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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 l.jpg

Experiment 3:

STEREOCHEMISTRY AND MOLECULAR MODELING OF CYCLOALKANES


Objectives l.jpg
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.





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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!


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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!


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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.


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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.


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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.


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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?


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Disubstituted cyclohexane

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 3.1

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


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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


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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.


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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.


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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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