Stereochemistry Flashcards
What is specific rotation? How can it be used to distinguish two compounds?
In chemistry, specific rotation ([α]) is a property of a chiral chemical compound. It is defined as the change in orientation of monochromatic plane-polarized light, per unit distance–concentration product, as the light passes through a sample of a compound in solution.
It can distinguish between two optically active compounds (with differing specific rotation), or one optically active compound and one non optically active compound.
What is a enantiomer?
Enantiomers are two stereoisomers that are related to each other by a reflection: They are mirror images of each other, which are non-superimposable. Human hands are a macroscopic analog of stereoisomerism. Every stereogenic center in one has the opposite configuration in the other. Two compounds that are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. As a result, different enantiomers of a compound may have substantially different biological effects. Pure enantiomers also exhibit the phenomenon of optical activity and can be separated only with the use of a chiral agent. In nature, only one enantiomer of most chiral biological compounds, such as amino acids (except glycine, which is achiral), is present.
What is a diastereomer?
Diastereomers (sometimes called diastereoisomers) are stereoisomers that are not enantiomers. Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocenters and are not mirror images of each other.
Diastereomers are stereoisomers not related through a reflection operation. They are not mirror images of each other. These include meso compounds, cis–trans (E-Z) isomers, and non-enantiomeric optical isomers. Diastereomers seldom have the same physical properties. In the example shown below, the meso form of tartaric acid forms a diastereomeric pair with both levo and dextro tartaric acids, which form an enantiomeric pair.
What are meso compounds?
A meso compound is an achiral compound that has chiral centers. It is superposable on its mirror image and is optically inactive although it contains two or more stereocenters.
What is a conformer?
A conformer usually results from the rotation of a sigma bond. Pi bonds cannot rotate and therefore only single C-C bonds can have conformers.
What are gauche, antistaggered and eclipsed conformers?
Gauche: Largest groups are staggered (not in same plane), but still next to each other.
Anti-staggered: Largest groups are maximally staggered
Eclipse: Largest groups overlap
True or false? Diastereomers only include stereoisomers with chiral centres.
False!
Diastereomers can also refer to cis/trans alkenes.
How do you know what direction a chiral compound rotates plane polarized light?
You cannot tell by S or R designation alone! You must measure the rotation by experimentation to know.
When known, rotation is given by:
Clockwise: d/+
Counterclockwise: l/+
