2-Butanol chirality center

In order of decreasing precedence the four substitu ents attached to the chirality center of 2 butanol are... 

The spatial aiiangement of substituents at a chirality center is its absolute configuration. Neither the sign nor the magnitude of rotation by itself can tell us the absolute configuration of a substance. Thus, one of the following structures is (-l-)-2-butanol and the other is (—)-2-butanol, but without additional infonnation we can t tell which is which. 

Because hydrogenation of the double bond does not involve any of the bonds to the chirality center, the spatial ariangement of substituents in (-l-)-3-buten-2-ol must be the sane as that of the substituents in (-l-)-2-butanol. The fact that these two compounds have the sfflne sign of rotation when they have the sane relative configuration is established by the hydrogenation experiment it could not have been predicted in advance of the experiment. 

Most of the biochemical reactions that take place in the body, as well as many organic reactions in the laboratory, yield products with chirality centers. Fo example, acid-catalyzed addition of H2O to 1-butene in the laboratory yield 2-butanol, a chiral alcohol. What is the stereochemistry of this chiral product If a single enantiomer is formed, is it R or 5 If a mixture of enantiomers i formed, how much of each In fact, the 2-butanol produced is a racemic mix ture of R and S enantiomers. Let s see why. 

B The fourth possibility arises in chiral molecules, such as (R)-2-butanol. The two — CH2- hydrogens at C3 are neither homotopic nor enantiotopic. Since replacement of a hydrogen at C3 would form a second chirality center, different diastereomers (Section 9.6) would result depending on whether the pro-R or pro-S hydrogen were replaced. Such hydrogens, whose replacement by X leads to different diastereomers, are said to be diastereotopic. Diastereotopic hydrogens are neither chemically nor electronically equivalent. They are completely different and would likely show different NMR absorptions. 

There is another point of nomenclature that must be discussed, namely where a chiral center is involved. Taking the simple case of 2-butanol, CH3CH(OH)CH2CH3, we can explain the point as follows (Scheme 1). Two configurations are possible at the chiral center. In both the R and the S series, three conformations are possible. The +sc form in the R series and the -sc form in the S series are enantiomers and their free energies must be the same under achiral conditions. However, the - sc form in the R and that in the S series differ in free energies. Therefore, it is not sufficient to call a conformation -sc if a chiral center is involved. In this case we may have to call such conformations - sc(R) and — sc(S) to distinguish them. 

Conversion of the unknown to, or formation of the unknown from, a compound of known configuration without disturbing the chiral center. See the glyceraldehyde-glyceric acid example above (p. 108). Since the chiral center was not disturbed, the unknown obviously has the same configuration as the known. This does not necessarily mean that if the known is R. the unknown is also R. This will be so if the sequence is not disturbed but not otherwise. For example, when (/ )-l-bromo-2-butanol is reduced to 2-butanol without dis-... 

SAMPLE SOLUTION (a) The highest ranking substituent at the chirality center of 2-methyl-1-butanol is CH2OH the lowest is H. Of the remaining two, ethyl outranks methyl. 

Using the Fischer projection of (R)-2-butanol that was given at the top of this page, explain how each of the following affects the configuration of the chirality center. 

AMYL ALCOHOLS. Amyl alcohol describes any saturated aliphatic alcohol containing five carbon atoms. This class consists of three pentanols, four substituted butanols, and a disubshtuted propanol. i.e.. eight structural isomers CvHdO four primary, three secondary, and one tertiary alcohol. In addition, 2-pentanol, 2-methyl-l-butanol. and 3-methyl-2-butanol have chiral centers and hence two enantiomeric forms,... 

The first atoms in two or more substituents often are identical, in which case it is necessary to explore further and compare the atomic numbers of the second attached atoms. Precedence is given to the substituent with a second atom of higher atomic number. For example, in 2-butanol, CH3CH (OH)CH2CH3, two of the groups at the chiral atom have carbon as the first atom. We therefore must compare the other atoms bonded to these two carbons. It is convenient to represent the arrangement at the chiral atom as shown in 7, where the first atoms are shown attached to the chiral center and the second atoms are listed in their priority order thus, (C,H,H) for ethyl and (H,H,H) for methyl ... 

When there is more titan one chiral center in a molecule, the number of possible stereoisomers increases. Since each chiral center can have either the R or S configuration, for a molecule of n chiral centers, there will be 2" possible stereoisomers. Thus 3-pheny 1-2-butanol has two stereogenic centers and four possible stereoisomers. These are shown below with the configuration of each chiral center designated. 

A reaction of an achiral molecule may introduce a chirality center, producing a chiral product. For example, reaction of the following ketone with hydrogen in the presence of a catalyst results in addition of the hydrogen to the carbon-oxygen double bond, producing 2-butanol ... 

The product is the pure 5-ester. No new chirality centers are formed during the reaction, and the configuration at the chirality center of (S)-2-butanol is unchanged. 

Takahashi et al. [21] reported the synthesis of isomeric butanols from the tertiary amine, (S)-4-amino-l-phenylbutanone [Eq. (6)]. The stereochemistry formed at the new chiral center (2-5- or 2-R-) was dependent on the reaction conditions. 

Let us take as an example ( — )-2.-methyl-l-butanol (the enantiomer found in fusel oil) and accept, for the moment, that it has configuration III, which we would specify S. We treat this alcohol with hydrogen chloride. and obtain the alkyl chloride, l-chloro-2-methylbutane. Without knowing the mechanism of this reaction, we can see that the carbon-oxygen bond is the one that is broken. No bond to the chiral center is broken, and therefore configuration is retained, with... 

Or, we oxidize (-)-2-methyl-l-butanol with potassium permanganate, obtain the acid 2-methylbutanoic acid, and find that this rotates light to the right. Again, no bond to the chiral center is broken, and we assign configuration V to ( + )-2-methylbutanoic acid. 

Reactions in which bonds to chiral centers are not broken can be used to get one more highly important kind of information the specific rotations of optically pure compounds. For example, the 2-methyl-1-butanol obtained from fusel oil (which happens to have specific rotation -5.756°) is optically pure—like most chiral compounds from biological sources—that is, it consists entirely of the one enantiomer, and contains none of its mirror image. When this material is treated with hydrogen chloride, the l-chloro-2-methylbutanc obtained is found to have specific rotation of 4-1.64°. Since no bond to the chiral center is broken, eveyy... 

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