3-Methylhexane enantiomers

New chiral centers are produced by addition reactions to other trigonal centers as well. Hydrogenation of 3-methyl-3-hexene gives 3-methylhexane. Clearly the addition of hydrogen to one face of the planar olefinic system gives one enantiomer and addition to the opposite face gives the opposite enantiomer. Likewise reaction of styrene with chlorine or bromine (X2) or potassium permanganate produces products with a new chiral center. Formation of the two possible enantiomers results from addition to either face of the olefin. 

Advantage has been taken of the fact that the diastereomers (9) and (10) are often easily separated by silica gel chromatography, particularly when both enantiomers of a compound are desired in pure form. Nonselective addition of (5) to 2-hexanone followed by chromatographic separation of the diastereomeric carbinols and hydrolysis of each gave both (+)- and (—)-2-hydroxy-2-methylhexanal in optically pure form. ... 

Because addition converts sp hybridized carbons to s[ hybridized carbons, however, sometimes new stereogenic centers are formed from hydrohalogenation. For example, Markovnikov addition of HCl to 2-ethyl-1-pentene, an achiral alkene, forms one constitutional isomer, 3-chloro-3-methylhexane. Because this product now has a stereogenic center at one of the newly formed sp hybridized carbons, an equal amount of two enantiomers—a racemic mixture—must form. 

Equal but opposite specific rotations opposite R/S specifications all other properties the same. 4. (a) Screw, scissors, spool of thread (b) glove, shoe, coat sweater, tied scarf (c) helix, double helix (d) football (laced), golf club, rifle barrel (e) hand, foot, ear, nose, yourself. 5. (a) Sawing (b) opening milk bottle (c) throwing a ball. 7. (a) and (b) 3-Methylhexane and 2,3-dimethylpentane. 8. a, b, e, k, 2 pairs enantiomers c, d, h,... 

Figure 15.9 Two chiral molecules. A, 3-Methylhexane is chiral because C-3 is bonded to four different groups. These two models are optical isomers (enantiomers). B, The central C in the amino acid alanine is also bonded to four different groups.

Models are essential now, as you must be absolutely certtun of the difference between 3-methylpentane and 3-methylhexane. The more symmetric 3-methylpentane is achiral, whereas 3-methylhexane and your hand are chiral. The two stereoisomers of 3-methylhexane are related in exactly the same way your two hands are. A chiral compound can be defined as a molecule for which the mirror image is not superimpos-able on the original. The two nonsuperimposable mirror images of 3-methylhexane are examples of enantiomers. 

There are times when a three-dimensional representation is important, hut the subject is a racemic mixture of the two enantiomers rather than one enantiomer. Unless both enantiomers are drawn, the figure appears to focus only on a single enantiomer. Our molecule, 3-methylhexane, illustrates this point in Figure 4.16. If we are talking about a racemic mixmre of 3-methylhexane but want to show the three-dimensional structure of this molecule, we should, stricdy speaking, draw both of the enantiomers. In practice this is rarely done, and you must be alert for the problem. Unless optical activity is specifically indicated, it is usually the racemate that is meant. 

Now let s look at how the two enantiomers of a chiral molecule differ from each other chemically. We still can t discuss specific chemical reactions, but we can make some important general distinctions. Note that there is no difference in how the two enantiomers of 3-methylhexane interact with a Spaldeen, an old-fashioned smooth... 

Figure 4.24 elaborates this subject—the interactions of chiral and achiral objects— by showing the two enantiomers of 3-methylhexane approaching an achiral molecule, propane. 

FIGURE 4.24 The two enantiomers of the chiral molecule 3-methylhexane interact in exactly the same way with the achiral molecule propane. 

In each instance, the 3-methyl group of 3-methylhexane approaches the hydrogen of propane and both the ethyl and propyl groups are opposite methyl groups of propane. Because of the symmetry of propane, one can always find exactly equivalent interactions for the two enantiomers and propane, no matter what approach you pick. Try it with models. 

Now imagine your hands, or the two enantiomers of 3-methylhexane, interacting with a chiral object. The Spaldeen with the label will do, but lets use a more common object, such as the shell in Figure 4.25. 

For the molecular counterpart of what Figure 4.25 shows, look at the approach of the two enantiomers of 3-methylhexane to another chiral molecule, (i )-rcbutyl alcohol (Rg. 4.26). As the (5) enantiomer approaches the alcohol, propyl is opposite methyl and ethyl is opposite H. For the (R) enantiomer, the interactions are propyl with H and ethyl with methyl. The two approaches are very different. Indeed, there is no approach to the chiral alcohol that can give identical interactions for the two enantiomeric 3-methylhexanes. Try some. Contrast this situation wth the approaches to the achiral propane shown in Figure 4.24. The two enantiomers interact identically with achiral propane but differently with the chiral molecule (i )-rcc-butyl alcohol. This situation is general for any chiral molecule The enantiomers have identical chemistries with achiral reagents but different chemistries with chiral ones. 

Stereoisomers have the same connectivity, but differ in the arrangement of their parts in space. Two kinds of stereoisomers exist enantiomers and diastereomers. Enantiomeric molecules are nonsuperimposable mirror images of each other. Simple examples are (R)- and (iS )-3-methylhexane. Slightly more complicated are (25,35)-dichlorobutane and its enantiomer, (2i ,3i )-dichlorobutane. These pairs are not structural isomers because they have the same connectivity (Fig. 4.55). 

Some of the physical and chemical properties of enantiomers are indistinguishable from one another. For example, both of the optical isomers of 3-methylhexane have identical freezing points, melting points, and densities. However, the properties of enantiomers differ from one another in two important ways (1) in the direction in which they rotate polarized light and (2) in their chemical behavior in a chiral environment. 

The lower-molecular-weight thiols and sulfides are most notorious for their foul smell. Ethanethiol is detectable by its odor even when it is diluted in 50 million parts of air. The major volatile components of the skunk s defensive spray are 3-methyl-1-butanethiol, trans-2-butene-l-thiol, and tra i-2-butenyl methyl disulfide. The all-too-familiar human BO (body odor) emanating from sweaty armpits was analyzed by chemists in the perfume industry in 2004. The major chemical culprit is 3-mercapto-3-methylhexan-l-ol, specifically the obnoxious 5-enantiomer. It is excreted admixed with 25% of its mirror image, which, curiously, has a fruity odor. 

In the preceding section, we saw that Sj 2 reactions at chiral centers occur with inversion of configuration. In contrast, an S l reaction at a chiral center reaction usually gives a mixture of enantiomers. For example, (5)-3-bromo-3-methylhexane reacts with water, a poor nucleophile, to give a racemic mixture of 3-methyl-3-hexanol. The reaction occurs by way of an achiral carbocation intermediate with a plane of symmetry (Figure 10.4). Because the carbocation intermediate has a plane of symmetry, the nucleophile can attack equally well from either side of the plane to give a racemic mixture. 

Apart from the rate law, what other evidence is there for the planar carbocation Again, we can use a chiral haloalkane and investigate the chirality of the product. When one of the enantiomers of 3-bromo-3-methylhexane reacts... 

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