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An introduction to the 3D orientation of atoms, focusing on enantiomers, chiral centers, and optical activity.
Why can your right hand never fit perfectly into a left-handed glove, even though they look identical? In chemistry, this 'handedness' is the difference between a life-saving medicine and a dangerous toxin.
At the core of stereochemistry is the chiral center (or stereocenter). This is typically a carbon atom with hybridization that is bonded to four different groups. Because of its tetrahedral geometry, these four groups can be arranged in two distinct ways that are mirror images of each other. We call these molecules chiral. If a molecule has a plane of symmetry, it is achiral (not chiral). In complex rings, you must look 'around the path' of the ring to see if the two directions are different. If the paths are identical, the carbon is not a chiral center.
Consider 2-butanol: . 1. Locate the carbons. , , and are bonded to multiple hydrogens (not 4 different groups). 2. Look at . It is bonded to: a Hydrogen (), a Hydroxyl group (), a Methyl group (), and an Ethyl group (). 3. Since all four groups are unique, is a chiral center.
Quick Check
How many chiral centers are present in a molecule of 2-chloropropane?
Answer
Zero. The central carbon is bonded to two identical methyl groups.
When a molecule has more than one chiral center, the relationships become more complex. Enantiomers are total mirror images; every chiral center is inverted (e.g., becomes ). They have identical physical properties (boiling point, solubility) except for their interaction with polarized light. Diastereomers are stereoisomers that are not mirror images. This happens when at least one, but not all, chiral centers are inverted (e.g., vs ). Unlike enantiomers, diastereomers have different physical properties and can be separated easily in a lab. The maximum number of stereoisomers is calculated as , where is the number of chiral centers.
For a molecule with 3 chiral centers: 1. Calculate the maximum isomers: possible stereoisomers. 2. If you have the isomer, its enantiomer is . 3. Any other combination, like , is a diastereomer of the original.
Quick Check
True or False: Enantiomers can be separated by simple distillation because they have different boiling points.
Answer
False. Enantiomers have identical physical properties like boiling points; diastereomers have different ones.
Chiral molecules possess optical activity, meaning they rotate the plane of plane-polarized light. We measure this using a polarimeter. If a compound rotates light clockwise, it is dextrorotatory (+); counter-clockwise is levorotatory (-). A 50/50 mixture of two enantiomers is called a racemic mixture and shows zero optical activity because the rotations cancel out. This is vital in medicine: often, only one enantiomer of a drug (like Ibuprofen) is active, while the other may be inactive or even harmful.
Which property differs between two enantiomers in a standard laboratory environment?
How many stereoisomers are possible for a molecule with 4 chiral centers?
A racemic mixture is optically inactive.
Review Tomorrow
In 24 hours, try to sketch a molecule with two chiral centers and label the relationship between its and forms.
Practice Activity
Look up the structure of Thalidomide and identify why its stereochemistry led to a medical catastrophe in the 1950s.