Hydrogenation of Alkenes
Reaction Type: Electrophilic Addition
Summary
- Alkenes can be reduced to alkanes with H2 in the presence of metal catalysts such as Pt, Pd, Ni or Rh.
- The two new C-H σ bonds are formed simultaneously from H atoms absorbed into the metal surface.
- The reaction is stereospecific giving only the syn addition product.
- This reaction forms the basis of experimental "heats of hydrogenation" which can be used to establish the stability of isomeric alkenes.
Related reactions
CATALYTIC HYDROGENATION
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Step 1:
Hydrogen gets absorbed onto the metal surface.
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Step 2:
Alkene approaches the H atoms absorbed on the metal surface.
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Step 3:
C=C reacts with the H atoms on the surface forming the two new C-H σ bonds.
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© Dr. Ian Hunt, Department of Chemistry, University of Calgary
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Chapter 9 : Alkynes
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Hydrogenation of Alkynes
Reaction Type: Electrophilic Addition
Summary
- Alkynes can be partially reduced to cis-alkenes with H2 in the presence of poisoned catalysts.
(eg. Pd / CaCO3 / quinoline which is also known as Lindlar's catalyst)
- Alkynes can be reduced to alkanes with H2 in the presence of catalysts (Pt, Pd, Ni etc.)
- The new C-H σbonds are formed simultaneously from H atoms absorbed onto the metal surface.
- The reaction is stereospecific giving only the syn addition product.
- The "poisoned" catalyst prevents over-reduction, which would give the alkane by reducing the alkene
Related reactions
CATALYTIC HYDROGENATION
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Step 1:
Hydrogen gets absorbed onto the metal surface.
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Step 2:
Alkyne approaches the H atoms absorbed on the metal surface.
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Step 3:
C≡C reacts with the H atoms on the surface forming the two new C-H σ bonds generating the alkene.
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Dissolving Metal Reduction of Alkynes
Reaction Type: Addition
Summary
- Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
- This reaction is stereospecific giving only the trans-alkene via an anti addition.
- Note that the stereochemistry of this reaction complements that of catalytic hydrogenation (syn)
- The reaction proceeds via single electron transfer from the Na with "H" coming from the NH3
- These reaction conditions do not reduce alkenes, so the reaction stops at the alkene.
Related reactions:
MECHANISM FOR THE REDUCTION OF ALKYNES WITH Na / NH3
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Step 1:
Sodium transfers an electron to the alkyne giving a radical anion.
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Step 2:
The radical anion removes a proton from the ammonia in an acid / base reaction.
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Step 3:
A second atom of sodium transfers another electron to the alkyne giving a vinly carbanion. The stereochemistry is established at this step - the alkyl groups being trans.
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Step 4:
The carbanion removes a proton from the ammonia in an acid / base reaction to give the trans-alkene.
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www.chem.ucalgary.ca /courses/351/cary/ch06;http://pages.towson.edu/laden/orgrxs/alkene/alkenerx.htm
Moderation Team
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