2.3- Dibromobutane stereoisomers

Problem 5.30 For the following compounds, draw projection formulas for all stereoisomers and point out their R,S specifications, optical activity (where present), and meso compounds (a) 1,2,3,4-tetrahydroxybutane, (b) l-chloro-2,3-dibromobutane, (c) 2,4-diiodopentane, (d) 2,3,4-tribromohexane, (e) 2,3,4-tribromopentane. 

C is correct. 2,3-dibromobutane has two enantiomers and a meso compound. Four stereoisomers are possible according to the 2" formula, but since there is a meso, only 3 exist. 

Dibromobutane has fewer than 2 stereoisomers. It has two asymmetric carbons (C2 and C3), so the 2 rule predicts a maximum of four stereoisomers. The four permutations of (R) and (S) configurations at C2 and C3 are shown next. Make molecular models of these structures to compare them. 

There are only three stereoisomers of 2,3-dibromobutane because two of the four structures are identical. The diastereomer on the right is achiral, having a mirror plane of symmetry. The asymmetric carbon atoms have identical substituents, and the one with (R) configuration reflects into the other having (S) configuration. It seems almost as though the molecule were a racemic mixture within itself. 

In dealing with processes where stereoisomer formation is subject to some kind of control, two terms have been used—stereoselective and stereospecific. These terms have caused some problems, and to some extent have been used synonymously. The meanings attached to these terms by chemists are unambiguous [52]. A stereospecific process is one in which a particular stereoisomer reacts to give one specific stereoisomer (or a racemate) of a product. Thus, two starting materials, differing only in stereoisomerism, must be converted into stereoisomerically different products. A classical example of chemical stereospecificity is the free radical addition of HBr to 2-bromo-2-butene. At — 78°C, the (E)-olefin, 24, forms meso-2,3-dibromobutane, 25, while the (Z)-isomer, 26, yields ( )-tAreo-2,3-dibromobutane, 27 plus 28. (Note in addition that this addition is regiospecific—2,2-dibromobutane is not formed.)... 

Whereas 2,3-dibromopentane has two stereogenic centers and the maximum of four stereoisomers, 2 -dibromobutane has two stereogenic centers but fewer than the maximum number of stereoisomers. 

To find and draw all the stereoisomers of 2,3-dibromobutane, follow the same stepwise procedure oudined in Section 5.7. Arbitrarily add the H, Br, and CH3 groups to the stereogenic centers, forming one stereoisomer A, and then draw its mirror image B. A and B are nonsuperimposable mirror images—enantiomers. 

Because one stereoisomer of 2,3-dibromobutane is superimposable on its mirror image, there are only three stereoisomers and not four, as summarized in Figure 5.9. 

The reactant, too, exists as diastereomers a pair of geometric isomers. If we start with, say, ci5-2-butene, which of the stereoisomeric products do we get A mixture of all of them No. cw-2-Butene yields only racemic 2,3-dibromobutane none of the meso compound is obtained. A reaction that yields predominantly one stereoisomer (or one pair of enantiomers) of several diastereomeric possibilities is called a stereoselective reaction. 

Draw all the stereoisomers of each of the molecules (a)- e) assign configuration to stereogenic centres and say whether each stereoisomer is chiral (a) 2,3-dibromobutane (b) 2-bromo-3-chlorobutane (c) the monomethyl ester of tartaric acid (2,3-dihy-droxybutanedioic acid) (d) 2,3-difluoropentane (e) 1,3-dichloro-cyclopentane. 

An example of a compound with two asymmetric carbons that has only three stereoisomers is 2,3-dibromobutane. 

Figure 3-2. Examples of stereoisomers. D- and L-alanine are configurational enantiomers (I), cis- and frfl 5-dibromoethylene (II), threose (III, left), and erythrose (III, right) are configurative diastereoisomers. Both atropoisomers of 2,8-dinitro-6,12-dimethyl biphenyl (IV) and both conformers of 2,3-dibromobutane shown (V) are conformational enantiomers.

