Unit 4: Stereochemistry
The three-dimensional shape of a molecule, not just its bonds: chirality, mirror images, and the rare case where both collapse into one.
8 topics · 27 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Chirality and Stereocenters
- How it worksHow does a carbon bonded to four different groups become a molecule's stereocenter?
- Worked exampleFind every stereocenter in 3-chloro-2-butanol, CH3CHClCHOHCH3.
- Common mistakeWhy isn't a molecule with a stereocenter automatically guaranteed to be chiral overall?
- Going furtherWhat happens to a molecule's chirality when one of the four different groups on its stereocenter becomes a duplicate?
Your turn1 of 2
How many stereocenters does 3-chloro-2-butanol, CH3CHClCHOHCH3, have, and which carbons are they?Assigning R and S Configuration
- How it worksHow do you rank the four groups on a stereocenter by priority and read off R or S from their arrangement?
- Worked exampleFind the R or S configuration of the stereocenter in bromochlorofluoromethane, CHBrClF.
- Worked exampleFind the R or S configuration of the stereocenter in 2-butanol, CH3CH(OH)CH2CH3.
Your turn1 of 2
A stereocenter's lowest-priority group points away from the viewer. Looking from the side opposite it, the remaining three groups go from highest to middle to lowest priority in a clockwise direction. What is this stereocenter's configuration?Enantiomers and Optical Activity
- How it worksHow does a chiral molecule rotate the plane of polarized light as it passes through a solution of it?
- Worked exampleFind the specific rotation of a compound if a 2 g/mL solution in a 10 cm tube rotates light by 20 degrees.
- Common mistakeWhy doesn't a pair of enantiomers have different melting points or boiling points from one another?
- Going furtherWhat happens to the optical rotation you measure when equal amounts of two enantiomers get mixed together?
Your turn1 of 2
A solution with concentration 2.94 g/mL is placed in a polarimeter tube 15 cm long. The observed rotation is 56 degrees. Using specific rotation = observed rotation / (path length in dm times concentration in g/mL), what is the compound's specific rotation?Diastereomers
- How it worksHow does matching some stereocenters while flipping others make two molecules diastereomers instead of enantiomers?
- Worked exampleFind the relationship between (2R,3R)-tartaric acid and (2R,3S)-tartaric acid: enantiomers or diastereomers.
- Common mistakeWhy aren't diastereomers just a weaker, watered-down version of being enantiomers?
Your turn1 of 2
What is the relationship between (2R,3R)-tartaric acid and (2R,3S)-tartaric acid?Meso Compounds
- How it worksHow does an internal mirror plane make one half of meso-tartaric acid cancel the other half's rotation?
- Worked exampleFind whether the (2R,3S) stereoisomer of 2,3-dibromobutane is chiral or meso.
- Common mistakeWhy doesn't having two stereocenters guarantee that a molecule like meso-tartaric acid must be chiral?
- Common mistakeWhy isn't meso-tartaric acid simply a 50/50 mix of the two tartaric acid enantiomers?
Your turn1 of 2
2,3-dibromobutane has two stereocenters that carry identical substituent sets, since the molecule is symmetric end-to-end. Is the (2R,3S) stereoisomer chiral or meso?Fischer Projections
- How it worksHow do the horizontal and vertical lines of a Fischer projection encode which groups point toward you and away?
- Worked exampleFind the R or S configuration directly from the Fischer projection of D-glyceraldehyde.
- Common mistakeWhy can't you rotate a Fischer projection 90 degrees in the plane of the page without changing what it means?
Your turn1 of 2
In the Fischer projection of D-glyceraldehyde, CHO is on top, OH is on the right, H is on the left, and CH2OH is on the bottom. What is this stereocenter's configuration?Racemic Mixtures and Resolution
- How it worksHow does reacting a racemic mixture with one pure enantiomer of a resolving agent split it into separable diastereomers?
- Worked exampleFind the enantiomeric excess of a sample that is 80 percent one enantiomer and 20 percent the other.
- Going furtherWhat happens to a drug's effect on the body when only one enantiomer in a racemic mixture is biologically active?
Your turn1 of 2
A sample contains 84 percent of one enantiomer and the rest as the other enantiomer. What is the sample's enantiomeric excess, in percent?Molecules with Multiple Stereocenters
- How it worksHow does the 2 to the n rule count the maximum number of stereoisomers for a molecule with n stereocenters?
- Worked exampleFind the maximum number of stereoisomers possible for a compound with four independent stereocenters.
- Ties togetherHow does spotting multiple stereocenters connect to predicting the exact stereochemistry SN2 and SN1 leave behind?
Your turn1 of 2
A molecule has 4 independent stereocenters, with no internal symmetry that would collapse any stereoisomers into meso compounds. What is the maximum number of stereoisomers it can have?