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: D-arabinose is an aldopentose, a five carbon sugar with the general open chain skeleton $CHO-(CHOH)_3-CH_2OH$.

Step 2: For any straight chain aldose with $n$ carbons, the terminal carbons are fixed: C1 is the carbonyl carbon (never chiral) and the last carbon is a $CH_2OH$ group with two identical hydrogens (never chiral).

Step 3: All the interior carbons, from C2 through C(n minus 1), each carry one $OH$, one $H$, and two different chain segments, so each interior carbon is a stereocenter. The number of such interior carbons is $n - 2$.

Step 4: For arabinose, $n = 5$, so the number of chiral carbons is $5 - 2 = 3$, corresponding to C2, C3 and C4.

\[ \boxed{5-2=3} \]