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Meso compoundFrom Wikipedia, the free encyclopedia Jump to: navigation, search 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.[1][2] This means that despite containing two or more stereocenters (chiral centers) it is not chiral. A meso compound is superimposable on its mirror image, and it does not produce a "(+)" or "(-)" reading when analyzed with a polarimeter.[3] For example, there are 3 isomers of tartaric acid (depicted below), there is a meso compound (the 2R,3S and 2S,3R isomers are equivalent) and the optically active pair of levotartaric acid (D(S,S)-()-tartaric acid) and dextrotartaric acid (L-(R,R)-(+)-tartaric acid). In the meso compound an internal plane of symmetry exists, bisecting the molecule which is not present in the non-meso compounds. That is, on rotating the meso compound by 180 on a plane perpendicular to the screen, the same stereochemistry is obtained, again this is not seen in the non-meso tartaric acid. (seeFischer projection).[3]

It is a requirement for two of the stereocenters in a meso compound to have at least two substituents in common (though having this characteristic does not necessarily mean that the compound is meso). For example, in 2,4-pentanediol, both the second and fourth carbons, which are stereocenters, have all four substituents in common. Since a meso isomer has a superimposable mirror image, a compound with a total of n stereocenters cannot have 2n stereoisomers if at least one of the stereoisomers is meso.[4]

[edit] Cyclic meso compounds1,2-substituted cyclopropane has a meso cis-isomer (molecule has a mirror plane) and two transenantiomers:

The two cis stereoisomers of 1,2-substituted cyclohexanes behave like meso compounds at room temperature in most cases. At room temperature, most 1,2-disubstituted cyclohexanes undergo rapid ring flipping (exceptions being rings with bulky substituents), and as a result, the two cis stereoisomers behave chemically identically with chiral reagents. 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. In nearly all cases at room temperature, the two cis stereoisomers of 1,2-disubstituted cyclohexanes can be treated as chemically equivalent. [5] Also noteworthy is the fact that when a cyclohexane undergoes a ring flip, the absolute configurations of the sterocenters do not change.

DiastereomerFrom Wikipedia, the free encyclopedia (Redirected from Diasteromer) Jump to: navigation, search

Erythro redirects here. For the fictional planet, see Erythro (Asimov). Diastereomers (diastereoisomers) are stereoisomers that are not enantiomers.[1] Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocentersand are not mirror images of each other.[2] When two diastereoisomers differ from each other at only one stereocenter they are epimers. Each stereocenter gives rise to two different configurations and thus to two different stereoisomers.

Diastereomers

Diastereomers differ from enantiomers in that the latter are pairs of stereoisomers which differ in all stereocenters and are therefore mirror images of one another.[3] Enantiomers of a compound with more than one stereocenter are D-Threose D-Erythrose also diastereomers of the other stereoisomers of that compound that are not their mirror image. Diastereomers have different physical properties and different reactivity, unlike enantiomers. Cis-trans isomerism and conformational isomerism are also forms of diastereomerism. Diastereoselectivity is the preference for the formation of one or more than one diastereomer over the other in an organic reaction.

Contents[hide]y y y y y y

1 Erythro / threo 2 Multiple stereocenters 3 Example 4 Applications 5 See also 6 References

[edit] Erythro / threo

Two common prefixes used to distinguish diastereomers are threo and erythro. When drawn in the Fischer projection the erythro isomer has two identical substituents on the same side and the threo isomer has them on opposite sides. The names are derived from the diastereomeric aldoses erythrose(a syrup) and threose (melting point 126 C). Another threo compound is threonine, one of the essential amino acids. The erythro diastereomer is called allo-threonine.

L-Threonine (2S,3R) and D-Threonine (2R,3S)

L-allo-Threonine (2S,3S) and D-allo-Threonine (2R,3R)

[edit] Multiple stereocentersIf a molecule contains two asymmetric carbons, there are up to 4 possible configurations, and they cannot all be non-superimposable mirror images of each other. The possibilities continue to multiply as there are more asymmetric centers in a molecule. In general, the number of configurational isomers of a molecule can be determined by calculating 2n, where n = the number of chiral centers in the molecule. This holds true except in cases where the molecule has meso forms.

