Question

1. Write a mechanism for the reduction of vanillin using sodium borohydride. 2. Compare and contrast sodium borohydride with another reducing agent, lithium borohydride.

OH 1. NaBHa, EtOH 2. H3O OCH3 IOCH OH OCH3 OH

Answer 1

The dative bond between H-
The dative bond between H- rightarrow BH3 is stronger than that between H- rightarrow AlH3, as Boron is smaller sized and strongly holds the donated electron pair by the H- due to its high effective nuclear charge. As stronger dative bond is broken less readily so less readily H- rightarrow BH3 gives out H- for reduction hence, weaker reducing agent Most important is the co-ordinate bond between central atom (Al or B) and H^- hydride ion. Al/B have an empty orbital to which H^- donates an electron pair. Due to smaller size, the e^- - pair is more tightly held by the (nuclear charges) in Boran so, it loses h^- less readily than Al. so it is a weaker reducing agent. However Li^+ is a hard Lewis acid having higher +ve charge than Na^+ that stabilizes AlH_4^-
BH3 is stronger than that between H-
The dative bond between H- rightarrow BH3 is stronger than that between H- rightarrow AlH3, as Boron is smaller sized and strongly holds the donated electron pair by the H- due to its high effective nuclear charge. As stronger dative bond is broken less readily so less readily H- rightarrow BH3 gives out H- for reduction hence, weaker reducing agent Most important is the co-ordinate bond between central atom (Al or B) and H^- hydride ion. Al/B have an empty orbital to which H^- donates an electron pair. Due to smaller size, the e^- - pair is more tightly held by the (nuclear charges) in Boran so, it loses h^- less readily than Al. so it is a weaker reducing agent. However Li^+ is a hard Lewis acid having higher +ve charge than Na^+ that stabilizes AlH_4^-
AlH3, as Boron is smaller sized and strongly holds the donated electron pair by the H- due to its high effective nuclear charge.As stronger dative bond is broken less readily so less readily H- $\rightarrow$ BH3 gives out H- for reduction hence,weaker reducing agent