Syn and Anti Addition
Predicting Stereochemistry in Electrophilic Addition Reactions.
By chemca Team • Updated Jan 2026
Stereochemistry is crucial in addition reactions. Syn Addition involves adding two groups to the same face of the double bond. Anti Addition involves adding groups to opposite faces. The nature of the substrate (Cis/Trans) and the mode of addition determine whether the product is Meso or Racemic.
1. Syn Addition Reactions
Same Side Attack
A. Catalytic Hydrogenation:
Addition of $H_2$ on a metal surface ($Ni, Pt, Pd$) occurs from the same side.
Addition of $H_2$ on a metal surface ($Ni, Pt, Pd$) occurs from the same side.
$$ >C=C< + H_2 \xrightarrow{Pd/C} \text{Syn Alkane} $$
B. Hydroxylation (Syn-Diol Formation):
Reagents like Cold Alkaline $KMnO_4$ (Baeyer's Reagent) or Osmium Tetroxide ($OsO_4$) add two -OH groups to the same face via a cyclic ester intermediate.
Reagents like Cold Alkaline $KMnO_4$ (Baeyer's Reagent) or Osmium Tetroxide ($OsO_4$) add two -OH groups to the same face via a cyclic ester intermediate.
$$ \text{cis-2-Butene} \xrightarrow{KMnO_4, OH^-} \text{Meso-Butane-2,3-diol} $$
C. Hydroboration-Oxidation:
Adds H and OH in a Syn manner.
Adds H and OH in a Syn manner.
2. Anti Addition Reactions
Opposite Side Attack
A. Halogenation ($X_2 / CCl_4$):
Formation of a Cyclic Halonium Ion blocks one face, forcing the nucleophile ($X^-$) to attack from the back.
Formation of a Cyclic Halonium Ion blocks one face, forcing the nucleophile ($X^-$) to attack from the back.
$$ \text{cis-2-Butene} + Br_2 \rightarrow \text{Racemic (dl)-2,3-Dibromobutane} $$
B. Addition of Hypohalous Acid (HOX):
Adds OH and X in Anti manner.
Adds OH and X in Anti manner.
C. Hydroxylation via Epoxide:
Reaction with Peroxyacids ($RCO_3H$) followed by acid hydrolysis leads to Anti-Diols.
Reaction with Peroxyacids ($RCO_3H$) followed by acid hydrolysis leads to Anti-Diols.
$$ >C=C< \xrightarrow{RCO_3H} \text{Epoxide} \xrightarrow{H_3O^+} \text{Anti-Diol} $$
3. Stereochemical Mnemonics (Golden Rules)
Predicting the Product
Use these mnemonics to instantly predict whether the product is Meso or Racemic.
| Substrate | Mode of Addition | Product Stereochemistry | Mnemonic |
|---|---|---|---|
| Cis | Syn | Meso | CSM (Cis-Syn-Meso) |
| Cis | Anti | Racemic ($\pm$) | CAR (Cis-Anti-Racemic) |
| Trans | Anti | Meso | TAM (Trans-Anti-Meso) |
| Trans | Syn | Racemic ($\pm$) | TSR (Trans-Syn-Racemic) |
Note: "Meso" implies the retention of symmetry (Erythro form). "Racemic" implies formation of enantiomers (Threo form). This applies to symmetrical alkenes (e.g., But-2-ene). For unsymmetrical alkenes, "Meso" corresponds to Erythro (groups on same side in Fischer) and "Racemic" to Threo (groups on opposite sides).
4. Applied Examples
1. Maleic Acid (Cis) + $Br_2$ (Anti):
Rule: CAR (Cis + Anti = Racemic).
Product: $(\pm)$-2,3-Dibromosuccinic acid.
Rule: CAR (Cis + Anti = Racemic).
Product: $(\pm)$-2,3-Dibromosuccinic acid.
2. Fumaric Acid (Trans) + $Br_2$ (Anti):
Rule: TAM (Trans + Anti = Meso).
Product: Meso-2,3-Dibromosuccinic acid.
Rule: TAM (Trans + Anti = Meso).
Product: Meso-2,3-Dibromosuccinic acid.
3. Cis-But-2-ene + Cold KMnO4 (Syn):
Rule: CSM (Cis + Syn = Meso).
Product: Meso-Butane-2,3-diol.
Rule: CSM (Cis + Syn = Meso).
Product: Meso-Butane-2,3-diol.
Knowledge Check
Test your Stereochemistry skills
No comments:
Post a Comment