E-StoreReduction mechanims in Organic
The Mechanism of Reduction Reactions
Two fundamentally different reducing agents have been used to add hydrogen across a double bond. A metal can be used to catalyze the reaction between hydrogen gas and the C=C double bond in an alkene.
A source of the hydride (H-) ion, on the other hand, is used to reduce C=O double bonds.
The difference between these reactions is easy to understand. The first reaction uses a nonpolar reagent to reduce a nonpolar double bond. The atoms on the surface of a metal are different from those buried in the body of the solid because they cannot satisfy their tendency to form strong metal-metal bonds. Some metals can satisfy a portion of their combining power by binding hydrogen atoms and/or alkenes to the surface.
Adding one of the hydrogen atoms to the alkene forms an alkyl group, which can bond to the metal until the second hydrogen atom can be added to form the alkene.
Although the hydrogen atoms are transferred one at a time, this reaction is fast enough that both of these atoms usually end up on the same side of the C=C double bond. This can't be seen in most alkanes produced by this reaction because of the free rotation around C
C bonds. Reduction of a cycloalkene, however, gives a stereoselective product.
Reduction of an alkyne with hydrogen on a metal catalyst gives the corresponding alkane. By selectively "poisoning" the catalyst it is possible to reduce an alkyne to an alkene. Once again, the reaction is stereoselective, adding both hydrogen atoms from the same side of the C
C bond to form the cis-alkene.
Because it is a polar reagent, LiAlH4 won't react with a C=C double bond. It acts as a source of the H- ion, however, which is a strong Brønsted base and a strong nucleophile. The H- ion can therefore attack the + end of a polar C=O double bond.
The neutral AlH3 molecule formed when an AlH4- ion acts as a hydride donor is a Lewis acid that coordinates to the negatively charged oxygen atom in the product of this reaction. When, in a second step, a protic solvent is added to the reaction, an alcohol is formed.
SOURCE - Scribd
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