Robert C. Duty and Bernard L. Ryder Illinois State university Normal, 61761
I Grignard Dehydration Reactions
I.
An undergraduate organic experiment 6
A typical organic laboratory introduces the student to organometallic chemistry through an experiment utilizing a Grignard reaction. In this laboratory we have incorporated the Grignard reaction in a step synthesis that has been successful in demonstrating several experimental and instrumental techniques. The Grignard reaction was used to prepare atertiary alcohol, 2,3-dimethyl-2-butanol (bp 120-121°C) ( I ) ,from a secondary alkyl bromide, 2-bromopropane
dH, &H, This alcohol was chosen because i t is readily synthesized from relatively inexpensive starting materials. Furthermore, 2-bromopropane was converted to the Grignard reagent quite successfully by students within a relatively short period of time, provided adequate precautions for anhydrous reaction conditions were taken (see experimental section). The dehydration of the tertiary alcohol was accomplished with concentrated sulfuric acid and the olefins formed were distilled through a 6-in. fractionating column packed with stainless steel wool. All glassware was 3 14/20 equipment.
5
d Fraction li
Fractl
0
Gas Chromatograms. Fracfion A (bp 25-65'C) Fraction 0 (bp 65-70') Peaks 1 and 4--Acetone Peaks 2 and 5-2.3-Dimethyl-1-BUtene Peaks 3 and 6-2.3-Dinethyl-2-BUtene
CHa The four stabilizing.. alkvl . ..croups . on 2.3-dimethvl-2-butene control the course of the reaction. This is drarnn;ically illustrated bv the exoerimental fact that the statistically controlled produc~(2,3-dðy~-1-butene)is less favored. ~ f t h relative e number of 1"hydrogens and 3O hydrogens which were available for the deprotonation step controlled the product ratio, there would have been a 6:1 ratio of I-alkene to Palkene. Actually, gas chromatographic analysis proved the ratio to be reversed in favor of the more substituted alkene.
\
/
C=C'
\
(minor product)
CK CHs The dehydration step provides a mixture of two isomeric alkenes that is separated very effectively by gas chromatography into the more stable Saytzeff isomer (2); 2,3-dimethyl-2-butene, as the major product and the less stable Hofmann isomer (3), 2,3-dimethyl-1-butene, as the minor product.
Discussion and Resulls This two-step synthesis requires two 3-hour laboratory ~eriods.Students were allowed to comolete the Grignard synthesis in one 3-hour laboratory period and finish the dehvdration and necessarv distillations durine- the second laboratory period. The dehydration of the 2,3-dimethyl-2-butanol with concentrated HzS04 works satisfactorily if the student (1) avoids usine an excess of acid and (2) does not distill to dryness when isolating the alkenes. Observing these precautions minimizes charring in the reaction flask. Concentrated H3P04 is a good dehydrating agent for the alcohol, hut we experienced severe Volume 53, Number 7. July 1976 / 457
foaming with it during the distillations. Anhydrous oxalic acid (4) eliminates both the foaming problem and charring, but the problems involved in obtaining anhydrous oxalic acid (5)does not justify its use in an undergraduate laboratory experiment. In a class of 33 students the average per cent yield of alcohol was 40.9% (see Table 1). The averaee per cent vield of olefins. based on the dehydration step wai40.7%; based on the two: step svnthesis. the averaee vield was 15.9%. Conseauentlv. every student bhtained an adequate supply of final product for the eas chromatoeraphic analvsis and the chemical tests for unsaturation. The per cent yields given in Table 1are based on weight yields rather th& gas chromatograms (i.e., the students &re instructed to collect and weigh the fraction boiling between 80' and 125°C). Since the alcohol boils a t 120-121°C, the fraction was not pure alcohol. Therefore, the per cent yield, a s renorted in Table 1. should be lowered bv .ao~roximatelv .. 10%.In addition, the per cent yield of pure olefins was lower than indicated in Table 1 because the distillation did not vield isomerically pure fractions. Most of the chromatoerams of the olefin fractions. esoecially the lower boiling friction, showed an acetone peak Gee the fie.) which was separated distinctlv from the alkene isomer he lower boilkg fraction of olefms (hp range 2M5"C) contained most of the acetone as an impurity, varying from 1 1 4 2 % (average = 35%; median = 42%). T h e higher boiling fraction (bp range 65-78'C) contained very little acetone with the amount varying from 0-30% (average = 5.6%; median = 2.4%). As these results indicate, a stuaent with good laboratory technique can obtain excellent results which are confirmed unequivocally by ~-gas chromatographic analysis. In the separation of the isomeric alkenes by fractional distillation, the efficiency of the distillation is limited by the equipment used. In Table 2 the ratios of the two isomers are given. The more highly branched (thermodynamically stable) isomer, the 2-butene isomer, exceeds the l-hutene isomer in both the lower boiling and higher boiling fractions. The ratio of 2-butene to l-butene isomer in the first fraction was apnroximatelv 2:l and increased to 9:l in the second fraction. If one wanted to eliminate as much of the 2-butene isomer as possible from the first fraction, a decrease in the upper limit oi6S0C tuan upper limit cluscr to the bpof the butene isomer rhp 5fi°C) is recommended. However, the fractional amount ~
Table 1.
