Question 26.8: Assignment of Priorities and Configuration of a Chiral Cente......

Analyze
To assign the configuration at the stereocenter, we must first assign the priorities of the substituents. Then we determine the R or S configuration by viewing the molecule toward the atom of lowest priority.

Solve
(a) This molecule is 3-iodohexane with C3 as the chiral center. The order of priorities of atoms attached to C3 is

I > C (ethyl) = C (propyl) > H

In order to decide the ranking of the ethyl group relative to the propyl group, we go to the first point of difference in the chains of these substituents.

We see that the propyl group ranks higher than the ethyl group, so the order of priorities is

I > C (propyl) > C (ethyl) > H

Viewing the molecule toward the lowest priority H atom, we see that
the priorities decrease in a counterclockwise manner, so the configuration at the stereo center is S. The complete name of the molecule is (S)-3-iodohexane.

(b) This molecule is 4-bromobutan-2-ol (see Are You Wondering 26-1), with C2 as the chiral center. The order of priorities of atoms attached to C2 is

O > C (bromoethyl) = C (methyl) > H

In order to decide the ranking of the bromoethyl group relative to the methyl group, we go to the first point of difference in the chains of these substituents.

We see that the bromoethyl group ranks higher than the methyl group, so the order of priorities is

I > C (bromoethyl) > C (methyl) > H

Viewing the molecule toward the lowest priority H atom is not quite as straightforward as in part (a).
This is because the H atom is in the plane of the paper and thus not as far away from the viewers as possible. We can tackle this problem in one of two ways. The first requires some three-dimensional “vision,” or “stereoperception,” in that we imagine picking up the molecule by the methyl group and bromoethyl group and reorienting the molecule so that the H atom points away from us, to give

where we can now discern the sequence of priorities as clockwise, so the compound is (R)-4-bromobutan-2-ol.
Alternatively, we can switch a pair of groups so that the group of lowest priority is bonded by a dashed wedge.

Now draw the view of the molecule toward the group of lowest priority. Here the sequence of priorities is counterclockwise.
Because we switched two groups, we created the enantiomer of the molecule whose configuration we are actually seeking.
Thus, although in the switched molecule the order of priorities corresponds to an S configuration, the original molecule corresponds to the R configuration. Therefore, as determined previously, the molecule is (R)-4-bromobutan-2-ol.

Assess
When assigning priorities to groups, it may be necessary, as it was in (b), to work backward along the chain to find the first point of difference and then compare the atomic numbers of the atoms at that point.