Reducing agent LiAlH4 Vs NaBH4

Which is a better Reducing Agents? Explain?

 

Introduction

we are perhaps more familiar with oxidising agent such as KMnO4 and K2Cr2O7 but yet our JC teacher or notes hardly cover the reducing reagent part.

2017 GRC A-level paper, H2, P2 have a question about NaBH4, everyone know that LiAlH4 is a stronger Reducing agent but why?

Let me slowly explain using this.

 

 

Nucleophilic addition

  • The reduction of both R.A. are actually Nucleophilic addition with H- nucleophile generated
  • LiAlH4 is a stronger RA than NaBH4 due to

    • More electronegative of B than Al which attract H+ more towards them
    • bond length of B-H shorter than Al-h
    • thus Al-H bond is weaker than B-H
    • LiAlH4 is more likely to get oxidised thus a better R.A.
    • Li+ better in stabilising  O- ion than Na+