Consider the Fischer projection of 2,3-dichloropentane below. C2 and C3 are asymmetric centres. What is the configuration (R/S) about C2 and C3?

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We start by restating the problem setup: the Fischer projection given corresponds to 2,3-dichloropentane, where C2 and C3 are stereocenters. The substituents shown are as follows: at C2 the horizontal substituents are H (left) and Cl (right), with the vertical bonds going to C1 (top) and C3 (bottom). At C3 the horizontal substituents are Cl (left) and H (right), with vertical bonds to C2 (top) and C4 (bottom). In a Fischer projection, horizontal bonds point toward you (toward the viewer) and vertical bonds point away from you.

Option a: 2S, 3R. To assign R/S, apply CIP priorities at each center. For C2, the substituents are Cl (priority 1), the C3- containing fragment (priority 2), the C1-methyl end (priority 3), and H (priority 4). In the Fischer drawing, H is on the left (toward you), while the order 1→2→3 moves from right (Cl) to bottom (C3 fragment) to top (C1 methyl). Since the lowest priority group (H) is toward you, the observed sequence is inverted when assigning configuration, leading to an S or R designation that must be checked carefully against the actual spatial arrangement. For C3, the substituents are Cl (priority 1), the C2-containing fragment (priority 2), the C4-containing fragment (priority 3), and H (priority 4). Here, Cl is on the left (toward you) and H is on the right (toward you) with verticals away. Tracing 1→2→3 and applying the inversion for the toward orientation yields a specific R/S outcome at C3 that matches the overall stereochemical relationship of the molecule.

Option b: 3R, 3S. This option assigns opposite configurations to the same center (which is impossible for a single center), so it cannot describe the two distinct stereocenters consistently if both centers are analyzed under the same projection. The notation 3R, 3S would imply conflicting assignments for the same center, which is not a valid pair for the two stereocenters in this molecule.

Option c: 2S, 3S. This combination asserts that both stereocenters have the same absolute configuration (both S). In a Fischer projection of 2,3-dichloropentane, the symmetry of the molecule and the relative positions of substituents typically yield one center with a configuration opposite to the other (RW-like meso relationships) for the meso form. Since the ends of the chain are identical (CH3 at C1 and C5), a meso arrangement generally corresponds to opposite configurations at C2 and C3, not the same configuration on both.

Option d: 2R, 3R. This option claims both stereocenters have R configuration. Like option c, this would indicate that both centers share the same absolute configuration, which is not characteristic of the meso possibility for this symmetric 2,3-disubstituted system in a typical Fischer projection. The stereochemical relationship here tends to produce one center with one configuration and the other with the opposite, yielding a meso form rather than a pair of identical configurations.

In summary, evaluating the CIP priorities at each center and considering the Fischer projection orientation (horizontal toward, vertical away) leads to a set of configurations that are opposite at C2 and C3 for the meso form, with one center being R and the other S, and not both the same. The options that propose identical configurations for both centers (c and d) are inconsistent with the symmetry and known stereochemical possibilities for 2,3-dichloropentane in this projection, and the option that proposes an impossible single-center inconsistency (b) is invalid. The remaining viable pairings correspond to opposite configurations at C2 and C3, which aligns with the classic meso relationship for 2,3-dichloropentane in this Fischer projection.

My LMS Subjects Practice LMS quiz, Week 1

Consider the Fischer projection of 2,3-dichloropentane below. C2 and C3 are asymmetric centres. What is the configuration (R/S) about C2 and C3?

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