D. The second carbon in the longest chain will be the carbon bonded to the bromine. The Cahn-Ingold-Prelog rules can be used to assign priorities to the groups on this carbon: (1) bromine atom because it has the highest atomic number, (2) -CHCl(CH3) group because of the Cl atom, (3) -CH2OH group, (4) hydrogen atom because it has the lowest atomic number. The lowest priority group should be pointing away, and this is the top/bottom position of a Fischer projection, which is not the case here. One option is to rotate the groups so H is at the top position (to do this, hold the carbon chain steady and rotate the other three groups clockwise). Another option is to determine the absolute configuration as is and then switch the configuration to the opposite: going from highest to lowest priorities (excluding number four) moves clockwise (R). However, since we are looking in the wrong direction the configuration is actually S (choices A and B are incorrect).
The same strategy can be applied to C-3. The priorities are (1) chlorine atom because it has the highest atomic number, (2) -CHBr(CH2OH) because of the Br atom, (3) -CH3 group, (4) hydrogen atom. Going from 1-2-3 is counterclockwise (S). But since we are looking in the wrong direction the configuration is actually R (choice C is incorrect and D is correct).
9th EditionJohn E. McMurry
8th EditionJohn E. McMurry
4th EditionRandall Engel
2nd EditionDavid Klein