Some structures, be they chemical structures or everyday objects, are not superimposable upon their mirror images. Common examples are your hands and threaded bolts. Other objects, such as marbles or plain white coffee cups, can be perfectly superimposed upon their mirror image. These objects have a plane of symmetry: one half of the object is the mirror image of the other.
An object (molecule) that is not superimposable on its mirror image is said to be chiral. It does not possess a plane of symmetry. Structures (molecules) with a plane of symmetry are superimposable upon their mirror images; they are achiral.
Most chiral organic molecules contain one or more stereogenic centers. These are carbon atoms that are bonded to 4 different groups. Examples of chiral compounds, each with a stereogenic center, are shown on the next page:
The nonsuperimposable mirror images are called enantiomers. A mixture containing amounts of each enantiomer is a racemic mixture (a racemate). Resolution is the separation of a racemic mixture into individual enantiomers. Enantiomers are a form of isomers called stereoisomers. Stereoisomers have the same bonding order of atoms but differ in the way these atoms are arranged in space. Stereoisomers that are not mirror images are called diastereomers.
Plane-polarized light (light vibrating in only one plane) passed through a chiral substance emerges vibrating in a different plane. The enantiomer that rotates the plane of polarized light clockwise (to the right) as seen by an observer is dextrorotatory; the enantiomer rotating to the left is levorotatory. The symbols (+) and (–) designate rotation to the right and left, respectively. Because of this optical activity, enantiomers are called optical isomers. A racemic mixture (±) is optically inactive since it does not produce a net rotation of polarized light; the effects of the two enantiomers cancel each other. The specific rotation [α] is an inherent physical property of an enantiomer which depends on the solvent used, temperature, and wavelength of light used. It is defined as the observed rotation per unit length of light path per unit concentration (for a solution) or density (for a pure liquid) of the enantiomer; thus,
Relative and Absolute Configuration
Configuration is the 3-dimensional (spatial) arrangement of groups in a stereoisomer. Enantiomers have opposite configurations. For a compound with one stereogenic center to be converted into its enantiomer, bonds must be broken. Configurations may change as a result of chemical reactions. Because stereochemical changes often occur in reactions, it is vital to assign configurations. The sign of rotation cannot be used because there is no relationship between configuration (spatial arrangement) and sign of rotation.
The Cahn-lngold-Prelog rules are used to designate the configuration of each chiral C in a molecule in terms of the symbols R and S. These symbols come from the Latin: R from rectus (right) and S from sinister (left).
Step 1: Groups on the chiral C are assigned priorities based on atomic number of the atom bonded directly to the C, with higher priority being given to larger atomic numbers.
Step 2: If the first atom is the same in two or more groups, the priority is determined by comparing the next atoms in each of these groups. Thus, ethyl (—CH2CH3), with one C and two H's on the first bonded C, has priority over methyl (—CH3), with three H's on the C.
Step 3: When evaluating the priorities, a double bond counts like two bonds to that element, and a triple bond counts like three bonds to the given atom. For example, for a C=C double bond:
Step 4: Once priorities have been assigned, arrange the molecule so that the lowest-priority group projects behind the plane of the paper, leaving the other three groups projecting forward. Then, for the remaining three groups, if the sequence of decreasing priority, (1) to (2) to (3), is counterclockwise, the configuration is designated S; if it is clockwise, the configuration is designated R. The rule is illustrated for 1-chloro-1- bromoethane below. Both configuration and sign of optical rotation are included in the complete name of a species, e.g., (S)-(+)-1-chloro-1- bromoethane.
We assign priority numbers to produce:
Arranging the molecule to put the lowest priority group behind the plane of the paper:
The resulting arrangement shows a counterclockwise sequence, and therefore this molecule has the S absolute configuration.
Molecules with Multiple Stereogenic Centers
Molecules containing n stereogenic centers can exist as a maximum of 2n stereoisomers. For example, there are 4 possible stereoisomers of 2,3-dihydroxypentane.
In this example, isomers I and II are enantiomers, and III and IV are enantiomers. The relationship between I and III, as well as between I and IV, is that they are diastereomers. Diastereomers are stereoisomers that are not enantiomers.
While it may appear that 2,3-dihydoxybutane can be drawn as 4 different isomers, closer inspection reveals that 2 of these are identical.
Rotation around the central C–C bond reveals that VI and VII have a plane of symmetry; thus they are therefore achiral and are identical. This type of isomer, an achiral diastereomer, is known as a meso compound.
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