The compound shown is but-3-yn-2-ol drawn with wedges. Which option gives its configuration together with the correct ranking of the two carbon substituents?

A $(S)$ ; ethynyl ranks above methyl because its carbon is attached to (C,C,C)
B $(R)$ ; methyl ranks above ethynyl because it has more hydrogens
C $(R)$ ; ethynyl ranks above methyl because its carbon is attached to (C,C,C)
D $(S)$ ; methyl ranks above ethynyl because it is saturated

Answer: A

At C2 the priorities are

OH

first. The ethynyl carbon is triply bonded to carbon, so with duplicate atoms it is attached to (C,C,C), whereas methyl carbon is attached to (H,H,H); ethynyl therefore ranks above methyl.

H

is last. In the drawing, with

H

away from the viewer,

OH \to C \equiv CH \to CH X_{3}

runs anticlockwise, so the configuration is

(S)

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The compound shown is but-3-yn-2-ol drawn with wedges. Which option gives its configuration together with the correct ranking of the two carbon substituents?

Answer: A

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