R. Gibbs and William P. Weber University of Southern Californib 10s Angeles, 90007
Oxymercuration-Demercuration An organic experiment involving the Markovnikov hydration of olefins
to yield alcohols is one of the mist basic reactions in organic chemistry. It is often the first examule of an el&rophilic adcktion to an alkehe presentei to students. However, it is a reaction which students seldom carry out in the laboratory. This is because the traditional method of hydrating an olefin, treatment with dilute sulfuric acid solution, while practical industrially is often difficult for students. Both the reaction temperature and the acid strength are critical for success. In addition, mixtures of products may result due to Wagner-Meenvein rearrangements of the carhonium ion intermediates. Perhaps the greatest advance in the last 15 years in laboratory scale synthetic methods has been the development of proced*es by Brown to hydrate olefins in high yield with control over the direction of the addition. Hvdroboration of a l-olefin followed bv oxidation with basic hydrogen mroxide of the intermediate trialkylborane re<s-in high yields of l-alcohol, the product of anti-Markovnikov addition of water to the carhon-carbon double bond ( I , ?2). 6 R--CH=CH2
+ B,H. d 2 (R--CHs-CH&B THF
The complementary procedure of oxymercuration of a l-olefin followed by in situ reduction of the organomercurial intermediate with sodium borohydride results in high yields of the 2-alcohol, the product of Markovnikov addition of water to the olefin (3-6).
A"
1
-av.A"
OH
Hydrohoration is unfortunately unsuitable for an undergraduate laboratory experiment due to the fact that the reaction must be carried out under a nitrogen atmosphere, since both excess diborane afld trialkylboranes are pyrophoric in air. On the other hand, we find that Brown's oxymercuration-demercuration procedure is adaptable as an undergraduate experiment. The reaction is rapid, clean, and goes in high yield. It is applicable to virtually any olefin. However, use of a terminal olefin t o demonstrate the direction of the addition seems indicated. Obvious precautions involved in working with toxic mercury salts must be observed. Finally, the high cost of mercuric acetate is not a problem since all the mercury is recovered as a shiny ball in the aqueous layer. Experimental Procedure Mercuric acetate 4.8 g (15 mmoles) is placed in a 250-ml Erlenmeyerflask. To this is added 20 nil of water. The flask is swirled to dissolve the mercuric acetate. Next, with continued swirling one adds 20 ml of tetrahydrofuran. The rapid formstion of a bright yellow finely divided precipitate is observed. This precipitate is pmbably mercuric oxide. Now add 16 mmoles of the desired olefia while swirling the flask (i-hexene, l-octene, and styrene are all satisfsctory). The yellow precipitate rapidly disappears and the solution becomes clear and colorless. This usually occurs in 1-5 mib The flask is then swirled for an additional 15 win. If the yellow color has not disappeared a t the end of this time, check that yon have added enough alefiri. During this time; prepare s. solution of s o d i u ~horohydride by dissolving 0.75 g.of sodium horohydride in 35 ml of 3 M sodium hydroxide. A 15-ml aliquot of this solution is then added to the Erlenmeyer flask slowly. The reaction is exothermic. Tlie temperature of the reaction should he maintained at about 25°C
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by cooling the Erlenmeyer flask in a pan of cold water. Immediately after the addition, a precipitate of mercury k formed turning the solution grey. After a few minutes, the grey solution clears with the formation of a hall of metallic mercury. The reaction is allowed to stand f o r 1hr to ensure completion of the reaction. At this point, sodium chloride is added until the aqueous layer is saturated. Fifteen milliliten of ether or methylene chloride are added, and the contents poured into a 125-ml separatory funnel. Be certain to save the mercury. The lower aqueous layer is drained off and discarded. The upper organic layer is then dried over anhydrous sodium sulfate. The organic layer is then filtered and the solvent removed by evaporation using a water aspirator and steam bath. The liquid remaining is product. The yields obtained average about 90%. Since the reaction is run on a small sample, i.e., about 1g of olefin, physical
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methods such as ir and nmr are very useful for proof of structure. If these are not available, the Lucas test will demonstrate that the alcohol formed from the 1-olefin is seoondrtry. This serves to prove the Markovnikov directionof the addition.
Literature Cited (1) Bnowr. H. C.. "Hydroboration:' W. A. Benjamin. Ino.. New Yorkr 1962.
G., A m Bnown, H. C.. 07&ReLIC1., 13, l(1963). (3) Bnowa, H. C., AND GEOOREOAN, P. J., JR., J. OW. Chcm., 35, 1844 (2) Zwemsh
(1970). (4) B n o w ~ .H. C., A N D G m a ~ e a P. ~ ~J.,,
. 1522 (1967).
JR..
J . A m w . Chem. Soc., 89,
(5) BROWN.H. C., HIMMEB.W. JAMES,J . Ainev. Chem. SOE.,89. 1524 (1907). (6) BROWN,H. C.. KIWIK*LII. J. H.,AND IRB(IAMI, S., J . Amw. Chem. Soc.. 89,1525 (1967).
