1559T_ch05_70-9810/29/053:49Pa9e81 ⊕ EQA Solufions to Problems 81 29.The stereocenter has been labeled with an asterisk ()ineach chiral molecule (a Not chiral (achiral)(2CH's on the tertiary carbon!) b入入√入Chiral Chiral (OBr入Br Not chiral (g),(),(i)Not chiral (all are planar molecules) (Ho -CHOHCH2NHCH;Chiral HO (k)Not chiral (1)Not chiral (m)CH2OH Chiral (has 2 stereocenters) (n)and (o)Not chiral.Both molecules (as HOCH 00 HO OH 30.and (e)the carbons with the -OH groups are the stereocenters:in (the stereocenter is C2. 31.Do you know how to rec counterclockwise directions from the ca n .2-dimethylcyclohexane in (a).conside dages?It bea wise.we first come to a CH2.whereas in the clockwise direction we The same sitution is found in(b).In (c).a difference between in Solutions to Problems • 81 29. The stereocenter has been labeled with an asterisk (*) in each chiral molecule. (a) (b) (c) (d) Not chiral (e) Chiral (f ) Not chiral (g), (h), (i) Not chiral (all are planar molecules) (j) Chiral (k) Not chiral (l) Not chiral (m) Chiral (has 2 stereocenters) (n) and (o) Not chiral. Both molecules (as drawn) have vertical planes of symmetry bisecting the rings. Make models! 30. Look for a stereocenter: a carbon atom with four different groups. Compounds (a), (c), and (e) are chiral. In (a) and (e) the carbons with the OOH groups are the stereocenters; in (c) the stereocenter is C2, the methyl-substituted carbon atom. 31. Do you know how to recognize chiral carbons in rings? The four groups on a ring carbon include the two external substituents, and the atoms of the cycle itself proceeding in clockwise and counterclockwise directions from the carbon under scrutiny. To take one example, cis-1,2-dimethylcyclohexane in (a), consider the topmost ring carbon (marked with a dot in the structure below): What are its four appendages? It bears an H and a CH3; that’s two. The other two are depicted by the curved arrows: the ring atoms proceeding clockwise and counterclockwise, respectively. Are they different? Yes; proceeding counterclockwise, we first come to a CH2, whereas in the clockwise direction we encounter a CHOCH3 first. The same situation is found in (b). In (c), a difference between ring atoms in H H CH3 CH3 O O HO OH H HOCH CH2OH * * HO HO CHOHCH2NHCH3 * Br Br Br Br * Br Br * Chiral Not chiral Not chiral (achiral) (2 CH3’s on the tertiary carbon!) 1559T_ch05_70-98 10/29/05 3:49 Page 81