A LITTLE LIGHTBULB MOMENT
Alkenes, alkynes and addition reactions
Catalytic hydrogenation adds both hydrogens to the same face
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The alkene lies on the metal surface and gets same-face hydrogens.
The idea behind the cue
Catalytic hydrogenation is syn addition because both hydrogens are delivered from the catalyst surface to the same side of the pi bond. This is a key stereochemical feature of metal-catalyzed addition to alkenes and alkynes.
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Short & simple
The alkene lies on the metal surface and gets same-face hydrogens.
Acronym
SS — Sleepy Sloths.
Story
Two hydrogen atoms arrive on a metal table at once. The alkene is pressed against the surface, so both new bonds form from the same side like butter being spread on one face of toast.
Connect the pieces
- S (Sleepy) → Syn addition: Both atoms are delivered to the same face of the pi bond.
- S (Sloths) → Surface delivery: In metal-catalyzed hydrogenation, both hydrogens come from the catalyst surface.
- Sleepy · Syn addition: Both atoms are delivered to the same face of the pi bond.
- Sloths · Surface delivery: In metal-catalyzed hydrogenation, both hydrogens come from the catalyst surface.
Know the limits
The syn pattern describes the addition step; stereochemistry may be unimportant if the product is achiral or fully saturated.
Further reading · OpenStax Organic Chemistry ↗Check your understanding
Catalytic hydrogenation of an alkene is best described as which type of addition?
A. Carbocation rearrangement
B. Syn addition
C. Anti addition
D. Markovnikov addition
Reveal explanation
Syn addition
Both hydrogens are delivered from the catalyst surface to the same face of the double bond, so the addition is syn.