1559T_ch05_70-9810/22/0520:19Pa9e73 EQA Keyso the Chopter·73 nical (carbon).We move out to the atom attached to cach of these carbons for First point of differenc ② ©H ©H ⊙CHh-ctc. ©H,-ctc CH: Ring CHa Ring C(CH)2 n each case the la orities are determined by moving one atom further out,as shown (circled): ©H -cH2-C--CH,-C-etc ⊙H Ethvl Ring CH Example:Determine R-Sdesignation for the marked carbon in OCH,CH, CHCH(CH3)2 OCH>CH Keys to the Chapter • 73 where the left-hand bond goes to the stereocenter and “etc.” means “continuing around the ring.” Priorities are assigned in accordance with rule 2 again, because in all three groups the first atom in the “chain” attached to the asymmetric carbon is identical (carbon). We move out to the atoms attached to each of these carbons for comparison (circled): In each case the largest of these atoms is carbon. No difference. However, in the case of the group at the right, the second largest is also carbon, whereas the second largest for the other two groups is hydrogen. The high￾est priority of these three groups is therefore the ring-contained OC(CH3)2OCH2Oetc. Second and third pri￾orities are determined by moving one atom further out, as shown (circled): Comparison here is straightforward. The CH2 in the ethyl is connected only to a simple CH3 group, whereas the CH2 of the ring is attached to a OC(CH3)2Oetc. group. So the latter one is higher in priority (C larger than H). Therefore we have: Example: Determine R–S designation for the marked carbon in C CHCH2CH3 CHCH(CH3)2 OCH2CH3 OCH2CH3 Cl H * C CH2CH3 (CH3)2C H2C H becoming C
b a b a c c d C (d in back) Counterclockwise
S C H3 H3 CH2 C C H H H CH2 C etc. First point of difference Ethyl Ring CH2 C H C H3 C H3 C H3 H C H C (CH3)2 etc. H C C H2 etc. First point of difference Ethyl Ring CH2 Ring C(CH3)2 1559T_ch05_70-98 10/22/05 20:19 Page 73