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Experiment 3:. STEREOCHEMISTRY AND MOLECULAR MODELING OF CYCLOALKANES. OBJECTIVES. To learn how to construct various cyclohexane conformers using handheld molecular models and the HyperChem molecular modeling program.

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### Experiment 3:

STEREOCHEMISTRY AND MOLECULAR MODELING OF CYCLOALKANES

OBJECTIVES

- To learn how to construct variouscyclohexane conformers using handheld molecular models and the HyperChem molecular modeling program.
- To determine the lowest energyconformation of the molecule by performing energy minimization calculations with HyperChem.
- To examine the individual factors that contribute to the overall energy of the system.

Conformations of Monosubstituted Cyclohexanes

- Although ring-flip occurs rapidly, the two conformers are not EQUAL!

This conformer has more diaxial interactions, therefore is higher in energy!

1,3-Diaxial Interactions

- Q: What causes the difference in energy between the conformers?
- Steric strain due to 1,3-diaxial interactions.

- Q: What is a 1,3-diaxial interaction?
- Atoms on C1 are too close to those on C3 and C5!

Disubstituted Cyclohexanes

- In disubstituted cyclohexanes, BOTH substituents experience steric interactions with axial groups.
- There are two isomers of 1,2-dimethylcyclohexane, cis and trans.
- Must consider the sum of all interactions.

Conformational Analysis of Disubstituted Cyclohexanes

- Q: What is conformational analysis?
- Assessing energy of cycloalkane by summing all steric interactions.

- Q: Why is it important?
- Can help predict which conformations are more favorable and more likely to exist.

OVERVIEW

- Sketch cyclohexane structures given the IUPAC name.
- Identify spatial relationship (cis/trans) between methyl groups on each structure using models.
- Rank stability of conformer before and after ring-flip.
- Use HYPERCHEM program to determine how much of each type of energy contributes to the overall energy of the molecule.
- Measure bond angles before and after geometry optimization to determine how angles change during energy minimization.

Part A: Conformational Stability with Models

- Using provided molecular models, build disubstituted cyclohexanes with given substitution pattern.
- Sketch cyclohexane in Table 3.1.
- Q: Are the methyl substituents cis or trans to one another?
- Q: Is this the most stable conformation as it is currently built, or would it be more stable after the ring flip occurs?

Cis or Trans?

Most stable conformation?

Structure

1,2-dimethylcyclohexane, both groups axial

trans

No: ring flips to more stable eq/eq conformer

1,2-dimethylcyclohexane, both groups equatorial

1,2-dimethylcyclohexane, one axial, one equatorial

1,3-dimethylcyclohexane, both groups axial

1,3-dimethylcyclohexane, both groups equatorial

1,3-dimethylcyclohexane, one axial, one equatorial

1,4-dimethylcyclohexane, both groups axial

1,4-dimethylcyclohexane, both groups equatorial

1,4-dimethylcyclohexane, one axial, one equatorial

Table 3.1Remember to include methyl substituents in the proper place and all of hydrogen atoms!

Part B: Conformational Analysis: HYPERCHEM

- E total = E bond stretch + E angle strain + E torsional strain + E VDW
- The HyperChem program will allow us to build the structure, then will perform energy minimization calculations in an effort to find the lowest energy conformation.
- We can keep track of the results by keeping a log of the file, which can be viewed to retrieve the desired results.

HyperChem refers to this as “dihedral” strain

Part B: Conformational Analysis: HYPERCHEM

- HyperChem will perform 2 kinds of calculations:
- SINGLE POINT
- determines the total energy of the molecule for a fixed geometry

- GEOMETRY OPTIMIZATION
- determines the lowest energy conformation by altering the molecular geometry.

- SINGLE POINT

Part B: Conformational Analysis: BOND ANGLES

- After the single point calculation has been performed, two bond angles will be determined:
- These angles will be measured again after the geometry optimization has been performed.

Part B: Using HyperChem…

- Build model with HyperChem.
- Start Log. Save to DESKTOP. Give file name such as “cis12sp”.
- Perform Single Point calculation.
- Stop Log.
- Open log file on DESKTOP. Record all required values.
- Measure bond angles.
- Deselect all atoms (NO GREEN!)
- Start Log. Save to DESKTOP. Give file name such as “cis12go”.
- Perform Geometry Optimization.
- Stop log.
- Open log file on DESKTOP. Record all required values.
- Measure bond angles.
- START OVER with next molecule!

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