Arranged differently Stereoisomers

The other broad class of isomers is stereoisomers. Within this class of isomers, the atoms are connected in the same way, but have different spatial arrangements. Within this class, chemists split them further into geometric and optical isomers. 

Geometric isomers have the same connections between the atoms, but they have different spatial orientations among the metal-ligand bonds. For example, with a square planar molecule such as Pt(NH3)Cl2, the ligands can either be situated across from each other (180 degrees apart) or next to each other (90 degrees apart). 

When the ligands are beside each other, the compound is a cis isomer. When the ligands are across from one another, the compound is a tnms isomer. Cis isomers have the ligands occupying adjacent comers of the molecule trans isomers have the ligands right across from each other in the same molecule. The names stem from Latin — cis means next to, and trans means across. 

A propeller is chiral, it just pushes water in different directions. In chemical terms, when a molecule is left-handed it s denoted as A (capital lambda), and when it s right-handed it s denoted as A (capital delta). In the following figure, you can see how a three-bladed propeller is either left-handed (on top), or right-handed (on the bottom). 

Optical isomers are compounds that are not superimposable on their own mirror image. This can be a little tricky to conceptualize if you are new to the idea. Take a look at your hands and you can see that they are mirror images of each other, one is a reflection of the other. This characteristic is called chirality, and objects that display it are described as chiral. Now take one hand and place it on top of the other and try to match or superimpose it. You see that they don t match. Because of their chirality, your hands aren t superimposable on each other. 

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Because a hexose contains four chiral carbon atoms, there are 2 = 16 different possible arrangements of the hydroxyl groups in space, ie, there are 16 different stereoisomers. The stmctures of half of these, the eight D isomers, are shown in Figure 1. Only three of these 16 stereoisomers are commonly found in nature D-glucose [50-99-7] D-galactose [59-23-4] and D-mannose [3458-28-4]. 

Spatial congruence of C-H graphs is applied essentially only in chemical formulas which represent a compound of carbon atoms and atoms of valence 1 (or radicals of valence 1). In this case condition (IV), besides (I), (II), (III), adds another restriction not only the relationships are important but also the spatial arrangement of the bonds. The spatial interpretation of the congruence of C-H graphs coincides with the interpretation of the chemical formula as stereoformula. I use stereoisomers in this sense. For example, the number of different stereoisomers is equal to the number of spati-... 

Figure 16.18 summarizes the types of isomerism found in coordination complexes. The two major classes of isomers are structural isomers, in which the atoms are connected to different partners, and stereoisomers, in which the atoms have the same partners but are arranged differently in space. Structural isomers of coordination compounds are subdivided into ionization, hydrate, linkage, and coordination isomers. 

The most common states of a pure substance are solid, liquid, or gas (vapor), state property See state function. state symbol A symbol (abbreviation) denoting the state of a species. Examples s (solid) I (liquid) g (gas) aq (aqueous solution), statistical entropy The entropy calculated from statistical thermodynamics S = k In W. statistical thermodynamics The interpretation of the laws of thermodynamics in terms of the behavior of large numbers of atoms and molecules, steady-state approximation The assumption that the net rate of formation of reaction intermediates is 0. Stefan-Boltzmann law The total intensity of radiation emitted by a heated black body is proportional to the fourth power of the absolute temperature, stereoisomers Isomers in which atoms have the same partners arranged differently in space, stereoregular polymer A polymer in which each unit or pair of repeating units has the same relative orientation, steric factor (P) An empirical factor that takes into account the steric requirement of a reaction, steric requirement A constraint on an elementary reaction in which the successful collision of two molecules depends on their relative orientation. 

The term configuration signifies the spatial arrangement of atoms in stereoisomers which differ from one another because their atoms are arranged differently in space. Therefore this term is different from constitution or conformation. 

Given the molecular formula, we now have to describe how the different atoms are connected to each other. The description of the exact connection of the various atoms is commonly referred to as the structure of the compound. Depending on the number and types of atoms, there may be many different ways to interconnect a given set of atoms which yield different structures. Such related compounds are referred to as isomers. Furthermore, as we will discuss later, there may be several compounds whose atoms are connected in exactly the same order (i.e., they exhibit the same structure), but their spatial arrangement differs. Such compounds are then called stereoisomers. It should be pointed out, however, that, quite often, and particularly in German-speaking areas, the term structure is also used to denote both the connectivity (i.e., the way the atoms are connected to each other) as well as the spatial arrangement of the atoms. The term constitution of a compound is then sometimes introduced to describe solely the connectivity. 

Isomers in the broadest sense are compounds with the same molecular formula but different structures. Isomers are most broadly categorized as either structural isomers or stereoisomers. Compounds that are structural isomers have a different arrangement of atom connectivity relative to each other, whereas compounds that are stereoisomers have the same atom connectivity but the atoms still arrange differently in a spatial sense. We ll begin this section with structural isomers, for which one example is shown in Figure 8.15. 

The second class of isomers involves a more subtle difference in bonding. Stereoisomers are isomers in which all atoms are bonded in the same order but are arranged differently in space. There are two types of stereoisomers. One type occurs in alkenes, which contain double bonds. Two carbon atoms with a single bond between them can rotate freely in relationship to each other. However, when a second covalent bond is present, the carbons can no longer rotate they are locked in place, as shown in Figure 22-11. 

Stereoisomers have all atoms bonded in the same order but arranged differently in space. 

Stereoisomers, in which all atoms have the same partners (same connectivity) but some atoms are arranged differently in space. 

Molecules with the same connectivity but with some of their atoms arranged differently in space —> Geometrical isomers are stereoisomers with different arrangements in space on either side of a double bond, or above and below the ring of a cycloalkane. 

Isomers are molecules with the same molecular formula in which the atoms are arranged differently. There are two classes of chemical isomerism structural isomers and stereoisomers. 

If four different groups surround a central carbon atom, two (and only two) different stereoisomers are possible, as illustrated in Figure 3.6. These are known as enantiomers (or optical isomers). These two isomers are different because rotation about any of the bond axes of the molecule cannot bring all four groups of the / -isomer into coincidence with the S-isomer. There are not more than two any other arrangement of groups round the central carbon atom can always be rotated around a bond axis... 

Stereoisomers Molecules with the same atoms connected in the same order but arranged differently in space. Stereoselectivity The predominance of one of several possible stereoisomeric products. 

Structures 1 and 2 are isomers because they have the same empirical formula, but they have the same constitution (same point of attachment). According to the simple definition of an isomer in Chapter 4, this fact poses a problem. They do have the same points of attachment, but they differ in the spatial arrangement of the atoms, so they are stereoisomers. Therefore, nonsuperim-posable mirror images are different stereoisomers and they are recognized as enantiomers. In other words, an enantiomer is a stereoisomer that has a nonsuperimposable mirror image. 

Enantiomers 39 and 40 are two different stereoisomers. The presence of the second stereogenic center allows a different arrangement of atoms for 2,3-dichloropentane, shown in the new stereoisomers 41 and 42. Changing the stereochemistry at C2 in 39A from (S) to (R) leads to 41A [(2i ,3i2)-dichloropen-tane], which is different from either 39A or 40A. Making a model of all three compounds and attempting to superimpose them can prove this. 

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