Diastereomers enantiomers

If compounds have the same topology (constitution) but different topography (geometry), they are called stereoisomers. The configuration expresses the different positions of atoms around stereocenters, stereoaxes, and stereoplanes in 3D space, e.g., chiral structures (enantiomers, diastereomers, atropisomers, helicenes, etc.), or cisftrans (Z/E) configuration. If it is possible to interconvert stereoisomers by a rotation around a C-C single bond, they are called conformers. 

R Stereoisomers (Section 4.2) are compounds whose atoms are connected in the same order but with a different geometry. Among the kinds of stereoisomers we ve seen are enantiomers, diastereomers, and cis-trans isomers (both in alkenes and in cycloalkanes). Actually, cis-trans isomers are just another kind of diastereomers because they are non-mirror-image stereoisomers. 

This type of isomerism is possible not only in biphenyls, but also in compounds in which rotation about an sp2-sp2 bond is restricted and the two planes involving the sp2 center are noncoincident and substituted unsymmetrically. In addition to enantiomers, diastereomers are possible. There are some examples reported of restricted rotation about an aromatic ring-to-carbonyl bond or aromatic ring-to-nitrogen bond (28). Since these reports make no mention of diastereomers but only of enantiomers, they will receive no further mention here. 

The maximum number of stereoisomers is 2" where n is the number of nonidentical chiral centers. Figure 1-2 shows the four stereoisomers present in a molecule with two chiral centers. Non-superimposable mirror images are enantiomers, while the other species in the figure are diastereomers. Unlike enantiomers, diastereomers have different physical properties. 

As noted previously, the classification of stereoisomers preferred by contemporary organic chemists is the enantiomer-diastereomer dichotomy17 and this may be quite conveniently applied to coordination compounds. Thus, complexes (9a) and (9b) are enantiomers, but (9a) and (9c), and (9b) and (9c), are diastereomers. Older terminology might have led to the description of A and B as optical antipodes and to (A+B) and C as geometrical isomers. 

Determine the relationship between the pairs of compounds written as perspective formulas as being enantiomers, diastereomers, conformational isomers, cis-trans... 

This is a problem similar to 5-29, except that the structures are written mostly as projection formulas of the Fischer or Newman type. Determine the relationship between the pairs of compounds as one of the following identical, position isomers, enantiomers, diastereomers, conformational isomers, or cis-trans isomers. (D stands for deuterium, the hydrogen isotope of mass 2.)... 

A-l. For each of the following pairs of drawings, identify the molecules as chiral or achiral and tell whether each pair represents molecules that are enantiomers, diastereomers, or identical. 

A-9. How many stereoisomeric products are obtained from the reaction of (S )-3-chloro-1 -butene with hydrogen bromide What is their relationship (enantiomers, diastereomers) ... 

A-5. Write structural formulas for the a- and jS-methyl pyranosides formed from the reaction of D-mannose (see Problem A-2 for its structure) with methanol in the presence of hydrogen chloride. How are the two products related—are they enantiomers Diastereomers ... 

Given Fischer projections of two isomers, tell their relationship (for example, same structure, enantiomers, diastereomers). 

To define and illustrate enantiomers, diastereomers, and meso forms. 

Draw all the distinct stereoisomers for each structure. Show the relationships (enantiomers, diastereomers, etc.) between the isomers. Label any meso isomers, and draw any mirror planes of symmetry. 

Q Identify pairs of enantiomers, diastereomers, and meso compounds, and explain how they differ in their physical and chemical properties. 

Give the stereochemical relationships between each pair of structures. Examples are same compound, structural isomers, enantiomers, diastereomers. Which pairs could you (theoretically) separate by distillation or recrystallization ... 

Problem 5.26 State how each pair of compounds is related. Are they enantiomers, diastereomers, constitutional isomers, or identical ... 

Comparing Compounds Enantiomers, Diastereomers, and Constitutional Isomers... 

How is each compound related to the simple sugar D-erythrose Is it an enantiomer, diastereomer. or identical ... 

An acid-base reaction of (R)-sec-butylamine with a racemic mixture of 2-phenylpropanoic acid forms two products having different melting points and somewhat different solubilities. Draw the structure of these two products. Assign R and S to any stereogenic centers in the products. How are the two products related Choose from enantiomers, diastereomers, constitutional isomers, or not isomers. 

Both enantiomers of A -acetyl alanine have a free carboxy group that can react with an amine in an acid-base reaction. If a chiral amine is used, such as (/ )-a-methyibenzyiamine, the two salts formed are diastereomers, not enantiomers. Diastereomers can be physically separated from each other, so the compound that converts enantiomers into diastereomers is called a resolving agent. Either enantiomer of the resolving agent can be used. 

Classical Resolution and Variants. Resolution is the process by which a chiral recemic molecule is combined with a second chiral, but enantiomerically homogeneous, molecule. The resultant mixture of diastereomers is separated and the appropriate diastereomer is then cleaved to recover the resolving agent and the desired enantiomer. As opposed to enantiomers, diastereomers have different physical properties, for example, melting points and solubilities, thus allowing for separation. 

Problem 36.9 Besides threonine, there are four amino acids in Table 36.1 that can exist in more than two stereoisomeric forms, (a) What are they (b) How many isomers are possible in each case Indicate enantiomers, diastereomers, any meso compounds. 

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