how to identify meso compounds
Notice that the terms enantiomer and diastereomer refer to the relationship between two molecules (e.g. If you know the R,S notation, you can use this procedure. “this molecule is meso”). He is currently a chemistry professor at Iowa State University.
R or S) is very crucial in determining whether it is a meso compound or not. The upper right compound has a plane of symmetry passing through the central H & OH groups.
Have questions or comments? The stereochemistry of stereocenters should "cancel out". Notice anything about them? This essentially just means that there is perfect symmetry in the molecule.
The stereochemistry (e.g. For example, can you spot the planes of symmetry in each of the meso compounds shown in the last figure? Beside meso, there are also other types of molecules: enantiomer, diastereomer, and identical. A meso compound contains a plane of symmetry and so is achiral, regardless of whether the molecule has a chiral center.
Any molecule that contains a chiral center will be chiral, with one exception: a meso compound. R or S) is very crucial in determining whether it is a meso compound or not. If 2 chiral centres, R,S = S,R = meso. Remember, even though meso compounds might appear to be chiral because they have stereogenic centers, they are achiral and often the subject of “trick” questions on exams. How to Identify Meso Compounds. Try to see if you can mentally rotate one to fall on top of the other.
It’s often easier for students to understand if we start with an example: Try to draw the enantiomer of this molecule. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Answer key: A C, D, E are meso compounds. Determine if the following molecules are meso.
How to Identify Molecules as Meso Compounds, Organic Chemistry I For Dummies Cheat Sheet, How to Predict the Equilibrium Direction of an Acid-Base Reaction.
For instance, R cancels S out in a meso compound with two stereocenters. As mentioned above, a meso compound is optically inactive, so their stereochemistry should cancel out.
They do not necessarily have to be two stereocenters, but can have more. It is superimposed on its mirror image and is optically inactive despite its stereocenters. Meso compounds can be a bit deceptive.
To do this, we take the mirror image: Now look closely at these two molecules. “these two molecules are enantiomers of one another”); however, meso just refers to a single molecule (e.g. The lower compound has a plane of symmetry passing through the common side of the two benzene rings. In organic chemistry, you need to be able to spot planes of symmetry in molecules so you can determine whether a molecule with chiral centers will be chiral or meso. The two structures on the right are meso compounds, because the top half is a mirror image of the bottom half. Meso compounds are achiral compounds that has multiple chiral centers. Both molecules below are still meso. We want to hear from you. Thus, its is a meso compound. Trans-1,2-dibromocyclopentane, however, is chiral because no plane splits the molecule into two mirror-image halves. Remember, there is free rotation around single bonds. Now look at the mirror images of these two molecules in the second figure to prove this generality to yourself. For example, cis-1,2-dibromocyclopentane (shown in the first figure) is meso because a plane cuts the molecule into two halves that are reflections of each other. 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.
Meso compounds can exist in many different forms such as pentane, butane, heptane, and even cyclobutane. For example, you can’t say two molecules are “mesos” of each other, but you can (and often do) say that two molecules are enantiomers of one another. By definition, a molecule that’s not superimposable on its mirror image is a chiral molecule. As the molecule is rotated, its stereochemistry does not change. This molecule has a plane of symmetry (the horizontal plane going through the red broken line) and, therefore, is achiral; However, it has two chiral carbons and is consequentially a meso compound. These two halves reflect each other by the internal mirror. For instance, R cancels S out in a meso compound with two stereocenters. Here are some other meso compounds with their internal mirror planes shown: If you are ever given a chair conformation, ALWAYS convert it into its 2-D conformation before you do anything involving stereochemistry. It simply means there is no certain direction of rotation of the polarized light, neither levorotatory (-) and dexorotatory (+). Adopted or used LibreTexts for your course? Meso compounds can be a bit deceptive. Despite the presence of stereogenic centers, they are actually achiral. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot.
Molecules with planes of symmetry will always have superimposable mirror images and will be achiral.
If A is a meso compound, it should have two or more stereocenters, an internal plane, and the stereochemistry should be R and S. Tips: An interesting thing about single bonds or sp3-orbitals is that we can rotate the substituted groups that attached to a stereocenter around to recognize the internal plane. Even though the cis compound has two chiral centers (indicated with asterisks), the molecule is achiral because the mirror image is identical to the original molecule (and is, therefore, superimposable on the original molecule). Despite the presence of stereogenic centers, they are actually achiral. The ideal way to identify the line of symmetry is to analyse the compound … He received his PhD at the University of Maryland in 2007. Note that the mirror planes can bisect atoms. The distinctive feature of meso compounds is an internal mirror plane. Remember the internal plane here is depicted on two dimensions. What it means here is that when we have an internal plane that splits the compound into two symmetrical sides, the stereochemistry of both left and right side should be opposite to each other, and therefore, result in optically inactive. In general, a meso compound should contain two or more identical substituted stereocenters.
Any molecule that contains a chiral center will be chiral, with one exception: a meso compound. Also, it has an internal symmetry plane that divides the compound in half. By Arthur Winter . How to Identify Molecules as Meso Compounds; How to Identify Molecules as Meso Compounds. Identify the R and S stereochemistry at each chiral centre. The mirror images of achiral (meso) and chiral molecules. [ "article:topic", "Meso Compounds", "showtoc:no" ]. This vastly simplifies any work involving stereogenic centers. Therefore, this whole section will cover how to identify meso compounds. Try flipping one of the molecules backward or forward, and you’ll see it more clearly. For example: Another case is when we rotate the whole molecule by 180 degree. Cyclic compounds may also be meso. Look for an internal plane, or internal mirror, that lies in between the compound. Click Here For Enantiomer, Diastereomer, and Meso Practice Problems! A plane of symmetry is a plane that cuts a molecule in half, yielding two halves that are mirror reflections of each other. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. When the optical activity of a meso compound is attempted to be determined with a polarimeter, the indicator will not show (+) or (-). On the other hand, the trans stereoisomer has no plane of symmetry and is chiral. They are identical! By continuing to use the service, you agree to our use of cookies. However, in reality, it is three dimensions, so be aware of it when we identify the internal mirror. The line of symmetry (also known as the internal mirror plane) is sneaky in the example above. Therefore, this whole section will cover how to identify meso compounds. trans-1,2-dichloro-1,2-ethanediol (meso)-2,3-dibromobutane It’s often easier for students to understand if we start with an example: Try to draw the enantiomer of this molecule. The stereochemistry (e.g. Arthur Winter is a graduate of Frostburg State University, where he received his BS in chemistry. We use cookies for various purposes including analytics and personalized marketing.
Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. A meso compound contains a plane of symmetry and so is achiral, regardless of whether the molecule has a chiral center.
As mentioned above, a meso compound is optically inactive, so their stereochemistry should cancel out. Compounds that contain chiral centers are generally chiral, whereas molecules that have planes of symmetry are achiral and have structures that are identical to their mirror images.
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