RESEARCH
New Route to Methoxyphenylacetic Acids Chemists modify old reaction, develop one-step synthesis using aldehydes and haloform Updating a 40-year-old reaction, chemists at the University of Mary land have worked out a one-step syn thesis for α-methoxyphenylacetic acids —usually made by a variety of methods whose drawbacks have long plagued chemists. The new synthesis uses an aryl aldehyde, haloform, methanol, and potassium hydroxide, Dr. Wilkins Reeve told the Washington-Mary land Meeting-in-Miniature. Dr. Reeve and his associates—Ivan Cristoffel, Paul Pickert, Dr. Charles Woods, and Dr. Edward C o m p e r e base their method on a reaction pub lished in 1920. That scheme made sodium α-ethoxyphenylacetate by re acting phenyl trichloromethyl carbinol with ethanol and sodium ethoxide. Basically, the Maryland chemists do the same thing. But by starting with an aldehyde and chloroform or bromoform, they make the carbinol in situ, then allow the reaction to go on to the methoxyphenylacetate without isolating the intermediate. Yields Vary. Yields depend on the nature of substituent groups on the benzene ring. Benzaldehyde itself gives a 40% yield. The monochloro derivatives range from 70 to 7 9 % , while the monofluoros give 51 to 69% yields. The monomethyl compounds are somewhat lower, 24 to 4 9 % . The reaction doesn't work at all when the phenyl group carries strong electron-donating groups, Dr. Reeve says. No methoxyphenylacetate re sults from p-dimethylamino- or phydroxybenzaldehyde, for example. Steric hindrance also plays an im portant role. While the monochloro derivatives work well, 2,6-dichlorobenzaldehyde gives only 2% yield. Similarly, 2,4,6-trimethylbenzaldehyde gives 1%. This synthesis, Dr. Reeve points out, wouldn't be as practical as it is if it were not for a particular property of many α-methoxyphenylacetic acids ^formation of crystalline salts con 50
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taining equimolar amounts of the acid and its sodium salt. When this hap pens, it is easy to precipitate and pur ify the product in the form of the acid salt, he says. This property is shared by most of the derivatives substituted with methoxy, methyl, fluorine, chlo rine, or bromine in the m- or pposition. Growth Regulators. Although the methoxyphenylacetic acid synthesis is of general use to organic chemists, much of the current interest in these compounds comes from their ac tivity as growth regulators in plants. Phenylacetic acid itself isn't too good; it doesn't move around to various parts of the plant. But the a-methoxy derivatives are very potent and very mobile, Dr. Reeve says. They seem to move freely throughout a plant and even through the soil from one plant to another. When used in proper amounts, growth regulators can also be herbi cides. Many of the compounds pre pared by Dr. Reeve's group are being tested at the USDA's Plant Industry Station at Beltsville, Md. One promising weed killer: 3,4,-dichloroα-methoxyphenylacetic acid.
Silicones at Low Temperature Tests relate polymer structure to crystallization temperature range Exposure tests covering a range of time and temperature are best for determin ing the low temperature usefulness of silicone rubbers. Short term tests alone just don't give good indications, David B. Braun told the ACS Division of Rubber Chemistry, meeting in Buffalo, N.Y. The most reliable procedure, he says, seems to be the standard ASTM method for measuring Young's Modu
lus in flexure. He assumes a Young's Modulus of 10,000 p.s.i. as the upper limit of utility for the silicone rubbers, then plots the time to reach this value as a function of exposure temperature. This gives what he calls "a low tem perature service curve." The resulting graph can show how long the elas tomer is usably flexible at a given tem perature. Some tests that Mr. Braun reported ran as long as 300 hours. Changes in "polymer architecture" shift the temperature range where crystallization of silicone rubbers takes place. This can result from copolymerizing bulky groups onto the side of the polymer chain or building complex segments into the backbone to become "intrapolymer chain stiffeners," Mr. Braun says. When phenyl groups replace some of the methyl groups, the temperature range of crystallization drops drasti cally, says Mr. Braun, who is with Union Carbide's silicones division. For pure dimethyl silicone polymers, it runs between - 6 0 ° and - 7 8 ° F . The most effective concentration of phenyl groups (4.9 mole %) lowers the range to between —100° and — 175° F. Above or below the opti mum concentration, the crystallization temperatures are higher. Other Groups. Similar effects arise from a number of other pendant groups. Copolymerization of 10 mole % of β-phenylethyl methyl siloxane or 5.8 mole % of 7-cyanopropyl methyl siloxane lowers the crystallization range markedly. Higher concentra tions of cyano groups give similar re sults in both silicone and organic rub bers, Mr. Braun states. The nitrile groups contribute "at least some flexi bility" when both types of elastomers are cooled to their second order transi tion temperatures. He notes no crys tallization at all with higher concen trations of nitrile groups. Changes in viscosity-temperature characteristics decrease polymer mo bility, he says, and result in the ab sence of crystallization, since necessary orientation can't take place. In or ganic nitrile rubber, on the other hand, there seems to be a dilution ef fect like that in styrene-butadiene elastomers. Adding acrylonitrile keeps the £ratts-l,4-polybutadiene component dilute enough to keep it from crystal lizing. Bulky organic groups built into the siloxane chain can likewise lower the crystallization range.