[edit] ExampleTartaric acid contains two asymmetric centers, but two of the "isomers" are equivalent and together are called a meso compound. This configuration is not optically active, while the remaining two isomers are D- and L- mirror images, i.e., enantiomers. The meso form is a diastereomer of the other forms.

(natural) tartaric acid L-(+)-tartaric acid dextrotartaric acid

D-(-)-tartaric acid

mesotartaric acid

levotartaric acid

(1:1) DL-tartaric acid "racemic acid" The families of 4, 5 and 6 carbon carbohydrates contain many diastereomers because of the large numbers of asymmetric centres in these molecules.

[edit] ApplicationsAs stated, two enantiomers will have identical physical properties, while diastereomers will not. This knowledge is harnessed in chiral synthesis to separate a mixture of enantiomers. This is the principle behind chiral resolution. After preparing the diastereomers, they are separated by chromatography or recrystallization.

Cistrans isomerismFrom Wikipedia, the free encyclopedia (Redirected from Cis-trans isomerism) Jump to: navigation, search

Cis-2-butene

Trans-2-butene

In organic chemistry, cis-trans isomerism or geometric isomerism or configuration isomerism or E-Z isomerism is a form of stereoisomerism describing the orientation of functional groups within a molecule. In general, such isomers contain double bonds, which cannot rotate, but they can also arise from ring structures, wherein the rotation of bonds is greatly restricted. Cis and transisomers occur both in organic molecules and in inorganic coordination complexes. The terms cis and trans are from Latin, in which cis means "on the same side" and trans means "on the other side" or "across". The term "geometric isomerism" is considered an obsolete synonym of "cis-trans isomerism" by IUPAC.[1] It is sometimes used as a synonym for general stereoisomerism (e.g., optical isomerism being called geometric isomerism); the correct term for non-optical stereoisomerism is diastereomerism.

Contents[hide]y

1In organic chemistry

y y y y

1.1Comparison of physical properties 1.1.1Stability o 1.2E/Z notation 2Inorganic coordination complexes 3See also 4References 5External linkso

[edit]In organic chemistryWhen the substituent groups are oriented in the same direction, the diastereomer is referred to as cis, whereas, when the substituents are oriented in opposing directions, the diastereomer is referred to as trans. An example of a small hydrocarbon displaying cis-trans isomerism is 2butene. Alicyclic compounds can also display cis-trans isomerism. As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane:

trans-1,2-dichlorocyclohexane

cis-1,2-dichlorocyclohexane

[edit]Comparison of physical properties

Cis isomers and trans isomers often have different physical properties. Differences between isomers, in general, arise from the differences in the shape of the molecule or the overall dipole moment.

cis-2-pentene

trans-2-pentene

cis-1,2-dichloroethene

trans-1,2-dichloroethene

cis-butenedioic acid (maleic acid)

trans-butenedioic acid (fumaric acid)

Oleic acid

Elaidic acid

These differences can be very small, as in the case of the boiling point of straight-chain alkenes, such as 2-Pentene, which is 37C in the cis isomer and 36C in the trans isomer.[2] The differences between cis and trans isomers can be larger if polar bonds are present, as in the 1,2dichloroethenes. The cis isomer in this case has a boiling point of 60.3C, while the trans isomer has a boiling point of 47.5C.[3] In the cis isomer the two polar C-Cl bond dipole moments combine to give an overall molecular dipole, so that there are intermolecular dipole-dipole forces (or Keesom forces) which add to the London dispersion forces and raise the boiling point. In the trans isomer on the other hand, this does not occur because the two C-Cl bond moments cancel and the molecule is non-polar. The two isomers of butenedioic acid have such large differences in properties and reactivities that they were actually given completely different names. The cis isomer is called maleic acid and the trans is named fumaric acid. Polarity is key in determining relative boiling point as it causes increased intermolecular forces, thereby raising the boiling point. In the same manner, symmetry is key in determining relative melting point as it allows for better packing in the solid state, even if it does not alter the polarity of the molecule. One example of this is the relationship

between oleic acid and elaidic acid; oleic acid, the cis isomer, has a melting point of 13.