How Can Alkynes Be Reduced to Alkenes and Alkanes

Propose a synthesis for each alkyne starting with acetylene and any necessary organic and inorganic reagents. 

In the previous section, we saw how terminal alkynes can be used to form C—C bonds and synthesize larger alkynes. In this section, we will learn how alkynes can be reduced to alkanes and alkenes. Because of the rich number of reactions available to alkenes, we can now use these two reactions in tandem to synthesize a large variety of compounds  

Treatment of an alkyne with Hg in the presence of a transition metal catalyst, most commonly Pd, Pt, or Ni, results in the addition of two moles of H2 to the alkyne and its conversion to an alkane. Catalytic reduction of an alkyne can be brought about at or slightly above 

Reduction of an alkyne occurs in two stages first, addition of one mole of H2 to form an al-kene and then addition of the second mole of H2 to the alkene to form the alkane. In most cases, it is not possible to stop the reaction at the alkene stage. However, by careful choice of catalyst, it is possible to stop the reaction at the addition of one mole of hydrogen. The catalyst most commonly used for this purpose consists of finely powdered palladium metal deposited on solid calcium carbonate that has been specially modified with lead salts. This combination is known as the Lindlar catalyst. Reduction (hydrogenation) of alkynes over a Lindlar catalyst is stereoselective syn addition of two hydrogen atoms to the carbon—carbon triple bond gives a cis alkene  

Because addition of hydrogen in the presence of the Lindlar catalyst is stereoselective for syn addition, it has been proposed that reduction proceeds by simultaneous or nearly simultaneous transfer of two hydrogen atoms from the surface of the metal catalyst to the alkyne. Earlier we presented a similar mechanism for the catalytic reduction of an alkene to an alkane (Section 5.6). 

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