4- Chloro-2-butanol

Epoxy compounds are prepared by heating halohydrins with strong caustic solutions and, where possible, distilling the product as it is formed. By this procedure, 3-chloro-2-butanol yields a mixture of cis-and frans-2,3-epoxybutane (90%), which can be readily separated by fractional distillation. Another example is the conversion of 2-chloro-cyclohexanol to cyclohexene oxide (73%). The reaction is included in an excellent discussion of the chemistry of ethylene and trimethylene oxides, ... 

The topicity concept is also important in the reactions of trigonal centers, such as carbonyls and alkenes. In consideration of carbonyls, for example, the two faces are homotopic in a symmetrically substituted ketone, such as acetone or 2-pentanone, because the molecule has C2 symmetry. However, the faces are enantiotopic in an unsymmetrically substituted ketone, such as 2-butanone 190. While the reaction with hydride ion on the top face of the carbonyl group forms (M)-2-butanol 191, the reaction on the bottom face forms (5)-2-butanol 192. Extending this argument further, the two faces are diastereotopic in an unsymmetrical ketone bearing a chiral center elsewhere in it, e.g., (7 )-3-chloro-2-butanone 193. The delivery of hydride ion to the top face of the carbonyl group forms 2(/ ),3(/f)-3-chloro-2-butanol 194 and the delivery to the bottom face forms 2(,S ),3(A )-3-chloro-2-butanol 195. The molecules 194 and 195 are diastereoisomers. 

Many organic compounds have more than one asymmetric carbon. The more asymmetric carbons a compound has, the more stereoisomers are possible for the compound. If we know how many asymmetric carbons a compound has, we can calculate the maximum number of stereoisomers for that compound a compound can have a maximum of 2 stereoisomers (provided it doesn t have any other stereocenters), where n equals the number of asymmetric carbons. For example, 3-chloro-2-butanol has two asymmetric carbons. Therefore, it can have as many as four (2 = 4) stereoisomers. The four stereoisomers are shown both as perspective formulas and as Fischer projections. 

The four stereoisomers of 3-chloro-2-butanol consist of two pairs of enantiomers. Stereoisomers 1 and 2 are nonsuperimposable mirror images. They, therefore, are enantiomers. Stereoisomers 3 and 4 are also enantiomers. Stereoisomers 1 and 3 are not identical, and they are not mirror images. Such stereoisomers are called diastere-omers. Diastereomers are stereoisomers that are not enantiomers. Numbers 1 and 4,2 and 3, and 2 and 4 are also diastereomers. (Cis-trans isomers are also considered to be diastereomers because they are stereoisomers that are not enantiomers.)... 

When Fischer projections are drawn for stereoisomers with two adjacent asymmetric carbons (such as those for 3-chloro-2-butanol), the enantiomers with similar... 

Problem 17.47. Draw the stereoisomers, if any, of each of the following (a) 1,1-dimethyl-l-propanol, (b) 2-methyl-l-propanol, (c) 1-propanol, id) 1-methyl-1-propanol, (e) 3-chloro-2-butanol, (/) 2,3-butanediol. 

Following are stereorepresentations of the four stereoisomers of 3-chloro-2-butanol ... 

For example, 3-chloro-2-butanol has two asymmetric centers. Therefore, it can have a maximum of four (2 = 4) stereoisomers. The four stereoisomers are shown here, both as perspective formulas and as Fischer projections. 

When Fischer projections are drawn for stereoisomers with two adjacent asymmetric centers (such as those for 3-chloro-2-butanol), the enantiomers with the hydrogens on the same side of the carbon chain are called the erythro enantiomers (see Problem 48), whereas those with the hydrogens on opposite sides are called the threo enantiomers. Therefore, 1 and 2 are the erythro enantiomers of 3-chloro-2-butanol (the hydrogens are on the same side), whereas 3 and 4 are the threo enantiomers. 

---