Submitted by Chessie E. Rehberg
Checked by H. R. Snyder and Fred E. Boettner.
1. Procedure
Six hundred and seventy-five grams (6 moles) of 80% lactic acid (Note
1),
300 ml. of benzene, and
5 ml. of concentrated sulfuric acid are placed in a
3-l. three-necked round-bottomed flask having a
capillary ebullition tube in one neck. The flask is attached to a
Vigreux column 75 cm. in length, having at its top a
trap of the Dean and Stark type, preferably the Barrett modification carrying a
stopcock (Note
2), and a
reflux condenser. The mixture is refluxed, and the aqueous layer is withdrawn from the trap until liberation of water becomes slow or until noticeable darkening of the material begins (Note
3). Then
1394 g. (1635 ml. or 24 moles) of allyl alcohol is added (Note
4), and refluxing and removal of water are continued until production of water ceases (Note
5). The acid catalyst is then neutralized with anhydrous
sodium acetate (18 g.) (Note
6), and the fractionating column is replaced by a short distillation head leading to a
3-l. round-bottomed receiving flask cooled in salt and ice. The liquid is distilled rapidly under diminished pressure, first at the
water pump and finally at a pressure of about 5 mm. A viscous residue (about 130 g.) containing inorganic salts is discarded. The distillate is fractionated through the column used in the first steps. The trap at the reflux condenser is replaced with a still head arranged for controlled partial take-off. The first fraction is the
benzene-allyl alcohol binary azeotrope, which distils at
76.8° and contains 17.4% of the alcohol. Frequently a small amount of
allyl ether is obtained at
94–95°.
Allyl alcohol is then distilled at
97° until most of it is removed and the temperature of the residue in the pot reaches about 120°. The pressure is reduced to about 50 mm. while the remainder of the alcohol is removed. Finally the
allyl lactate is distilled at
56–60°/8 mm. (Note
7). It amounts to
693–710 g. (
89–91%) (Note
8).
2. Notes
1. The
lactic acid used was the commercial edible grade and was almost colorless. The checkers used
635.6 g. of 85% lactic acid,
U.S.P. The lower grades of commercial acid probably would give somewhat lower yields.
2. A suitable piece of apparatus is listed in Catalog LP-34,
p. 43, of the Corning Glass Works.
3. Generally, about 35 ml. of water per mole of
80% lactic acid is obtained at this stage, and 4–6 hours of refluxing is required for its removal. The linear
lactic acid polymer thus produced contains approximately 3
lactic acid units.
4. A large excess of alcohol is essential for a high yield of
ester. Thus, when the ratio of alcohol to acid is 4:1, the yield is
90%; when it is 3:1, the yield is
85–88%; when it is 2:1, the yield is about
65%; and when it is 5:3, the yield is
58–60%.
5. The checkers found this distillation to require about 16 hours. The distillate is the ternary azeotrope. It consists of 8.6% water,
1 9.2% allyl alcohol, and 82.2%
benzene, and boils at 68.2°. The aqueous layer contains some
allyl alcohol, though this loss is insignificant, since only about 15 ml. of the aqueous layer is obtained from each mole of
lactic acid used.
6. It is essential to neutralize any strong acid present before distilling
lactic ester; otherwise, condensation by ester interchange occurs, with liberation of alcohol and production of "polylactic acid," a linear polyester. Other neutralizing agents, such as alkali or alkaline-earth hydroxides or carbonates, doubtless could be used satisfactorily instead of
sodium acetate.
7.
Allyl lactate is a clear, colorless, mobile liquid boiling at
60°/8 mm.,
79°/25 mm., and
175–176°/740 mm. Other properties are
d420 1.0452;
nD20 1.4369.
3. Discussion
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