Question

How many chiral carbons are there in D-arabinose?

• A. 2
• B. 3
• C. 4
• D. 1

Hint:

Count only the carbons attached to four different groups; check the two end carbons carefully.

Answer 1

The Correct Option is B

Step 1: Write the open chain structure of D-arabinose, an aldopentose: \( CHO-CHOH-CHOH-CHOH-CH_2OH \), numbered C1 to C5.

Step 2: A carbon is a stereocenter only if it is tetrahedral (sp3) and carries four different groups.

Step 3: C1 is the aldehyde carbon (\( CHO \)); it is not tetrahedral, so it cannot be chiral. C5 is \( CH_2OH \); it carries two identical hydrogen atoms, so it is also not chiral.

Step 4: C2, C3 and C4 each carry an \( -OH \), an \( -H \), and two different carbon chain fragments on either side (the chain differs in length and substitution going toward C1 versus toward C5), so each of these three carbons is a genuine stereocenter.

Step 5: Counting C2, C3 and C4 gives the total number of chiral carbons in D-arabinose.

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