Organic Chemistry 1 · Stereocenters and configuration
Assigning the drawn acid
Conditions: Fischer convention: horizontal bonds point toward the viewer; vertical bonds point away. Condensed terminal groups retain the indicated connectivity.
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The question
Assign the configuration at the central carbon in the displayed lactic-acid projection.
Reveal the answer and explanation
Answer and explanation
Correct answer S.
OH > CO₂H > CH₃ > H; the clockwise page order is inverted because H points toward the viewer.
Not this one R.
This is the uncorrected page result; priority 4 is horizontal rather than pointing away.
Not this one No descriptor is possible because CO₂H and CH₃ both start with carbon.
Their next-atom lists distinguish the carbon ligands.
Worked solution
O in OH gives priority 1. The carboxyl carbon compares as [O,O,O], beating methyl [H,H,H], giving priorities 2 and 3. H is priority 4. Since H is horizontal, invert the clockwise page trace to obtain S.
What you'll practice
Use the Fischer viewing convention to obtain an absolute descriptor.
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