What a learner can do afterwards
- Counts the maximum number of stereoisomers for a molecule and explains when the count falls short
- Classifies a pair as enantiomers or diastereomers and predicts which pair differs in melting point
- Identifies a meso compound and explains why an internal mirror plane cancels the rotation
- Converts between a wedge and dash drawing and a Fischer projection without losing the assignment
1 · Read
With more than one stereocentre the mirror image rule stops being the whole story. A molecule with n stereocentres can have at most 2 to the power n stereoisomers. Never quote the maximum before checking for duplicates, because symmetry can make some arrangements identical. Pairs split into two camps. Enantiomers mirror at every centre, while diastereomers differ at some centres but not all.
Only diastereomers differ in melting point, boiling point and most spectra. Enantiomers share every scalar property. So a melting point gap proves you hold diastereomers, not enantiomers. That single test sorts many pairs, and it decides whether two samples can be separated by ordinary means like crystallisation.
Tartaric acid shows the great trick. One form carries stereocentres yet is achiral, because an internal mirror plane makes one half the mirror of the other and the rotations cancel. That is a meso compound. It sits beside a true enantiomer pair. Hunt for the plane of symmetry in a flat drawing. Top reflecting perfectly into bottom means meso.
Read a Fischer cross correctly. Vertical bonds sink back and horizontal bonds reach forward. To convert from wedges and dashes, fix the carbon backbone upright first, then lay left and right groups on the horizontal arms without swapping sides. One accidental swap inverts that centre, while two swaps restore it.
Count 2 to the power n, cut for symmetry, test melting points for diastereomers, and read Fischer crosses without swapping.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.