Lecture Notes: Addition Reactions

Overview

Addition reactions involve two groups being added to adjacent carbons with the pi bond being removed.
Opposite of elimination reactions, where two groups are removed from adjacent carbons and replaced by a pi bond.
Result: sp2 carbons become sp3 carbons.

Hydrogenation: Addition of hydrogen atoms to the carbons in a double bond.
Catalyst: Typically involves a transition metal catalyst, often platinum.
Mechanism:
- Two hydrogen atoms are added to the two carbons in the double bond.
- Involves heterogeneous catalysis (catalyst is in a different phase than the substrate).
- The substrate approaches the catalyst surface, and hydrogen atoms are added to the substrate from the same side.

Addition reactions can be stereospecific, meaning the spatial orientation of the added groups matters.
Syn Addition: Groups are added to the same side of the substrate.
- Example: Hydrogenation over platinum metal is a syn addition.
- The added hydrogen atoms are on the same side, leading to implied hydrogens on wedge bonds and methyl groups on dashes.
Anti Addition: Groups are added to opposite sides of the substrate (not covered in detail in this lecture).

Syn and Anti: Describe the mechanism of operation of a reaction (context of the reaction).
Cis and Trans: Describe the spatial relationship between groups on a molecule (independent of reaction context).
- Syn addition usually results in groups being cis to each other in the product.

Addition reactions convert sp2 carbons to sp3 carbons.
Hydrogenation is an example of a stereospecific addition reaction (syn addition).
Distinction between syn/anti and cis/trans is crucial for understanding reaction mechanisms and product spatial orientation.

Addition reactions are important in organic chemistry for modifying molecular structures.
Stereospecificity plays a key role in determining the final product configuration.

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