPLE

(CH₃)₃CBr + H₂O → (CH₃)₂C=CH₂ + HBr

Two steps, carbocation intermediate, rearrangements possible

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OrgoChem II

Aromatics, enolates, carboxylic acid derivatives, alcohols, ethers, and amines

Alcohols

Aldehyde (1°) or ketone (2°), carboxylic acid (1° with strong oxidant)

EXAMPLE

CH₃CH₂OH + PCC → CH₃CHO (aldehyde)

Primary → aldehyde → carboxylic acid, secondary → ketone

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Acid-Catalyzed Dehydration

Alkene, Zaitsev product

EXAMPLE

CH₃CH₂CH(OH)CH₃ + H₂SO₄ → CH₃CH=CHCH₃ + H₂O

E1 mechanism, carbocation intermediate, rearrangements possible

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Amines

Reductive Amination

Amines

REAGENTS

Carbonyl + amine + NaBH₄ or NaBH₃CN

RESULT

Amine (primary, secondary, or tertiary)

EXAMPLE

CH₃CHO + NH₃ + NaBH₄ → CH₃CH₂NH₂ (primary amine)

Imine formation, then reduction

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Aromatic Chemistry

C₆H₆ + HNO₃ + H₂SO₄ → C₆H₅NO₂ + H₂O

Electrophilic aromatic substitution, meta-directing

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Friedel-Crafts Alkylation

C₆H₆ + CH₃CH₂Cl + AlCl₃ → C₆H₅CH₂CH₃

Electrophilic aromatic substitution, carbocation rearrangements possible

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Carboxylic Acids & Derivatives

Nucleophilic Acyl Substitution

Carboxylic Acids & Derivatives

REAGENTS

Nucleophile + carboxylic acid derivative

RESULT

New carboxylic acid derivative

EXAMPLE

CH₃COCl + CH₃OH → CH₃CO₂CH₃ + HCl

Tetrahedral intermediate, reactivity: acid chloride > anhydride > ester > amide

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Enolates

Aldol Condensation

Enolates

REAGENTS

Base (OH⁻ or LDA), then heat

RESULT

α,β-Unsaturated carbonyl

EXAMPLE

2 CH₃CHO + OH⁻ → CH₃CH=CHCHO (aldol condensation)

Enolate formation, then aldol addition, then dehydration

Alkoxide base (same as ester), then H₃O⁺

RESULT

β-Keto ester

EXAMPLE

2 CH₃CO₂Et + EtO⁻ → CH₃COCH₂CO₂Et (β-keto ester)

Enolate formation, then nucleophilic acyl substitution

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Ethers & Epoxides

Williamson Ether Synthesis

Ethers & Epoxides

REAGENTS

Alkoxide + alkyl halide

RESULT

Ether

EXAMPLE

CH₃CH₂O⁻ + CH₃Br → CH₃CH₂OCH₃ + Br⁻

SN2 mechanism, primary halide best, works for symmetrical and unsymmetrical ethers

Acid or base + nucleophile

RESULT

1,2-Diol or substituted alcohol

EXAMPLE

Epoxide + H₂O → 1,2-Diol (anti addition)

Regioselectivity depends on acid vs base conditions

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