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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

Experiment 3:

STEREOCHEMISTRY AND MOLECULAR MODELING OF CYCLOALKANES

objectives
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
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
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
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
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
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
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?
table 3 1

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!

part b conformational analysis hyperchem
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 hyperchem14
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.
part b conformational analysis bond angles
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
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!