A solution of
20.5 g. (0.1 mole) of α-acetaminocinnamic acid (p. 1) in
150 cc. of glacial acetic acid (Note
1) is placed in the bottle of a
Burgess-Parr reduction apparatus,
0.5 g. of platinum oxide catalyst
(Org. Syn. Coll. Vol. I, 1941, 463) is added, and the mixture is shaken in an atmosphere of
hydrogen under an initial pressure of 40 lb. per sq. in. until the calculated amount of gas is taken up; usually about two hours is required (Note
2) and (Note
3). When the reduction is complete, the catalyst is removed by suction filtration and washed with a little water. The combined filtrate and washings are evaporated to dryness under diminished pressure on a
water bath.
The crystalline residue (Note
4) is taken up in
400 cc. of 1 N hydrochloric acid, transferred to a 1-l. flask fitted with a reflux condenser, and boiled for ten hours (Note
5). The resulting solution is evaporated to dryness under diminished pressure on the water bath; to the residue 100 cc. of water is added slowly through a dropping funnel at about the same rate as that at which it distils, in order to remove the excess
hydrochloric acid as completely as possible. The residue is then taken up in 30–40 cc. of boiling water, and the
pH of the solution is adjusted until it is basic to
Congo red, but still acid to litmus, by careful addition of concentrated
ammonia and
acetic acid (Note
6). Then two volumes of
95 per cent alcohol is added to aid in the separation of the
phenylalanine. The mixture is placed in the refrigerator for a day, after which the product is transferred to a
Büchner funnel, and washed first with three 25-cc. portions of ice-cold water and then with alcohol. The yield is
14.5 g. The filtrate is evaporated to dryness under reduced pressure on the water bath, and the residue is extracted with about 70 cc. of ice-cold water in three or four portions. The insoluble material, after washing with
95 per cent alcohol, is added to the main fraction of
phenylalanine. The total yield is
16 g. (Note
7).
The combined fractions weighing about
16 g. are dissolved in a minimum amount of boiling water (Note
8), two volumes of
95 per cent alcohol is added, and the flask is placed in a refrigerator overnight to complete crystallization. The
phenylalanine is transferred to a Büchner funnel, washed with several small portions of ice-cold water, and finally with alcohol. The yield is
10.5–11 g. By concentrating the filtrate and washings further,
3–3.5 g. of product can be obtained conveniently. The total yield is
14–14.3 g. (
85–86 per cent of the theoretical amount) of analytically pure
phenylalanine.