8/11/2019 NABH4 reduction
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Chem 216 S11 Notes - Dr. Masato Koreeda
Date: May 10, 2011
Topic: _Experiment 4__ page 1 of 3.
Experiment 4: NaBH4Reduction of an Unknown Aromatic Ketone[See:
Eges book, Section 14.4, pp 545-549]
Two commonly used hydride-based reducing agents:
Na+ B-H
H
H
H- mild reducing agent- relatively stable reagent (against
moisture, air)
electronegativity values: H 2.1; B 2.0; Al 1.5
|!e.n.| for B-H: 0.1|!e.n.| for Al-H: 0.6
1. Sodium borohydride (NaBH4)
Li+ Al-H
H
H
H
- powerful reducing agent
- reactsviolentlywith water, ROH to produce H2 gas
- Reactions with LiAhH are usually carried out in a polar
aprotic solvent such as anhydrous tatrahydrofuran
(THF) and anhydrous (diethyl) ether (CH3CH2OCH2CH3)
2. Lithium aluminum hydride (LiAlH4)
more polarized, more on the H a stronger H donor
O
O
THF (diethyl) ether
In addition, the size differences between B and H and Al and H
should make the dissociation of H- more effective for the
Al-Hbond.
In addition, the difference in the coordination power of Na+and
Li+(stronger) on the carbonyl oxygen
further contributes to make the reactivity of
LiAlH4stronger.
Reduction with LiAlH4requires an aqueous (usually acidic)
workup.
Carboxylates/esters vs ketones/aldehydes
ease of reduction
CR O
O
carboxylate
CR O
O
ester
R' CR
O
ketone
CR H
O
aldehyde
NaBH4reduces
LiAlH4reduces
(carboxylic acid)*
*
CR O
O
H
AlH
H
H
H
CR O
O
Al
H
H
HLi
Li
+ H2H3O
+-workupRCH2OH
C=O C more electrophilic
R'
