Atoms in Space

Kekule paved the way for this paper a year earlier, when he added a fifth type to Gerhardt s four. The new type involved the combination of four atoms or radicals with carbon. Because the simplest molecule of this type is methane or marsh gas, CH4, it was known as the marsh gas type. Kekule also insisted that his types were not merely characterized by similarity of formula and prop- 

In the 1850s and 1860s chemists wrestled with ways of representing this idea in the formulas that they wrote. The problems were obvious. How could chemists represent the connections, links, or, as they became known in the 1860s, bonds between different atoms when it was difficult for them to find agreement about the arrangement of atoms in a molecule How were atoms linked Most chemists were reluctant to commit themselves to representations of what, a few years previously, would have been dismissed as the wildest speculation. 

One of the leading chemists in this line was the English chemist Edward Frankland (1825—99), who had studied in Germany with Robert Bunsen and 

In contrast to what some chemists saw as the false promise of ball-and-stick models, formulas using straight lines to link atoms indicated merely the order in which atoms were j oined. Thus methane had a carbon atom attached by single bonds to four separate hydrogen atoms. It could be represented on the page by the graphic formula 

Carbon had a valence of four and could be linked to other atoms, including other carbon atoms, by more than one bond. Thus in ethane, C2H4, each carbon atom was joined by two bonds, or a double bond, to the other carbon atom, and by a single bond to each of two atoms of hydrogen  

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Stereoisomers are isomers that have the same con stitution but differ in the arrangement of their atoms in space... 

Stereochemistry (Chapter 7) Chemistry in three dimensions the relationship of physical and chemical properties to the spatial arrangement of the atoms in a molecule Stereoelectron ic effect (Section 5 16) An electronic effect that depends on the spatial arrangement between the or bitals of the electron donor and acceptor Stereoisomers (Section 3 11) Isomers with the same constitu tion but that differ in respect to the arrangement of their atoms in space Stereoisomers may be either enantiomers or diastereomers... 

Concepts in stereochemistry, that is, chemistry in three-dimensional space, are in the process of rapid expansion. This section will deal with only the main principles. The compounds discussed will be those that have identical molecular formulas but differ in the arrangement of their atoms in space. Stereoisomers is the name applied to these compounds. 

In Chapter 3 another type of isomerism, called stereoisomerism, will be introduced. Stereoisomers have the sane constitution but differ in the anangement of atoms in space. 

The cis and trans forms of 1,2-dimethylcyclopropane are stereoisomers. Stereoisomers are isomers that have then- atoms bonded in the sane order—that is, they have the sane constitution, but they differ in the anangernent of atoms in space. Stereoisomers of the cis-trans type are sometimes refened to as geometric isomers. You learned in Section 2.18 that constitutional isomers could differ in stability. What about stereoisomers ... 

Stereochemistry refers to chemistry in three dimensions. Its foundations were laid by Jacobus van t Hoff" and Joseph Achille Le Bel in 1874. Van t Hoff and Le Bel independently proposed that the four bonds to carbon were duected toward the corners of a tetrahedron. One consequence of a tetrahedral ariangement of bonds to carbon is that two compounds may be different because the ariangement of their atoms in space is different. Isomers that have the same constitution but differ in the spatial ariangement of their- atoms are called stereoisomers. We have already had considerable experience with certain types of stereoisomers—those involving cis and bans substitution patterns in alkenes and in cycloalkanes. 

The minor images of bromochlorofluoromethane have the sane constitution. That is, the atoms are connected in the sane order. But they differ in the anangement of then-atoms in space they are stereoisomers. Stereoisomers that are related as an object and its nonsuperimposable minor image are classified as enantiomers. The word enantiomer describes a paiticulai- relationship between two objects. One cannot look at a single molecule in isolation and ask if it is an enantiomer any more than one can look at an individual human being and ask, Is that person a cousin Fuithennore, just as an object has one, and only one, minor image, a chiral molecule can have one, and only one, enantiomer. 

