When we do that, the left and right sides are mirror images of each other, so the smiley face has an internal mirror plane or a plane of symmetry. What are the Similarities Between Achiral and Meso – Outline of Common Features 4. A meso compound contains a plane of symmetry and so is achiral, regardless of whether the molecule has a chiral center. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. This means that despite containing two or more stereogenic centers, the molecule is not chiral. Key Terms: Achiral, Chemical Formula, Chiral, Chiral Center, Chirality, Inversion Center, Meso, Mirror Image, Molecular Formula. In Organic Chemistry, a meso compound is a compound with two or more stereocentres that has an internal plane of symmetry. Look for an internal plane, or internal mirror, that lies in between the compound. A plane of symmetry is a plane that cuts a molecule in half, yielding two halves that are mirror reflections of each other. For example, in 2,4-pentanediol, both the second and fourth carbon atoms, which are stereocenters, have all four substituents in common. They are part of the IUPAC nomenclature used to specify the position of any non-hydrogen substituents in an aromatic hydrocarbon. The stereochemistry of stereocenters should "cancel out". It is seen in acridines and calixarenes. For example: Upper Saddle River NJ. These two halves reflect each other by the internal mirror. Meso compounds are achiral compounds that has multiple chiral centers. It is superimposed on its mirror image and is optically inactive despite its stereocenters. W.H. There are many structures and objects around us that are symmetric, like buildings, houses, and even our faces. [ "article:topic", "Meso Compounds", "showtoc:no" ]. The meso compound must not be confused with a 50:50 racemic mixture of the two optically-active compounds, although neither will rotate light in a polarimeter. As the molecule is rotated, its stereochemistry does not change. This internal plane cuts the molecule into two halves, each of which is a mirror image of the other. Meso means that a molecule has an internal plane of symmetry. A meso compound is "superposable" on its mirror image (not to be confused with superimposable, as any two objects can be superimposed over one another regardless of whether they are the same). Tacticity (from Greek τακτικός taktikos "of or relating to arrangement or order") is the relative stereochemistry of adjacent chiral centers within a macromolecule. Of the four colored pictures at the top of the diagram, the first two represent the meso compound (the 2R,3S and 2S,3R isomers are equivalent), followed by the optically active pair of levotartaric acid (L-(R,R)-(+)-tartaric acid) and dextrotartaric acid (D-(S,S)-(-)-tartaric acid).  At low temperatures, however, this is not the case, as the activation energy for the ring-flip cannot be overcome, and they therefore behave like enantiomers. 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. Two objects can be superposed if all aspects of the objects coincide and it does not pro… The prefixes ipso-, meso-, and peri- describe ring substitutions in organic chemistry. A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active. Missed the LibreFest? 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. When the optical activity of a meso compound is attempted to be determined with a polarimeter, the indicator will not show (+) or (-). A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active. It has an internal plane of symmetry and we can superimpose it on its own mirror image. 1,2-substituted cyclopropane has a meso cis-isomer (molecule has a mirror plane) and two trans-enantiomers: The two cis stereoisomers of 1,2-substituted cyclohexanes behave like meso compounds at room temperature in most cases. However, in reality, it is three dimensions, so be aware of it when we identify the internal mirror. It is superimposed on its mirror image and is optically inactive although it contains two or more stereocenters. Answer key: A C, D, E are meso compounds. It is superimposed on its mirror image and is optically inactive despite its stereocenters. Two objects can be superposed if all aspects of the objects coincide and it does not produce a "(+)" or "(-)" reading when analyzed with a polarimeter.. Ultraviolet–visible spectroscopy of stereoisomers, https://en.wikipedia.org/w/index.php?title=Meso_compound&oldid=954163065, Creative Commons Attribution-ShareAlike License, This page was last edited on 1 May 2020, at 00:40. Beside meso, there are also other types of molecules: enantiomer, diastereomer, and identical. Remember the internal plane here is depicted on two dimensions. For more information contact us at firstname.lastname@example.org or check out our status page at https://status.libretexts.org.