Mirror twins and how to label them

Science · Chemistry · ages 18-20

Name ______________________ Date ____________

With the lowest priority hidden behind, the trace 1 to 2 to 3 runs clockwise. What is the label?
- R
- S
- Neither, redraw first
What makes a carbon a stereocentre?
- Any carbon with a double bond
- A carbon with four different groups
- Any carbon inside a ring
A racemic mixture shows a strong net rotation in a polarimeter.
Circle one: True False
Two attachments tie at the first atom and one has a double bond. How do you break the tie?
- Rank by group size instead
- Duplicate the atoms to break the tie
- Give up and call it achiral
You trace R with the lowest priority pointing toward you. What is the true label?
- Keep the R you first read
- Swap R and S at random
- Flip it: the true label is S
A pure enantiomer rotates 40 units. A sample rotates 20. What is the enantiomeric excess?
Answer: ______________
Two pure enantiomers go through the polarimeter one after another. What do you see?
- Identical readings, since spectra match
- Zero for both, since they cancel
- Equal but opposite rotations
A student finds no stereocentres in a chiral ring compound. How many did the student most likely miss?
- One, hiding inside the ring
- Zero, the molecule is achiral
- Three, counting every carbon

Answer key

For grown-ups. Fold this page away before handing over the rest.

R · Clockwise 1 to 2 to 3 spells R.
A carbon with four different groups · Four distinct attachments make the mirror image non-superimposable.
False · A 50:50 mix cancels exactly and reads zero.
Duplicate the atoms to break the tie · Duplicated atoms let atomic numbers settle double bond ties.
Flip it: the true label is S · Reading from the front reverses the trace direction.
50 · Half the pure rotation means 50 percent excess of one twin.
Equal but opposite rotations · Twins match in everything except the sign of rotation.
One, hiding inside the ring · Rings hide stereocentres from hasty counters.