The experimental facts that led van t Hoff and Le Bel to propose that molecules having the same constitution could differ in the anangement of their atoms in space concerned the physical property of optical activity. Optical activity is the ability of a chiral substance to rotate the plane of plane-polarized light and is measured using an instrument called a polarimeter. (Figure 7.5). 

Enantiomers can have striking differences, however, in properties that depend on the anangement of atoms in space. Take, for example, the enantiomeric forms of carvone. (/ )-(—)-Carvone is the principal component of spearmint oil. Its enantiomer, (.S)-(-F)-car-vone, is the principal component of caraway seed oil. The two enantiomers do not smell the same each has its own characteristic odor. 

Section 7.5 Relative configuration compares the ariangement of atoms in space to some reference. The prefix cis in d5-4-methylcyclohexanol, for example, describes relative configuration by referencing the orientation of the CH3 group to the OH. Absolute configuration is an exact description of the ariangement of atoms in space. 

In Section 18-6.3 the composition of proteins was given. They are large, amide-linked polymers of amino acids. However, the long chain formula (Figure 18-14, p. 348) does not represent all that is known about the structure of proteins. It shows the covalent structure properly but does not indicate the relative positions of the atoms in space. 

The Lewis structures encountered in Chapter 2 are two-dimensional representations of the links between atoms—their connectivity—and except in the simplest cases do not depict the arrangement of atoms in space. The valence-shell electron-pair repulsion model (VSEPR model) extends Lewis s theory of bonding to account for molecular shapes by adding rules that account for bond angles. The model starts from the idea that because electrons repel one another, the shapes of simple molecules correspond to arrangements in which pairs of bonding electrons lie as far apart as possible. Specifically ... 

Van tHolF, J. H. 90%), Die Lagerung der Atome im Raume [The Arrangement of Atoms in Space], Braunschweig Vieweg. 

This is very different from the case with single bonds, which are freely rotating aU of the time. But a double bond is the result of overlapping p orbitals, and double bonds cannot freely rotate at room temperature (if you had trouble with this concept when you first learned it, you should review the bonding structure of a double bond in your textbook or notes). So there are two ways to arrange the atoms in space cis and trans. If you compare which atoms are connected to each other in each of the two possibilities, yon will notice that all of the atoms are connected in the same order. The difference is how they are connected in 3D space. This is why they are called stereoisomers (this type of isomerism stems from a difference of orientation in space— stereo ). 

Stereoisomers differ only in arrangement of their atoms in space. 

In 1877, Hermann Kolbe (of the University of Leipzig), one of the most eminent organic chemists of the time, criticized van t Hoffs publication on The Arrangements of Atoms in Space. as a childish fantasy. 

A satellite, e.g. for radio or TV communication, needs to be robust to withstand its environment in space. In particular, it needs to be protected from the tremendous gravitational forces exerted during take off, from the deep vacuum of space, and from atoms in space. 

The hydrogen atoms in space form a hydride with the materials on the surface of the satellite. 

For a particular conformation c of a molecule, the positions of all (united) atoms in space as well as the chain conformers are known. The potential energy of this conformation is therefore just the sum of the contributions, as given by equation (9) for all the united atoms and a particular energy quantity per gauche bond in the chain. The statistical weight for this conformation is proportional to the Boltzmann factor containing this segment potential ... 

Isomers are compounds which have identical formulas but differ in the nature or sequence of bonding of their atoms or the arrangement of the atoms in space. There is a variety of different types of isomerism. 

According to Thiele, the explanation of the phenomenon of 1 4-addition is that the fields of force, which surround the carbon atoms separated by double bonds, partially cancel each other between C2 and C3 because of the proximity of these atoms in space, so that a higher chemical potential exists at C4 and C4 than at C2 and C3. Consequently the places at which addition occurs preferentially are C4 and C4. 

Wurtz, Introduction, 255. "Let us guard against envisioning the preceding formulas as representing really the positions of atoms in space" (Legons elementaires, 226). 

Grimaux said, "None of us imagine, with figures traced in a plane, to determine the real place of atoms in space.. . . These formulas recall to us the reactions themselves."96... 

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