Organic Chemistry
Chirality Chirality
Organic Chemistry - Ch 11 547 Daley & DaleyOpen a jar of spearmint oil. Smell it. Then taste it. Next, do thesame with a container of caraway seed. As you can observe, their odorand taste are very different. However, the principal component for theflavor and odor of both are enantiomers of each other. (R)-carvone isthe principal component of spearmint oil. Its enantiomer, (S)-carvone,is the principal component of caraway oil. The only difference betweenthe two enantiomers is the arrangement of their atoms in space.CH 3CH 2CH 3CH 2OOCH 3(R)-CarvoneCH 3(S)-CarvoneThe distinctive odor of each is the result of the odor receptorsin the nose that appear to be based on chiral receptor sites. Thus, thenose perceives odors based on the fit of the odor-causing molecule intothe receptor sites located in the nose. This behavior is similar tofitting a key into a lock—only the key designed for a specific lock fits.Likewise, the odor-causing enantiomers fit only into the appropriatelyshaped odor receptors in the nose. Therefore, (R)-carvone fits into adifferent receptor than does its enantiomer, (S)-carvone.Biological systems commonly exhibit high levels of chiralrecognition. Chiral recognition takes place when a chiral receptor orreagent interacts specifically with one enantiomer of a chiral molecule.A striking example of chiral recognition occurred with the drugthalidomide, a sedative and mild hypnotic introduced to the Europeanmarketplace in 1956.O ONHOThalidomideNHOImmediately, thalidomide became popular with pregnant women tocontrol morning sickness because, even in large doses, it was notwww.ochem4free.com 5 July 2005
Organic Chemistry - Ch 11 548 Daley & Daleylethal and testing had indicated no undesirable side effects. At aboutthe same time, however, the number of congenitally deformed babiesincreased. The most seriously deformed babies were born withphocomelia, or seal limbs. That is, they either had no limbs or hadshortened ones causing the hands and feet to grow close to the body.Other serious defects included absent or malformed external ears,fusion defects of the eye, and the absence of the normalgastrointestinal tract openings. After five years, medical researchersfinally traced the problem to the use of thalidomide during the firstthree months of pregnancy, and thalidomide was removed from themarket.Thalidomide had been tested, so what happened to cause thisproblem? True, the (R) isomer of thalidomide, the medically effectiveisomer, was made in a laboratory and was tested for negative sideeffects, and there were none. However, when manufactured underproduction conditions, the reaction temperature was increased tospeed up the reaction time, thus producing a mixture of the (R) and (S)isomers. This was discovered later through more testing along withthe fact that the (S) isomer of thalidomide was a teratogen. Ateratogen is a chemical substance that causes birth defects. Not allbabies whose mothers took thalidomide were born with defects,however. The fetus was susceptible only at specific points in the firstthree months of its development. Therefore, if the mother did not usethe drug at those specific points, there were no problems. If just thepure (R) isomer of thalidomide had been manufactured and ingested,thousands of babies would not have been born deformed.11.4 Optical IsomerismOne way to differentiate between the two enantiomers in a pairof enantiomers is to observe the differences in how they react withanother asymmetric compound. Another way is to contrast theirinteractions with a plane of polarized light. With ordinary light, thelight waves vibrate at all angles in an infinite number of planes ofvibration. Passing ordinary light through some polarizing materialallows only one of these orientations to pass through. This light isplane-polarized light. See Figure 11.8.www.ochem4free.com 5 July 2005
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<strong>Organic</strong> <strong>Chemistry</strong> - Ch 11 548 Daley & Daleylethal and testing had indicated no undesirable side effects. At aboutthe same time, however, the number of congenitally deformed babiesincreased. The most seriously deformed babies were born withphocomelia, or seal limbs. That is, they either had no limbs or hadshortened ones causing the hands and feet to grow close to the body.Other serious defects included absent or malformed external ears,fusion defects of the eye, and the absence of the normalgastrointestinal tract openings. After five years, medical researchersfinally traced the problem to the use of thalidomide during the firstthree months of pregnancy, and thalidomide was removed from themarket.Thalidomide had been tested, so what happened to cause thisproblem? True, the (R) isomer of thalidomide, the medically effectiveisomer, was made in a laboratory and was tested for negative sideeffects, and there were none. However, when manufactured underproduction conditions, the reaction temperature was increased tospeed up the reaction time, thus producing a mixture of the (R) and (S)isomers. This was discovered later through more testing along withthe fact that the (S) isomer of thalidomide was a teratogen. Ateratogen is a chemical substance that causes birth defects. Not allbabies whose mothers took thalidomide were born with defects,however. The fetus was susceptible only at specific points in the firstthree months of its development. Therefore, if the mother did not usethe drug at those specific points, there were no problems. If just thepure (R) isomer of thalidomide had been manufactured and ingested,thousands of babies would not have been born deformed.11.4 Optical IsomerismOne way to differentiate between the two enantiomers in a pairof enantiomers is to observe the differences in how they react withanother asymmetric compound. Another way is to contrast theirinteractions with a plane of polarized light. With ordinary light, thelight waves vibrate at all angles in an infinite number of planes ofvibration. Passing ordinary light through some polarizing materialallows only one of these orientations to pass through. This light isplane-polarized light. See Figure 11.8.www.ochem4free.com 5 July 2005
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