Organic Syntheses, CV 3, 146
Submitted by Chessie E. Rehberg
Checked by H. R. Snyder and Fred E. Boettner.
1. Procedure
In a
2-l. two-necked round-bottomed flask having a capillary ebullator tube in one neck (Note
1) are placed
371 g. (5 moles) of n-butyl alcohol,
861 g. (10 moles) of methyl acrylate,
20 g. of hydroquinone, and
10 g. of p-toluenesulfonic acid (Note
2). The flask is attached to an
all-glass fractionating column, preferably one without packing such as the Vigreux type (Note
3), and the solution is heated to boiling in an
oil bath. The column is operated under total reflux until the temperature of the vapors at the still head falls to 62–63°, which is the boiling point of the
methanol-
methyl acrylate azeotrope (Note
4). This azeotrope is then distilled as rapidly as it is formed, the temperature at the still head not being allowed to exceed 65°. When the production of
methanol has become very slow (6–10 hours), the excess
methyl acrylate is distilled, and the
butyl acrylate is then distilled, preferably at 10–20 mm. It boils at
39°/10 mm.,
84–86°/101–102 mm., and at about
145° at atmospheric pressure. The yield is
500–600 g. (
78–94%) (Note
5).
2. Notes
1. The capillary is used to introduce a gas to prevent bumping and superheating during the vacuum distillation of the product. As air has some tendency to catalyze polymerization of the acrylic ester, if it is introduced through the capillary the amount must be as small as possible. The gas introduced should be an inert one, such as
carbon dioxide or
nitrogen. If polymeriaztion is troublesome, it may be advantageous to pass in a slow stream of
carbon dioxide through the capillary during the entire reaction period.
2.
Sulfuric acid is also a very satisfactory catalyst; aluminum alkoxides also are useful, especially when the alcohols would be adversely affected by strong acids. Sodium alkoxides produce undesirable side reactions and give lower yields. When alkaline catalysts are employed, an alkaline polymerization inhibitor, such as
p-phenylenediamine or
phenyl-β-naphthylamine, should be used instead of
hydroquinone.
3. The fractionating column should be one that can be cleaned readily if a polymer is formed in it. A large number of plates is not required, though the column should be capable of separating the
methanol-methyl acrylate azeotrope (b.p.
62–63°) from
methyl acrylate (b.p.
80°), and
butanol (b.p.
117°) from
butyl acrylate (b.p.
145°). The necessity of effecting the latter separation can be practically eliminated by allowing the reaction to go virtually to completion, all the
butanol thus being consumed. This can be done by extending the reaction period as long as reaction occurs and by adding a considerable excess of
methyl acrylate. Instead of the twofold excess specified, three or four times the theoretical amount may be used with benefit. The larger amount is especially desirable when the acrylate of a relatively unreactive alcohol is being prepared.
4. The
methanol-
methyl acrylate azeotrope contains about
45% methyl acrylate, which can be recovered by washing out the
methanol with a large volume of water or brine; the acrylate is purified by drying and distilling. An inhibitor, such as
hydroquinone, should always be added to any acrylic ester before attempting to distil it, and, unless it is stored in a refrigerator, the distilled ester should not be kept more than a few hours without the addition of a small amount (0.1–1.0%) of an inhibitor.
5. Yields of the primary alkyl acrylates vary somewhat, owing to occasional losses through formation of polymer, but are usually in the range of 85–99%. Some secondary alcohols react very slowly, others readily. The method has been applied to more than fifty alcohols, some of which (with percentage yields) are listed below:
ethyl,
99%;
isopropyl,
37%;
n-amyl,
87%;
isoamyl,
95%;
n-hexyl,
99%;
4-methyl-2-pentyl,
95%;
2-ethylhexyl,
95%;
capryl,
80%;
lauryl,
92%;
myristyl,
90%;
allyl,
70%;
furfuryl,
86%;
citronellyl,
91%;
cyclohexyl,
93%;
benzyl,
81%;
β-ethoxyethyl,
99%;
β-(β-phenoxyethoxy) ethyl (from diethylene glycol monophenyl ether),
88%.
3. Discussion
This preparation is referenced from:
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