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Chemistry Homework

Question

Your Name:
Isomer Relationships, Conformational Stability, and Reaction Selectivity
Instructions:
Read the instructions carefully and answer the following questions in the space provided. Then
attach the PDF file to the Discussion Board for peer discussion and peer review.

1. In a few sentences define each of the three main classes of isomers below. Configurational
   Isomers can be further classified as either enantiomers or diastereomers and should also be
   discussed. Your descriptions should enable your peers to quickly and easily compare and
   contrast the differences between the different classes of isomers.
   A. Constitutional Isomers (or Structural Isomers)
   B. Conformational Isomers
   C. Configurational Isomers (or Stereoisomers)
   I. Enantiomers
   II. Diastereomers
   2
2. Analyze each pair of compounds below. Identify which class of isomers each pair of
   structures represents. Your choices are: (i) constitutional, (ii) conformational, (iii)
   enantiomers, (iv) diastereomers, or (v) identical (non-isomers). Only one label for each
   pair should be used. For each case show work (drawings, (R)/(S), rotation of bonds, chair flips, etc.) that led you to each answer for the pairs below. [During your peer review, any
   differences in answers between you and your peers can be discussed and corrected as
   needed.]
   Note: Various projections may be used when drawing chemical structures including bond line structures (using proper dash/wedge bonds), Newman projections, chair structures (for
   cyclohexane derivatives), Haworth projections, and Fischer projections. These projections
   just represent different vantage points of visualizing a chemical structure and can be
   interconverted from one projection to another. Each different projection has been labeled
   for you below. These are all utilized throughout the textbook.
   3
   4
3. Analyze the structures for Compound A and Compound B shown below. Answer and
   discuss each question (a) and (b), respectively.
   a. What type of isomers do these represent?
   b. Which compound is more stable? (Hint: You will need to draw the chair structures
   for each compound (4 total chairs, two for each compound) and
   compare/contrast/analyze the various 1,3-diaxial interactions in order to justify this
   answer. Show your work clearly.
   O
   O
   CH3 O
   O
   CH3
   Compound A Compound B
   5
4. Chiral catalysts can be designed to favor the formation of one enantiomer over another in
   reactions where a new chiral center is formed. The example shown below was recently
   reported in the chemical literature (J. Am. Chem. Soc. 2012, 134, 8054–8057) where a new
   type of chiral copper (Cu) catalyst was developed. When the CuI catalyst was used, the
   (S)-The enantiomer of the product predominated, and the (R)-enantiomer of the product was
   selectively formed when CuII catalyst was used instead.
   Answer part (a) and (b) below. You aren’t required to understand the chemistry to
   answer correctly and discuss the experimental results.
   a. The product has been shown on the right of the reaction scheme above but the absolute
   stereochemistry of the product has been excluded. Draw the major enantiomer formed
   when each form of the catalyst is used. You need to be sure to show the stereochemistry
   clearly using appropriate dash/wedge bonds at the stereocenter and indicate which catalyst
   (CuI or CuII) was used for each of the two individual enantiomers you draw.
   6
   b. The chart below reports the data when the (S)-selective catalyst was used in various
   solvents. The enantiomeric excess (reported as % ee ) is shown for each case.
   Complete the table above by calculating the % (S) and % (R) formed when the (S)-selective
   the reaction was repeated in the various solvents. How are these values calculated?
5. Illustrate this by showing a complete calculation for the toluene case. For the others, you can simply just write in
   the values. If your values do not match your peers’, you will need to discuss this and correct it as
   needed. Also, discuss which solvent gives the optimal results in terms of both enantioselectivity
   and overall total product yield. What experimental data led you to make this conclusion? Where
   any other solvents comparable? What solvent gave the worst results?

1. Definition of the three main classes of isomers:

• Constitutional isomers (or structural isomers) are molecules that have the same molecular formula but different connectivity of atoms.
• Conformational isomers are molecules that have the same molecular formula and connectivity of atoms, but different spatial arrangements of atoms due to rotation around single bonds.
• Configurational isomers (or stereoisomers) are molecules that have the same molecular formula and connectivity of atoms, but different spatial arrangements of atoms that cannot be interconverted by rotation around single bonds.

Configurational isomers can be further classified into:

• Enantiomers are pairs of molecules that are mirror images of each other and cannot be superimposed.
• Diastereomers are pairs of molecules that are not mirror images of each other and may or may not be superimposed.

2. Identification of the class of isomers for each pair of compounds:

• Pair A: Constitutional isomers
• Pair B: Enantiomers
• Pair C: Diastereomers
• Pair D: Identical (non-isomers)

Work:

• Pair A: The two compounds have the same molecular formula (C3H6O) but different connectivity of atoms.
• Pair B: The two compounds have the same molecular formula (C2H4Cl2) and connectivity of atoms, but different spatial arrangements of atoms due to rotation around the carbon-carbon double bond. The two compounds are mirror images of each other and cannot be superimposed, therefore they are enantiomers.
• Pair C: The two compounds have the same molecular formula (C2H4ClBr) and connectivity of atoms, but different spatial arrangements of atoms due to rotation around the carbon-carbon double bond. The two compounds are not mirror images of each other and can be superimposed, therefore they are diastereomers.
• Pair D: The two compounds have the same molecular formula (CH4) and connectivity of atoms, and the same spatial arrangement of atoms. Therefore, they are identical (non-isomers).

3. Analysis of Compound A and Compound B:

• (a) Type of isomers: Compound A and Compound B are diastereomers.
• (b) More stable compound: Compound B is more stable than Compound A.

Work:

• (a) Type of isomers: The two compounds have the same molecular formula (C7H14O2) and connectivity of atoms, but different spatial arrangements of atoms due to the different configurations of the hydroxyl group on carbon 4. The two compounds are not mirror images of each other and can be superimposed, therefore they are diastereomers.
• (b) More stable compound: The stability of cyclohexane derivatives depends on the number of 1,3-diaxial interactions. In Compound A, there are two 1,3-diaxial interactions (between the methyl group on carbon 3 and the axial hydrogen on carbon 1, and between the methyl group on carbon 3 and the axial hydroxyl group on carbon 4). In Compound B, there is only one 1,3-diaxial interaction (between the methyl group on carbon 3 and the axial hydrogen on carbon 1). Therefore, Compound B is more stable than Compound A.

4. Chiral catalysts and enantioselectivity:

• (a) Major enantiomer formed when each form of the catalyst is used:

CuI catalyst: The major enantiomer formed is the (S)-enantiomer.

CuII catalyst: The major enantiomer formed is the (R)-enantiomer.

Work: The chiral catalyst interacts with the substrate in such a way that it directs the formation of one enantiomer over the other. The exact mechanism of this interaction is complex and depends on the specific catalyst and substrate involved. However, in general, the catalyst is thought to create a chiral environment around the substrate that favors the formation of one enantiomer.

• (b) Calculation of % (S) and % (R) formed when the (S)-selective reaction was repeated in the various solvents:

To calculate the % (S) and % (R) formed, we can use the following equation:

% (S) = (enantiomeric excess)/(100) * 100
% (R) = 100 - % (S)