Question

a) Explain briefly the difference between the two center electron pair bond and a three center electron pair bond. 

b) Compare NaBH₄ and LiAlH₄ as reducing agents. 

c) Which of the above two would you use in the following conversions:

i) R—CH=CH—CHO → R—CH=CH—CH₂OH

ii) R—CH₂—COOEt → R—CH₂—CH₂OH

Answer 1

a) In a two center electron pair bond, a pair of electrons binds two atoms, whereas in a three center electron pair bond a pair of electrons binds three atom together.

b) LiAlH₄ is a more versatile reducing agent than NaBH₄; it reduces as many as sixty functional groups in organic chemistry. On the other hand, NaBH₄ is more selective. For example, NaBH₄ reduces >C=O group in aldehyde and ketones to alcohols, but functional groups such as >C=C<, —COOH and —NO₂ are not normally attacked.

c) i) NaBH₄ ii) LiAlH₄