A familiar example of a stereoselective reaction would be the formation of a higher yield of f rans-2-butene than cis-2-butene in an E2 reaction, no matter whether the starting material is (R)- or (S)-2-bromobutane. The addition of bromine to trans-2-butene to produce mcso-2,3-dibromobutane or the addition of bromine to the cis isomer to produce an equimolar mixture of the two enantiomers of 2,3-dibromobutane is a stereospecific reaction. Note that a reaction that gives only one of a pair of eirantiomers is not necessarily stereospecific. A yeast-mediated reduction of 3-chloropropiophenone gives (S)-3-chloro-l henylpropan-l-ol, with no evidence for formation of the R enantiomer. Because the reactant cannot exist as stereoisomers, it is not possible for stereoisomerically different reactants to give stereoisomerically different products, and the reaction can only be considered stereoselective, not stereospecific. 

To understand this, let us write stereochemical formulas for 2,3-dibromobutane. We begin in the same way as we did before. We write formulas for one stereoisomer and for its mirror image ... 

The reactants CM-2-butene and trawr-2-butene are stereoisomers they are diastereo-mers. The product of reaction 1, (2i ,35)-2,3-dibromobutane, is a meso compound, and it is a stereoisomer of both of the products of reaction 2 (the enantiomeric 2,3-dibro-mobutanes). Thus, by definition, both reactions are stereospecific. One stereoisomeric form of the reactant (e.g., trans-2- mtene) gives one product (the meso compound), whereas the other stereoisomeric form of the reactant ( i--2-butene) gives a stereoisomeri-cally different product (the enantiomers). 

When a molecule has a structure with identical atoms or groups on the top and bottom (one side can be perfectly reflected into the other side), that molecule has a plane of symmetry (see Figure 9.9). When such symmetry occurs, the mirror image of one stereoisomer is superimposable on itself. Such a stereoisomer is called a meso compound. Because 2,3-dibromobutane has two stereogenic centers, the 2" rule predicts a maximum of four stereoisomers. Symmetry in one stereoisomer means that 46 is a meso compound, so there are only three stereoisomers (the two enantiomers 43 and 44 and the meso compomid 46). It is important to point out that 46 and 43 are diastereomers. Likewise, 46 and 44 are diastereomers, but 43 and 44 are enantiomers. 

The stereochemistry of 43 (38) is easy to see because the cyclohexane ring cannot rotate about individual C-C bonds. Does this same stereochemical bias occur with acyclic alkenes When an acyclic alkene reacts with a halogen, the product is an acyclic dihalide, and free rotation is possible about those bonds. However, the answer is yes Acyclic alkenes react with the same stereochemical bias because the mechanism of reaction of an alkene and diatomic halogen is the same for both acyclic and cyclic alkenes. This selectivity is demonstrated with the simple acyclic alkene, 2-butene however, an analysis requires an examination of each stereoisomer, cts-2-butene and rans-2-butene (see Chapter 9, Section 9.4). When cts-2-butene reacts with bromine, the product is a racemic mixture, (2S,3iS)-dibromobutane along with (2i2,3i2)-dibromobutane (see 44). Two new stereogenic centers are created by this reaction. (See Chapter 9, Section 9.3, to review absolute configuration.) When rans-2-butene reacts with bromine, however, the product is a racemic mixture of (2S,3i2)-dibromobutane and (2i ,3S)-dibromobutane (see 45), which are drawn a second time as the eclipsed rotamer (Chapter 8, Section 8.1) to show their relationship to 44. Dibromides 44 and 45 are diastereomers (Chapter 9, Section 9.5). 

Dibromobutane contains 2 chiral centers. Therefore, the possibility of 4 stereoisomers exists. However, the... 

The addition of bromine to 2-butene (Section 6.3D) gives 2,3-dibromobutane, a molecule with two chiral centers. Three stereoisomers are possible for this compound a meso compound and a pair of enantiomers (Section 3.4). We now ask the following questions ... 

Build a model of each of the four stoeoisomo-s of 2,3-dibromopentane. Why does 2,3-dibromopentane have four stereoisomers, wha-eas 2,3-dibromobutane has only three ... 

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