Organic Chemistry

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Organic Chemistry - Ch 11 561 Daley & DaleySee Section 1.7, page000, for more on van’tHoff and Le Bel.Section 7.7, page 000discusses the reactionof a ketone with ahydride nucleophile.Then, while looking through the microscope, he used a pair oftweezers to separate the crystals into two piles. When he comparedthe properties of the two piles, the only difference he found was theirinteraction with a plane of polarized light. The crystals in one pileturned the light clockwise; the crystals in the other pile turned thelight counterclockwise. He had managed to affect the first separationof enantiomers. It was this work by Pasteur that led to van't Hoff'sand Le Bel's suggestion in 1874 that four atoms bonded to carbonresulted in a tetrahedral structure.Enantiomers rarely crystallize in different forms, as doestartaric acid, so chemists must use other methods to separate them.The most common method is to react the racemic mixture with asingle pure enantiomer. Single pure enantiomers of optically activecompounds are generally isolated from biological sources becausebiological systems usually contain only one enantiomer of a pair. Thisreaction produces a pair of diastereomers that is separated by physicalmethods.The synthesis of (R)-2-butanol shows the resolution of a pair ofenantiomers. A convenient synthetic method for the synthesis of (R)-2-butanol is the addition of hydrogen to the carbonyl group of 2-butanone. The product is a racemic mixture of 2-butanol becauseneither starting material nor reagents are asymmetric. Unless you usean optically active starting material or reagent when synthesizing anasymmetric compound, the result is a racemic mixture of enantiomers.OCH 3 CH 2 CCH 3CH 31) LiAlH 4H OH2) H 3 OCH 2 CH 3+HOCH 3HCH 2 CH 3(S)-2-Butanol(R)-2-ButanolTo resolve this racemic mixture, you need an asymmetric reagent thatreacts with the alcohol to produce a pair of diastereomers. Anasymmetric carboxylic acid, such as Louis Pasteur’s (+)-tartaric acid,is a logical choice because it will react with the alcohol to form a pairof diastereomeric esters which can then be separated and hydrolyzed.Additionally, (+)-tartaric acid is readily available because wineriesproduce large quantities of it as a side product.www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 11 562 Daley & DaleyCOOHHHOOHHCOOH(+)-Tartaric acidFigure 11.9 shows the esterification reaction of the resultingdiastereomeric esters. Because diastereomers generally have differentphysical properties, they can be separated easily via distillation,chromatography, crystallization, or other physical methods. Forexample, the two diastereomeric esters of tartaric acid have meltingpoints that differ by about 20 o C. They also have different watersolubilities and different acid strengths. These differences allowseparation using ordinary physical methods. The separation is socomplete that you end up with each of the diastereomeric esters in thepure form.OCH 3HCOHOCH 3HCOOHHHOOHHCH 2 CH 3HCH 2 CH 3CH 3OHCH 2 CH 3+H OH HHO HCOOHHCOOH(R)-2-Butyl (+)-tartrateCH 3 OO CRacemic 2-butanol(+)-Tartaric acidCH 2 CH 3HOHHOHCOOH(S)-2-Butyl (+)-tartrateFigure 11.9. Reaction of (R)- and (S)-2-butanol with (+)-tartaric acid to produce (R)-and (S)-2-butyl (+)-tartrate. The two diastereomeric esters of tartaric acid can beseparated because they have different physical properties.A hydrolysis reaction is a good way to isolate the enantiomericalcohols. Hydrolysis of (R)-2-butyl tartrate gives (R)-2-butanol.www.ochem4free.com 5 July 2005
Page 1 and 2: Richard F. Daley and Sally J. Daley
Page 3 and 4: Organic Chemistry - Ch 11 532 Daley
Page 38: Organic Chemistry - Ch 11 567 Daley

Organic Chemistry

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