l11 - CH 203 O R G A N I C C H E M I S T R Y I The R,S...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
The R , S system © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I The R , S system Chirality centers We saw in a previous lecture that a molecule that has a center of symmetry or a plane of symmetry cannot be chiral and cannot have an enantiomer. There is another quick test of chirality: If a molecule has one and only one chirality center, it is guaranteed to be chiral and to have an enantiomer. A chirality center is an sp 3 hybridized atom bonded to four nonequivalent groups. An example of a chirality center is the C1 carbon in the 1-chloro-1- fluoroethane structures shown below: F Cl H 3 C H F Cl CH 3 H The C1 carbon is bonded to four non-equivalent groups: a chorine atom, a fluorine atom, a hydrogen atom and a methyl group. Note that the molecule has one and only one chirality center, no center of symmetry and no plane of sym- metry: it is chiral and has an enantiomer, namely, its non-superimposable mirror image. In the definition of the chirality center, we are careful to specify that the chirality center is bonded to four nonequivalent groups and not to four non- equivalent atoms. The C3 carbon in the 1-chloro-3-methylhexane structure be- low is a chirality center even though it is bonded to three carbon atoms and a hydrogen atom: what makes the C3 carbon a chirality center is that it is bonded to four nonequivalent groups, namely, (1) CH 2 CH 2 Cl, (2) CH 2 CH 2 CH 3 , (3) CH 3 , and (4) H. Cl H 3 C CH 3 H Chirality centers are not restricted to carbon atoms: there are boron, nitro- gen, oxygen, phosphorus and silicon chirality centers. The requirements are that the atom be sp 3 hybridized and bonded to four nonequivalent groups.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
The R , S system © Bruno I. Rubio 2 Problem Circle the chirality centers in these drugs and biomolecules. (a) O OH OH HO HO OH ! -D-glucose (b) N H 3 C O O O OCH 3 cocaine (c) O O CH 3 O O H 3 C O N CH 3 heroin (d) N S CH 3 CH 3 CO 2 H O N H O NH 2 HO amoxicillin (e) H 3 C CH 3 CH 3 H 3 C HO H 3 C cholesterol (f) H 3 N N CO 2 O H HS O N H CO 2 glutathione Answer (a) O OH OH HO HO OH (b) N H 3 C O O O OCH 3 (c) O O CH 3 O O H 3 C O N CH 3 (d) N S CH 3 CH 3 CO 2 H O N H O NH 2 HO
Background image of page 2
The R , S system © Bruno I. Rubio 3 (e) H 3 C CH 3 CH 3 H 3 C HO H 3 C (f) H 3 N N CO 2 O H HS O N H CO 2 If a molecule has more than one chirality center, all bets are off: that molecule may or may not be chiral. Both Structure A and Structure B shown be- low have two chirality centers, but Structure A has a plane of symmetry that disqualifies it from being chiral: it does not have an enantiomer. In con- trast, Structure B has no center of symmetry and no plane of symmetry: it is chiral and has an enantiomer. Cl Cl A Cl Cl B Problem How is Structure A stereochemically related to Structure B? Which is more stable? Answer They are diastereomers. Structure B is more stable because its chlo- rine atoms are farther apart and therefore do not interfere with each other sterically.
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 22

l11 - CH 203 O R G A N I C C H E M I S T R Y I The R,S...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online