Per
Cent Yields of Alcohol and Hexvlenes % Yield Ranae
Median
Product
Averaae
2.3-dimethyl-2butanola 2.3-dimethyl-Zbutene and 2-3-dimethyl-1 butenen 2.3-dimethyl-Zbutene and 2.3-dimethyl-lbuteneb
40.9
96.3-10.8
33.9
15.9
34.1-2.2
15.6
40.7
96.2-5.3
41.7
a Bared en 0.214 mole of 2-bromouro~ane. b Based an amount o f alconol prepared by each student. Table 2.
Ratio of Isomer in
Two
Lower Boiling Fraction lbu range 2 5 ' - 6 5 ' ~ ) l-butene isomer 1%) Range
Average Median
17-60 37 38
2-bufene iromer ( % ) 83-40 63 62
a Population: 26 rtudentr.
458 / Journal of Chemical Education
Distillation Fraction@
collected will be correspondingly smaller and more difficult to collect. This alteration of procedure, however, is not recommended for first year organic students. The step synthesis of an olefin has been given high ratings by the students because it reinforces manyof the basic principles which are stressed in the first semester organic lecture course. First, i t allows the student to perform a Grignard synthesis with virtually no chance of failures because of unrkactive starting mate&ls. Second, it introduces the student to a classical mechanism with sulfuric acid as a protonating and dehydrating agent in the organic chemist~laboratory. Third, the student has the opportunity to assemble and use hotha fractional and a simple distillation apparatus. The effectiveness of the distillation technique, as performed by the student, is revealed dramatically to him by the gas chromatograms. T h e gas chromatographic analysis of the olefins provides the students with a clear picture to illustrate the careful and accurate distillations, as distinguished from those that are less accurate and hurriedly performed. Experlrnental
Preparation of 2.3-Dimethyl-2-Butanol Attaeh to a 250-ml round bottom flask (T 14/20),a Claisen adapter, a droooine funnel. and a condenser orotected with a CaCl?- d .~ .. n e tube ,\dd 6.0 g (0.25 g atom, of (;rignard magnesium turnings to the flask and mount the apparatus h ~ g henough lo permit the plnrement of an ice bath under the renetmn flask, if needed Flame the entire apparatus using a Bunsen burner with a small flame. After all Bunsen flames have been extinguished in the laboratory and the glassware has ewled, add to the flask 50 mlof anhydrousether through the dropping funnel. Clase the stopcock of the funnel and add a solution of 20 ml(26.2 e: 0.214 mole) of 2-bromoorooane . .. . . in 30 ml of anhydrous ethyl ether. Add the solution dropwise at swh n rnte na tuallow themixture toreact with the magnesium towstain autendg reflux of the other a h e n t . Do not add more than 10% oithe 2-bromopropane solution until certain the Grignard reaction has started. There may be an induction period prior to the onset ofthe Grignard reaction. If the solution does not become grayish with bubbling, raise the reflun condenser from the round bottom flask and carefullycrush n few metal turnines n with a drv , elass stirrine rod. Add the rest of the solution at arawluprrmil arteady reflux. HRVCR mirrureof crushed ice and water ready to cool the rvund hutlorn f l s k if the reflux rntc is excessive or the condenser begins to flood. After the addition of 2-bromopropane solution is complete, apply heat, using a Thermo-wellor other convenient heat source, tomaintain a vigorous reflux rate for about 15mi". During this reflux period, add to the closed dropping funnel a mixture of 17 ml (14.3 g; 0.247 mole) of analytical reagent grade acetone dissolved in 30 ml of anhydrous ethyl ether. At the end of the refluaing period, add the acetone-ether solution dropwise to maintain a gentle reflux. Be ready with the ice bath if coaling is necessary. At the end of the addition, heat the reaction mixture under gentle reflux for an additional 15 mi". After cooling the flask, pour the contents over 150ml of ice in a 400-ml beaker. While vigorously stirring the ice mixture, add concentrated HCI slowly until the mixture is acidic to litmus paper. At this point the grayish mixture will lose its gelatinous consistency. Pour the contents of the beaker into a 250-ml separatory funneland discard any unreacted magnesium metal. Separate the lower aqueous layer from the organic layer and wash the aqueous layer with two separate 30-ml portions of USP diethyl ether. Combine the ether extracts with the original organic layer and dry with anhydrous Na.2301. Decant the dried solution into a dry 250-ml round bottom flask and add 5 ml of o-xylene as a chaser solvent for the alcohol. Assemble the fractional distillation1apparatus and collect two fraetions boiling from room temperature to 80°C, and 80-125'C. respectively. The second fraction will contain the'al~ohol.~
~ . -~ . ~
~
~~
~~~
~
-
Higher Boiling Fracrion l b u range 6 5 ' - 7 8 ' ~ ) l-butene iromer ( % I
2-burene isomer 1%)
1-56
99-44
11
89
8
92
'Fractionating columns were packed with stainless steel wool.
Note to instructor: At this point each student was required to turn in 1 ml of his or her distilled alcohol. This permitted the instructor to check the purity of each student's aleohbl andlor provide an unfortunate student with new starting material far the dehydration experiment in case he or she lost his or her alcohol sample.
DehNration of 2,3-Dimethyl-2-Butanol Pour the alcohol fraction into a round bottom flask assembled for total reflux and add 1 ml of concentrated HISOlfor everv 10 ml of
DC 200 substrate. Flow rate was 60 mllmin and the temperature was 65°C. The gas chromatograph was s Gow-Mac Model 69-100 with a Heath Model SR 255-10 mV recorder,
Volume 53. Number 7,July 1976 / 459
