What Are Stereoisomers

Stereoisomers Isomers that have the same molecular formula and the same connectivity, but different orientations of their atoms in space. 

Stereoisomers have the same molecular formula and the same connectivity of atoms in their molecules, but different three-dimensional orientations of their atoms in space. The one example of stereoisomers we have seen thus far is that of cis-trans isomers in cycloalkanes (Section 3.7) and alkenes (Section 4.1C)  

Different compounds with the same molecular formula 

Isomers with a different connectivity of their atoms 

Isomers with the same connectivity of atoms, but a different orientation of their atoms in space 

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The CIS and trans forms of 1 2 dimethylcyclopropane are stereoisomers Stereoisomers are isomers that have their atoms bonded m the same order—that is they have the same constitution but they differ m the arrangement of atoms m space Stereoiso mers of the cis-trans type are sometimes referred to as geometric isomers You learned m Section 2 18 that constitutional isomers could differ m stability What about stereoisomers We can measure the energy difference between as and trans 1 2 dimethylcyclo propane by comparing their heats of combustion As illustrated m Figure 3 20 the two compounds are isomers and so the difference m their heats of combustion is a direct measure of the difference m their energies Because the heat of combustion of trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) less than that of its cis stereoisomer it follows that trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) more stable than as 1 2 dimethylcyclopropane... 

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 ... 

What are the configurations (R or S) of the chiral carbons in each stereoisomer Does internal rotation affect the configuration of a chiral atom Why or why not ... 

How many stereoisomers of 1,2-cyclopentanediol are there What are their stereochemical relationships Are they enantiomers ... 

The four stereoisomers of 2-amino-3-hydroxybutanoic acid can be grouped into two pairs of enantiomers. The 2R,3R stereoisomer is the mirror image of 25,35, and the 2R,35 stereoisomer is the mirror image of 25,3R. But what is the relationship between any two molecules that are not mirror images What, for example, is the relationship between the 2R,3R isomer and the 2R,3S isomer They are stereoisomers, yet they aren t enantiomers. To describe such a relationship, we need a new term—diastereomer. 

Problem 14.28 ( ) Give the structural formula of the epoxide formed when m-chloroperoxybenzoic acid reacts with (i) c -2-butene and (ii) rrfl 5-2-butene. (The epoxides are stereoisomers.) (b) What can you say about the stereochemistry of the epoxidation (c) Why is a carbocation not an intermediate ... 

Now that we can generate the stereoisomers of compounds with more than one chiral center, it is appropriate to ask what are the relationships between these isomers. Thus 2-bromo-3-acetoxy butane has two chiral centers and four stereoisomers, shown below. 

How many stereoisomers result from the non-selective reduction of the following substituted cyclobutanedione with LiAlH4 What are the topicities of the hydrogen atoms at positions 1 and 3 in the reaction products ... 

When similar reactions are carried out with trans-1 -bromo-2-fluoro-cyclohexane, the result of the dehydrohalogenation is quite different, and the products are compounds W and X. What are these compounds, and how can the difference between the products of elimination of hydrogen halides from either stereoisomer be accounted for ... 

There are four stereoisomers for 2,3-dibromopentane enantiomers A and B, and enantiomers C and D. What is the relationship between two stereoisomers like A and C A and C represent the second broad class of stereoisomers, called diastereomers. Diastereomers are stereoisomers that are not mirror images of each other. A and B are diastereomers of C and D, and vice versa. Figure 5.8 summarizes the relationships between the stereoisomers of 2,3-dibromopentane. 

But what about stereoisomers Not too long ago the question was considered largely theoretical, since many synthetic drugs were racemates for which there were no practical means of resolution. Efforts to prepare and isolate pure enantiomorphs were generally perceived as interesting chemical exercises of little practical pharmaceutical importance because it was presumed that both enantiomorphs were equally active, or one of the pair was totally inert, or the enantiomorphs would spontaneously race-mize in solution. Now we know better. The d versus I forms of amphetamine are perhaps one of our best and oldest examples of important pharmacological differences between enantiomorphs of the same agent. 

In this chapter, we shall learn how to predict the existence of the kinds of stereoisomers called enantiomers and diastereonwrs, how to represent and designate their structures, and, in a general w ay, how their properties will compare. Then, in following chapters, we shall begin to use what we learn ip this one. In Secs. 5.5-5.6, we shall learn about the kind of stereoisomers called gfo/nc/nc isomers. In Chapter 7, the emphasis will shift from what these stereoisomers are, to how they are formed, what they do, and what they can tell us. 

What is the relationship between these two salts They are not superimpos-able, since the acid portions are not superimposable. They are not mirror images, since the base portions are not mirror images. The salts are stereoisomers that are not enantiomers, and therefore are diaster comers. 

Frobieiii 9.15 (a) Addition of iCCIa to cyclopcntenc yields a single compound. What is it (b) Addition of CBrC l to cyciopentene yields a mixture of stereoisomers. In light of (a), how do you account for this What are the isomers likely to be Hint Use models.)... 

It can be seen that the stereoisomers with configuration (17 ,27 ) and (15,25) are related as enantiomers, and the (17 ,25) and (15,27 ) stereoisomers are also related as enantiomers. What then is the relationship between the stereoisomers of 1 with configurations (17 ,25) and (17 ,27 ), for example With respect to each other, these have one stereogenic centre of common, (17 ), configuration, and the other of opposite, (25) and (27 ), configuration. Clearly these stereoisomers are not enantiomers. Instead, they are related as diastereoisomers and the same can be said for the (15,27 ) and (15,25) stereoisomers. The most concise definition, given in the plural, is diastereoisomers are stereoisomers that are not enantiomers . 

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