Molecular Modeling Postlab – Flashcards
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The gauche conformation of butane occurs when the dihedral angle between the C1-C2 bond and the C3-C4 bond is 60°. There is steric strain in the molecule because of the proximity of the two methyl groups. Would the steric strain in this molecule increase or decrease if the C1-C2-C3 bond angle (and the C2-C3-C4 bond angle) was increased to more than 109.5° ?
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By increasing the bond angle to more than the regular tetrahedral 109.5°, the steric strain will decrease because the methyl groups are moving further apart from each other.
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The gauche conformation of butane occurs when the dihedral angle between the C1-C2 bond and the C3-C4 bond is 60°. There is steric strain in the molecule because of the proximity of the two methyl groups. When you consider that E(strain) = E(stretch) E(angle) E(torsion) E(steric), would E(torsion) increase or decrease when the dihedral angle of gauche butane is increased from 60° to 65° ?
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If the dihedral angle is increased from 60° to 65°, then the steric strain on the molecule will decrease, meaning the methyl groups will move further apart from each other. In contrast, the torsional strain will increase because other the bonds in the molecule will move closer to being eclipsed. With an increase in torsional strain, this new conformation will be higher in energy.
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The gauche conformation of butane occurs when the dihedral angle between the C1-C2 bond and the C3-C4 bond is 60°. There is steric strain in the molecule because of the proximity of the two methyl groups. Would the magnitude of this steric strain increase or decrease if the dihedral angle was increased from 60° to 65°?
answer
By increasing the bond angle from 60° to 65°, the magnitude of steric strain will decrease because the methyl groups are moving further apart. The torsional strain will increase because the neighboring hydrogen atoms are moving closer to each other with the dihedral angle increase of 5°. As a result, the total energy of the molecule will increase due to an increase in torsional strain.
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Identify the gauche interactions between methyl groups in the conformation of trans-1,2-dimethylcyclohexane.
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In the conformer of trans-1,2-dimethylcyclohexane, the methyl groups are separated by a torsion angle of 180°. There are no gauche interactions in the molecule shown.
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Identify the gauche interactions between methyl groups in the shown conformation of cis-1,2-dimethylcyclohexane.
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In cis-1,2-dimethylcyclohexane, the methyl groups are separated by a torsion angle of 60°. The steric repulsion caused by this gauche interaction destabilizes the molecule by 3.8 kJ/mol (0.9 kcal/mol).
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Identify the gauche interactions between methyl groups in the conformation of trans-1,2-dimethylcyclohexane.
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In the conformer of trans-1,2-dimethylcyclohexane, the methyl groups are separated by a torsion angle of 60°. This gauche interaction destabilizes the molecule by 3.8 kJ/mol (0.9 kcal/mol).
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Identify the atoms that participate in 1,3-diaxial interactions in the shown conformation of cis-1,2-dimethylcyclohexane.
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In cis-1,2-dimethylcyclohexane, one methyl group is axial and one methyl group is equatorial. The axial methyl group experiences a steric repulsion with two axial hydrogens on the same face. The steric repulsion caused by these two 1,3-diaxial interactions destabilizes the molecule by 2 × 0.9 kcal/mol.
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Identify the atoms that participate in 1,3-diaxial interactions in the shown conformation of trans-1,2-dimethylcyclohexane.
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In the diaxial conformer of trans-1,2-dimethylcyclohexane, the methyl groups are separated by a torsion angle of 180°. Each axial methyl group experiences a steric repulsion with two axial hydrogens on the same face. The steric repulsion caused by these four 1,3-diaxial interactions destabilizes the molecule by 4 × 0.9 kcal/mol.
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Identify the atoms that participate in 1,3-diaxial interactions interactions in the shown conformation of trans-1,2-dimethylcyclohexane.
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In the diequatorial conformer of trans-1,2-dimethylcyclohexane, the methyl groups are separated by a torsion angle of 60°. There are no 1,3-diaxial interactions in the molecule shown.
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Considering that gauche interactions and 1,3-diaxial interactions both contribute 3.8 kJ/mol to the strain energy for a conformation, calculate the total strain energy for the shown conformation of trans-1,2-dimethylcyclohexane.
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In the diequatorial conformer of trans-1,2-dimethylcyclohexane, the methyl groups are separated by a torsion angle of 60°. This gauche interaction destabilizes the molecule by 3.8 kJ/mol (0.9 kcal/mol). There are no 1,3-diaxial interactions.
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Considering that gauche interactions and 1,3-diaxial interactions both contribute 3.8 kJ/mol to the strain energy for a conformation, calculate the total strain energy for the shown conformation of trans-1,2-dimethylcyclohexane.
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In the diaxial conformer of trans-1,2-dimethylcyclohexane, the methyl groups are separated by a torsion angle of 180°. There are no gauche interactions between the methyls in the molecule shown, but there are four 1,3-diaxial interactions for a total strain energy of 15.2 kJ/mol. (4 × 3.8 kJ/mol).
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Considering that gauche interactions and 1,3-diaxial interactions both contribute 3.8 kJ/mol to the strain energy for a conformation, calculate the total strain energy for the shown conformation of cis-1,2-dimethylcyclohexane.
answer
In cis-1,2-dimethylcyclohexane, the methyl groups are separated by a torsion angle of 60°. The steric repulsion caused by this gauche interaction destabilizes the molecule by 3.8 kJ/mol. There are also two 1,3-diaxial interactions for an additional 7.6 kJ/mol.
