SolutionsReduction Reactions
1. Diphenylacetylene
Predict the result of the following reactions:
The Lindlar catalyst adds both hydrogens to the same face (syn addition, cis alkene); sodium in liquid ammonia transfers the electrons one at a time and gives the more stable trans alkene.
2. Synthesis of a pheromone
How would you prepare (Z)-tricos-9-ene, CH3(CH2)7CH=CH(CH2)12CH3, the sex pheromone of the housefly, from an alkyne?
By hydrogenation of tricos-9-yne, CH3(CH2)7C≡C(CH2)12CH3, with H2 and Lindlar catalyst, which gives the cis (Z) alkene.
3. Why does it stop?
Why does hydrogenation with the Lindlar catalyst stop at the alkene and not go on to the alkane?
The catalyst is “poisoned” (palladium on CaCO3 with lead acetate and quinoline): it keeps enough activity to hydrogenate the triple bond, which is adsorbed more strongly on the metal, but not to hydrogenate the double bond.