Methylhexanes 3-methylhexane, chirality

Intramolecular hydrogen bonding is present in the chiral diastereomer of 225 5 tetra methylhexane 3 4 diol but absent in the meso diastereomer Construct molecular models of each and suggest a reason for the difference between the two... 

Methylhexane undergoes radical bromination to yield optically inactive 3-bromo-3-methylhexane as the major product. Is the product chiral What conclusions can you draw about the radical intermediate ... 

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. 

Abstraction of a hydrogen atom from the chirality center of (S)-3-methylhexane produces an achiral radical intermediate, which reacts with bromine to form a 1 1 mixture of R and S enantiomeric, chiral bromoalkanes. The product mixture is optically inactive. 

The chiral tertiary alcohol (/ )-3-methyl-3-hexanol reacts with HBr by an SnI pathway. HBr protonates the hydroxyl group, which dissociates to yield a planar, achiral carbocation. Reaction with the nucleophilic bromide anion can occur from either side of the carbocation to produce ( )3-bromo-3-methylhexane. 

As shown in Table 1, the optical purity of the product [(S)-5-methylhexane-3-ol] increases as the chain length of the N-n-alkyl substituent increases and reaches a peak of 93% ee at a chain length of four carbons (Table 1, entry 4). Thus, among M,M-di-n-alkylnorephedrines examined, (IS,2R)-N,N-di-n-butylnorephedrine (DBNE) (2) is the best chiral catalyst precursor. 

Table 1 Effect of fV-Alkyl Substituents of (lS,2/ )-fV,fV-Dialkyl-norephedrine as Chiral Ligand for the Addition of Diethylzinc to 3-Methylbutanal to Yield (S)-5-Methylhexan-3-ol...

The reactions considered in the previous two sections involve additions toa achiral alkenes, and optically inactive products are formed in all cases. Whatfl would happen, though, if we were to carry out a reaction on a single enaa tiomer of a chiral reactant For example, what stereochemical result would J be obtained from addition of HBr to a chiral alkene, such as (fl>4-methy - 1-hexene The product of the reaction, 2-bromo-4-methylhexane, has two chirality centers and four possible stereoisomers. 1 ... 

What about the configuration at C2, the newly formed chirality center As illustrated in Figure 9.19, the stereochemistry at C2 is established by attack of Br ion on a carbocation intermediate in the usual manner. But this carbocation does not have a plane of symmetry it is chiral because of the chirality center at C4. Since the carbocation has no plane of symmetry, it is not attacked equally well from top and bottom faces. One of the two faces is likely, for steric reasons, to be a bit more accessible than the other face, leading to a mixture of R and S products in some ratio other than 50 50, Thus, two diastereomeric products, (2/ ,4fi)-2-bromo-4-methylhexane and (2S,4/i)-2-bromo-4-methylhexane, are formed in unequal amounts, and the mixture is optically active. 

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.

Identify the chiral centre in the compound 3-methylhexane and draw the products as mirror image form. 

In general, the observed rotations of organic compounds decrease with increasing temperature as a consequence of reduced density. Even when this is accounted for in the calculation of specific rotation, many substances show a decrease in rotation as the relative abundances of chiral conformers, some of which may have opposed rotations, become more nearly equal. However, the specific rotations of (+ )-3-methylhexane (1) and (+ )-3-methylheptane (2) are nearly constant with temperature, especially in solution. ... 

Examples of this type of reaction are Sjyfl reactions in which the leaving group departs from a chirality center. These reacdiions almost always result in extensive and sometimes complete racemization. For example, heating optically active (5)-3-bromo-3-methylhexane... 

The phenomenon of handedness, or chirality, actually surfaces long before we encounter a molecule as complicated as rflwr-l,2-dimethylcyclopropane. We went right past it when we wrote out the heptane isomers in Figure 2.45 (p. 82). One of these heptanes, 3-methylhexane, exists in two forms. To see this, draw out the molecule in tetrahedral form using C(3) as the center of the tetrahedron (Fig. 4.2a). 

Now consider Figure 4.4. Here we apply to 3-methylhexane the same rotation that we applied before to 3-methylpentane. In this case, we cannot superimpose the original molecule onto the newly rotated molecule no matter how many ways we try. The propyl group in one form winds up where the ethyl group is in the other. Indeed, no amount of twisting and turning can do the job The two are irrevocably different. 3-Methylhexane is chiral. 

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. 

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. 

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. 

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 addition generates one stereocenter (marked in each of the two products in the center of the scheme by an asterisk) therefore each product molecule is chiral (Section 5-1). Because the products form in equal amounts, the consequence is a racemic mixture of (R)- and (S)-3-methylhexane. 

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. 

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