A sawhorse projection of 2-bromobutane looks along the C2-C3 bond. The front carbon is C2: its three bonds point up (12 o'clock) to CH3, lower-left (8 o'clock) to Br and lower-right (4 o'clock) to H. The rear carbon is C3 (an ethyl group continues from it). What is the descriptor at C2?

cannot be assigned from a conformation

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A sawhorse projection of 2-bromobutane looks along the C2-C3 bond. The front carbon is C2: its three bonds point up (12 o'clock) to $CH X_{3}$ , lower-left (8 o'clock) to Br and lower-right (4 o'clock) to H. The rear carbon is C3 (an ethyl group continues from it). What is the descriptor at C2?

AR
Bcannot be assigned from a conformation
CZ
DS

Answer: A

The priorities at C2 are Br, then C3 (ethyl), then

CH X_{3}

, then H. The rear carbon C3 points away, so exchange H and C3: H is then directed away and C3 sits at 4 o\'clock. Br (8 o\'clock) to C3 (4 o\'clock) to

CH X_{3}

(12 o\'clock) runs anticlockwise, i.e. S for the exchanged structure. One exchange inverts the configuration, so the real C2 is R. Conformation does not change the descriptor.

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A sawhorse projection of 2-bromobutane looks along the C2-C3 bond. The front carbon is C2: its three bonds point up (12 o'clock) to CHX3​, lower-left (8 o'clock) to Br and lower-right (4 o'clock) to H. The rear carbon is C3 (an ethyl group continues from it). What is the descriptor at C2?

Answer: A

More questions on Fischer, Newman and sawhorse projections

A sawhorse projection of 2-bromobutane looks along the C2-C3 bond. The front carbon is C2: its three bonds point up (12 o'clock) to $CH X_{3}$ , lower-left (8 o'clock) to Br and lower-right (4 o'clock) to H. The rear carbon is C3 (an ethyl group continues from it). What is the descriptor at C2?