In the Fischer projection of 2-bromobutane, the vertical chain from top to bottom is CH3, the stereocentre C2, then CH2CH3. At C2 the bromine is on the left and the hydrogen on the right. This compound reacts with NaN3 in DMF by a clean SN2 reaction. In the Fischer projection of the product (same vertical chain), the azide group is on the (left / right) and the CIP descriptor of the product is (R / S).

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In the Fischer projection of 2-bromobutane, the vertical chain from top to bottom is $CH X_{3}$ , the stereocentre C2, then $CH X_{2} CH X_{3}$ . At C2 the bromine is on the left and the hydrogen on the right. This compound reacts with $NaN X_{3}$ in DMF by a clean SN2 reaction. In the Fischer projection of the product (same vertical chain), the azide group is on the (left / right) and the CIP descriptor of the product is (R / S).

Aright; $R$
Bright; $S$
Cleft; $R$
Dleft; $S$

Answer: B

The substrate is

(R)

-2-bromobutane (priorities Br, ethyl, methyl, H; Br on the left, ethyl at the bottom, methyl at the top, H on a horizontal bond: the counter-clockwise sequence seen from the front is reversed to clockwise, so

R

). Inversion is the mirror-image arrangement, so the azide is on the right and H on the left. Since

N X_{3} > C X_{2} H X_{5} > CH X_{3} > H

, the order is unchanged and the product is

(S)

-2-azidobutane.

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Answer: B

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