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figure 10-4. The aldotetroses contain two chiral carbon atoms, every of which may exist in one R- or S- form, giving rise to 4 stereoisomers. The 2 erythrose isomers room mirror images (enantiomers), as are the two threose isomers. Each erythrose is a diastereomer the the threoses, and vice versa. To draw these stereoisomers quickly and conveniently, we usage Fischer estimate formulas (Fig. 10-5). In this convention the carbon backbone is attracted vertically and the hydroxyl and hydrogen substituents are inserted horizontally to the ideal or left. This horizontal bond by definition project outward, towards the viewer, if the upright bonds are behind the aircraft of the paper, away from the viewer. The carbon atoms consisting of the backbone lie in ~ the intersections of the vertical and horizontal lines. The Fischer convention is portrayed in Fig. 10-5 because that (2R,3R)-erythrose.
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figure 10-5. In the Fischer convention the molecule is viewed from over with horizontal groups projecting the end of the page, vertical groups behind the page. Every time we include a chiral facility to a molecule, we double the possible number of stereoisomers. V 1 chiral center, there are 2 isomers, 2 chiral centers, 4 possible isomers, 3 centers, 8 isomers and also 4 centers, 16 feasible stereoisomers. For an arbitrarily number (n) of chiral centers in a molecule there are as numerous as 2n possible stereoisomers. Sucrose, v nine chiral carbons, has actually 29 stereoisomers, or 512. Glucose has four chiral carbons in that aldehyde form, and also so there room 24, or 16 possible stereoisomers the this formula, only among which is dextrose <(+)-glucose>. This 16 isomers are shown in Fig. 10-6. We could, that course, designate the stereohistoricsweetsballroom.comistry at every chiral carbon as R or S; in this method carbons 2 through 5 in dextrose might be labeled 2R,3S,4R,5R. More commonly, the sugars are divided into two classes, D- or L-, depending upon whether the hydroxyl ~ above the bottom chiral carbon is on the right (Dextro) or left (Levo) in the Fischer formulas. This has been excellent in Fig. 10-6.


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The symbols "D" and "C" are vestiges the the system used to designate configuration before the creation of R and S. None of these has anything to carry out with the authorize of rotation the the plane of the plane-polarized light, stood for by d-, l-, (+)-, or (-)-.
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