Quiz - Stereoisomers - 3 | Stereoisomers
Organic Chemistry 1 - Quiz - Stereoisomers - 3
Which of the following statements about chirality is incorrect?
The statement that a molecule with two stereocenters is always chiral is incorrect. While a molecule with two stereocenters can be chiral, it also can be achiral if it has an internal plane of symmetry, making it a meso compound.
When assigning absolute R/S configuration, which of the following factors is not considered?
Electronegativity is not considered when assigning the absolute configuration (R or S) to a stereocenter. The priority is determined by the atomic number of the atoms directly attached to the chiral center, and if there's a tie, by the atomic number of atoms further away, according to the Cahn-Ingold-Prelog rules.
How do enantiomers and diastereomers differ in terms of their physical properties?
Enantiomers are mirror images of each other and have nearly identical physical properties (such as boiling point, melting point, solubility), but they differ in the direction in which they rotate plane-polarized light. Diastereomers, which are not mirror images, have different physical and chemical properties.
Which property can be used to distinguish between a mixture of enantiomers and a racemic mixture?
Chromatography using a chiral stationary phase can distinguish between enantiomers due to their different interactions with the chiral environment, whereas other methods like melting point analysis, mass spectrometry, and infrared spectroscopy generally cannot differentiate between enantiomers or assess the composition of a racemic mixture.
In specific rotation values, a positive rotation indicates:
A positive specific rotation value indicates dextrorotary rotation, meaning the substance rotates polarized light in a clockwise direction.
What is the relationship between the components of a racemic mixture?
A racemic mixture consists of an equal amount (1:1 ratio) of a pair of enantiomers. They are mirror images of each other and not superimposable, which makes them optically active in opposite directions. The net optical activity of a racemic mixture is zero.
In an R-configured chiral center, what is the direction of rotation?
In an R-configured chiral center, the direction of rotation is clockwise.
What is the significance of enantiomeric excess (ee) in stereoisomeric mixtures?
Enantiomeric excess (ee) is significant because it indicates the purity of one enantiomer in a mixture, expressed as a percentage. It is calculated based on the optical activity and shows the excess of one enantiomer over an equal mixture of enantiomers (a racemic mixture).
In the context of optical activity, what is one main feature that distinguishes a racemic mixture from a meso compound?
Both racemic mixtures and meso compounds are optically inactive; however, the reason for this differs. A meso compound is optically inactive because its internal symmetry leads to compensation, whereas a racemic mixture's optical inactivity is due to the external cancellation of two enantiomers' rotation.
Which optical property of enantiomers can be quantified using a polarimeter?
A polarimeter measures the specific rotation of chiral molecules, which is a quantifiable property of enantiomers. The specific rotation helps to determine the degree to which a compound rotates plane-polarized light and can aid in identifying and characterizing the compound.