vogel_practical_ochem_3.pdf

11] MISCELLANEOUS REACTIONS 881
solution. Separate the ether layer, extract the aqueous layer with two
200 ml. portions of ether, and dry the combined ethereal solutions with
anhydrous magnesium sulphate. Distil off the ether and dry the residue
in a vacuum desiccator. The resulting o-aminobenzyl alcohol has m.p. 82°
and weighs 12-0 g.
VI,11. REDUCTIONS WITH POTASSIUM (OR SODIUM)
BOROHYDRIDE
Potassium and sodium borohydride show greater selectivity in action than
lithium aluminium hydride : thus ketones or aldehydes may be reduced to
alcohols whilst the cyano, nitro, amido and carbalkoxy groups remain unaffected.
Furthermore, the reagent may be used in aqueous or aqueous-alcoholic solution.
One simple application of its use will be described, viz., the reduction of
m-nitrobenzaldehyde to m-nitrobenzyl alcohol :
N02
N02
CHO ^ x &
CH2OH
m-Nitrobenzyl alcohol.* Clamp a 500 ml. three-necked flask,
equipped with a mechanical stirrer, a thermometer and a burette, above the
bench so that an ice bath can be placed beneath it. Place a solution of
15-1 g. of m-nitrobenzaldehyde (1) in 100 ml. of methanol in the flask and,
whilst stirring, add a solution of potassium borohydride (2*0 g. of KBH4
in 2 ml. of 2N sodium hydroxide diluted with 18 ml. of water) at the rate
of 0*5 ml. per minute, with occasional cooling to keep the reaction at
18-25°. When about three-quarters of the solution has been added,
there is no further tendency for the temperature to rise, and the addition
is stopped. Treat a small portion of the reaction mixture with dilute
sulphuric acid : hydrogen should be evolved.
Remove most of the methanol by distillation on a steam bath, and
dilute the residue with 100 ml. of water. Extract the mixture with ether,
wash the upper layer with water, and dry it rapidly with a little anhydrous
magnesium sulphate. Remove the ether by " flash distillation ", and
distil the residual pale yellow oil under diminished pressure. Collect the
m-nitrobenzyl alcohol at 183-185°/17 mm. ; it solidifies to a pale yellow
solid, m.p. 30°, when cooled in ice. The yield is 13 g.
Note.
(1) m-Nitrobenzaldehyde may be prepared as follows. Place 250 ml. of concentrated
sulphuric acid and 21*5 ml. of fuming nitric acid, sp. gr. 1 • 5, in a 500 ml.
three-necked flask fitted with a mechanical stirrer (unsealed) and a dropping funnel.
Stir and cool to 0° in a bath of ice and salt. Add 62 • 5 g. (60 ml.) of purified benzaldehyde
dropwise from the dropping funnel; do not allow the temperature to rise
above 5°. Then warm the mixture gradually to 40°, cool to room temperature, and
pour in a thin stream with vigorous stirring on to finely crushed ice. Filter through
a sintered glass funnel, wash with a little water, press out the oil with a wide glass
stopper, and dry in the air upon absorbent paper. The resulting crude m-nitrobenzaldehyde
weighs 55 g. and melts at 48-50°. Melt the crude solid under excess
of 10 per cent, sodium carbonate solution, stir, cool, filter and dry in the air : the
* The experimental details were kindly provided by the Chemical Research Laboratories
of May and Baker Ltd., Dagenham.