Our hands are said to be chiral objects because they are mirror-images of one another and are nonsuperimposable. In a similar way, molecules are said to be chiral if they have a nonsuperimposable mirror image known as the enantiomer. Optical Activity and Optically Active Molecules Visible light is composed of regions of oscillating electric and magnetic fields that are perpendicular with respect to one another.
Although light produced by the sun exists in its unpolarized form, we can polarize the light using a special device. This means that the polarizer device screens or blocks out all the electric and magnetic fields except the one that moves along one direction.
This light is called plane polarized light. The interesting thing about plane polarized light is that chiral molecules have the ability to rotate the plane-polarized light some amount of degrees when the light travels through that molecule. A chiral molecule that rotates light is said to be optically active.
If the molecule rotates the plane-polarized light in the clockwise direction, we call that rotation a dextrorotatory rotation. However, if the light rotates counterclockwise, we call that rotation levorotatory. Racemic Mixtures Recall that a chiral molecule is a molecule that contains two nonsuperimposible mirror-images of one another, each called an enantiomer. A racemic mixture is defined as a mixture of equal amounts of the two enantiomer forms of a given chiral molecule.
One interesting aspect about a racemic mixture is that it is optically inactive, meaning it does not rotate plane polarized light. Actually to be more technical, it does rotate light but it rotates it in both directions by equal degrees and so the net rotation is zero.
Absolute Configuration The absolute configuration gives us a way to differentiate between the two types of enantiomers of any chiral molecule. The first step in determining the absolute configuration is to determine where the stereogenic carbon is.
The stereogenic carbon is the carbon that is attached to four different groups. Once you have determine the stereogenic carbon, you're next step is to assign the priority for the four different groups attached to the stereogenic carbon using the cahn-ingold-prelog system.
If the top groups create an arrow that points counterclockwise, then the enantiomer has the S absolute configuration. On the other hand, if the arrow points clockwise, then the enantiomer has the R configuration. Absolute Configuration Examples In these several examples, our goal will be to determine the absolute configuration of the chiral molecule by locating the stereogenic carbon and using the Cahn-Ingold-Prelog priority system to prioritize the four different groups.
The highest priority group is given a 1 while the lower priority group is given a 4. Chemical Differences between Enantiomers One common physical difference between any two enantiomers of a given chiral molecule is the ability to rotate plain polarized light.
Remember, when one enantiomer rotates plane polarized light in one direction, the second enantiomer rotates plane polarized light in the opposing direction. Meso compounds are achiral optically inactive diastereomers of chiral stereoisomers.
Investigations of isomeric tartaric acid salts, carried out by Louis Pasteur in the mid 19th century, were instrumental in elucidating some of the subtleties of stereochemistry. Some physical properties of the isomers of tartaric acid are given in the following table. Fischer projection formulas provide a helpful view of the configurational relationships within the structures of these isomers. In the following illustration a mirror line is drawn between formulas that have a mirror-image relationship.
Beside meso, there are also other types of molecules: enantiomer , diastereomer , and identical. Determine if the following molecules are meso. Skip to main content. Chapter 4: Isomers. Search for:. Key Term Make certain that you can define, and use in context, the key term below. Introduction In general, a meso compound should contain two or more identical substituted stereocenters.
Identification If A is a meso compound, it should have two or more stereocenters, an internal plane, and the stereochemistry should be R and S. Look for an internal plane, or internal mirror, that lies in between the compound. The stereochemistry e. R or S is very crucial in determining whether it is a meso compound or not. As mentioned above, a meso compound is optically inactive, so their stereochemistry should cancel out.
For instance, R cancels S out in a meso compound with two stereocenters. Show Answer 1 has a plane of symmetry the horizonatal plane going through the red broken line and, therefore, is achiral; 1 has chiral centers. Thus, 1 is a meso compound. Show Answer This molecules has a plane of symmetry the vertical plane going through the red broken line perpendicular to the plane of the ring and, therefore, is achiral, but has has two chiral centers.
Thus, its is a meso compound. Other Examples of meso compounds Meso compounds can exist in many different forms such as pentane, butane, heptane, and even cyclobutane. Achiral Diastereomers meso-Compounds The chiral centers in the preceding examples have all been different.
Examples Problems Beside meso, there are also other types of molecules: enantiomer , diastereomer , and identical.
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