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## Experiment 3:

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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?**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**• 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.**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!