Sabatier-Senderens Reaction: Catalytic Hydrogenation
The Sabatier-Senderens Reaction is the catalytic hydrogenation of unsaturated hydrocarbons (alkenes or alkynes) to saturated hydrocarbons (alkanes) using Nickel ($Ni$) as a catalyst at elevated temperatures ($200-300^\circ C$).
1. General Reaction
Hydrogen gas adds across the pi-bonds of alkenes or alkynes.
Conditions:
- Reactants: Alkene/Alkyne + Hydrogen gas ($H_2$).
- Catalyst: Finely divided Nickel (often Raney Nickel).
- Temperature: High temperature is required ($200-300^\circ C$) for Nickel.
- Note: If Platinum ($Pt$) or Palladium ($Pd$) is used, the reaction occurs at room temperature (known simply as Catalytic Hydrogenation).
2. Detailed Mechanism
The reaction follows a Heterogeneous Catalysis mechanism involving surface adsorption.
Step 1: Adsorption
Hydrogen molecules ($H_2$) adsorb onto the metal surface, breaking the H-H bond and forming metal-hydrogen bonds. The alkene also adsorbs onto the surface.
Step 2: Hydrogen Transfer
One hydrogen atom transfers from the surface to one of the alkene carbons, forming a partially hydrogenated intermediate bonded to the surface.
Step 3: Final Addition & Desorption
The second hydrogen atom transfers to the other carbon. The saturated alkane product, having weak affinity for the catalyst, desorbs from the surface.
3. Stereochemistry: Syn-Addition
Syn-Addition Rule
Since both hydrogen atoms are transferred from the same face of the metal catalyst surface, the addition is stereospecifically Syn-Addition (cis-addition).
Example: Hydrogenation of 1,2-Dimethylcyclohexene yields cis-1,2-Dimethylcyclohexane.
4. Examples
A. Ethene to Ethane
$$ CH_2=CH_2 + H_2 \xrightarrow{Ni, \ \Delta} CH_3-CH_3 $$B. Propyne to Propane
Alkynes are reduced all the way to alkanes (unless poisoned catalysts are used).
$$ CH_3-C \equiv CH + 2H_2 \xrightarrow{Ni, \ \Delta} CH_3-CH_2-CH_3 $$5. Industrial Application: Hardening of Oils
This reaction is widely used in the food industry to convert liquid vegetable oils (polyunsaturated fatty acids) into semi-solid fats (saturated fats) like Vanaspati ghee or margarine.
Sabatier-Senderens Quiz
Test your concepts on Hydrogenation. 10 MCQs with explanations.
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