Catalytic hydrogenation involves addition of hydrogen to a gt C = Clt or -C -= C- bond in presence of a metal catalyst such as Ni, Pt, Pd, etc. These are called heterogeneous catalysts. Both H_2 and alkene/alkyne get adsorbed on the surface of the metal , and syn-addition of hydrogen atoms takes place. The relative rates of hydrogenation follows the order: -C-=C- gt gt C=C lt gt C=o lt C_6H_6 If sodium or lithium in liquid ammonia is used, anti-addition of H-atoms occurs across -C-=C-bond. These reducing agents can also be used to reduce double bonds of benzene rings and conjugated dienes but not of isolated dienes. Which of the following alkenes will react most readily with H_2 under catalytic hydrogeneration conditions ?

Catalytic hydrogenation involves addition of hydrogen to a gt C = Clt

1.	Catalytic hydrogenation involves addition of hydrogen to a gt C = Clt or -C -= C- bond in presence of a metal catalyst such as Ni, Pt, Pd, etc. These are called heterogeneous catalysts. Both H_2 and alkene/alkyne get adsorbed on the surface of the metal , and syn-addition of hydrogen atoms takes place. The relative rates of hydrogenation follows the order: -C-=C- gt gt C=C lt gt C=o lt C_6H_6 If sodium or lithium in liquid ammonia is used, anti-addition of H-atoms occurs across -C-=C-bond. These reducing agents can also be used to reduce double bonds of benzene rings and conjugated dienes but not of isolated dienes. Which of the following alkenes will react most readily with H_2 under catalytic hydrogeneration conditions ?
Answer» Solution :During catalytic HYDROGENATION , the hydrogens are transferred from the CATALYST to the same side of the double BOND thereby giving cis-alkenes. EVIDENTLY lesser the NUMBER of R substituents, lesser is the steric hindrance and hence faster is the rate of hydrogenation. Hence , option (b) is correct.