I I I
George H. Clelandl Occidental College
Los Angeles,California 90041
A Synthesis of oChlorocinnamic Acid Utilizing a Meerwein Reaction An undergraduate organic experiment
M a n y laboratory texts designed for undergraduate courses in organic chemistry include t h e synthesis of cinnamic acid or of some substituted cinnamic acid via t h e Perkin reaction. A few s u c h t e x t s contain cinnamic acid svntheses usine t h e Knoevenagel or related reactions. T h e ~ e l r w e i reaction n (I) is seldom encountered in u n d e r a r a d u a t e lahoratorv m a n u a l s
(2).
I n the oast several vears we have developed exoerimental procedur&, suitahle for a n undergraduate'coursd in organic c h e m ~ s t r vt.h a t involve d ~ a r o t i z a t i o no i a substituted aniline a n d then-a Meerwein reaction t o give a n a-bromohydrocinnamic acid ArNHz
-
ArNzBr,
+ CHz=CHCOOH
CuBr.HBr
+
acelona
ArCHzCH(Br)COOH
+ N2
followed by dehydrohromination of t h e n-hromo acid with a q u e o u s o r methanolic sodium o r potassium hydroxide a n d then acidification t o yield t h e corresponding cinnamic acid ArCHzCH(Br)COOH + 2 NaOH
-
H+
ArCH==CHCOONa, +
A~CH=CHCOOH T h e reaction sequence presented here involves t h e conversion of o-chloroaniline to o-chlorocinnamic acid.
Reagents Most of the reagents were used as received from commercial sources. Acrylicacid, if taken from a container that has been opened a number of times, should he distilled previous to use onto a small amount of a stabilizer such as p-methaxyphenol. Cuprous bromide u.ith some green color is quite satisfactcry since ropprr paqses hark ru'the and forth krwrcn WIIand C ~ ~ ~ I l ~ d ~ r i n ~ t h t . n r u r ihleerwein r reduced to Cwl) Iwa rractlnn w:th rencthm and. in addition. C u ~ l l is acetone (3). ~
~
a-Bromo-o-chlorohydrocinnarnicacid o-Chloroaniline (2.1 ml, 0.02 mole) is added from a bbret to a 250-300 ml erlenmeyer flask, 50 ml of acetone and then 1-8 ml of "48%"hydrohromic acid (0.060.07 mole) are introduced, and the mixture is swirled and cooled in a water-ice hath until the temperature reaches 14%(occasionally some of the amine hydrobromide separates a t this point; no problem is presented if swirling is vigorous during the next step). Diazotization is accomplished by swirling the mixture while 4.2-4.3 ml of 5.00 M sodium nitrite is added from a buret (this represents a slight excess of sodium nitrite; the excess sodium nitrite has no adverse effect). Acrylic acid (5 ml, -0.07 male) is introduced, a water-ice bath is made available, the temperature of the mixture is adjusted to 15-17°C. and 2&25 mg of cuprous hromide is added. (CAUTIONS.(1) The amount of catalystused should not he much more than that specified or else the ensuing reaction can become so vieorous as to cause same of the mixture to foam out of the reartion flask. ILI Hromoareu,ne ika rnlnur side product, hut it u u w s extreme rye and respiratory irritation. It is thrrefbrr best to keep all opwationn invdviny t h r Merwein rrartmn and rubseqwnt transfrrs undrr cltaecmtrol with rcipert torxpwuretothe Inhuratorv atrnosphrrc unril the Merrwein product hns heen isolated, u n h oisposal oiall ~nridcn~al f~ltratesin a hood., The mixture dnrkens on a d d i r ~ m of the catalyst and nitrogen is soon evolved. The mixture is swirled and the temoerature is allowed to reach about 20% From this noint to eovern the ternoerature of theemthermicree&x on it is exoedient . so that it dvrs nor ercerd IO0C hy application 131a watrr-,re hath and,
