The number of optically active compounds from the following is _________.

Solution

Chemistry ma’am · 3 steps

Do not just count chiral carbons; check each molecule for a mirror plane before you call it active.

1. The test. A compound is optically active only if it is chiral, meaning its mirror image cannot be placed on top of it. A molecule with chiral carbons can still be inactive if it has a plane of symmetry. Such a molecule is called meso.
2. The two symmetric ones. In the Fischer projection of butane-2,3-diol, both OH groups are on the same side. A horizontal plane between C2 and C3 reflects the top half onto the bottom half, so it is meso. Heptane-3,4,5-triol is drawn with no wedges, and its two halves around C4 are identical mirror images. A plane through C4 splits it, so it is also treated as meso.

   Compound	Chiral carbons	Mirror plane?	Optically active?
   butane-2,3-diol (Fischer, both OH on the right)	C2, C3	yes, between C2 and C3	no (meso)
   heptane-3,4,5-triol	C3, C5 (C4 is pseudo-asymmetric)	yes, through C4	no (meso)
   CH₃CH₂CH₂CH₂OH	none	not needed	no
   CH₃CH₂CH(Cl)CH₃	C2 (H, Cl, CH₃, C₂H₅)	no	yes
   CH₃CH₂CH₂CH₂Cl	none	not needed	no
   (CH₃)₂CHCH₂CH₂Cl	none (C3 carries two CH₃ groups)	not needed	no

3. So. Only 2-chlorobutane has a chiral carbon with no internal mirror plane. The number of optically active compounds is 1.

The common slip: 2. Counting the triol or the Fischer diol as active just because each has chiral carbons gives 2 or more. Both have a mirror plane, so neither counts.

The idea to keep. Two or more identical chiral centres arranged symmetrically usually mean a meso compound. Always look for a mirror plane before counting a molecule as optically active.

Answer from the NTA final answer key. Solution by PrepWiser.