Newman Projections Practice Problems
Construct a potential energy graph for n-pentane as it rotates about the bond between C2 and C3. Start with a 0° dihedral angle and rotate the front carbon.
Draw a potential energy diagram for the conformational analysis of n-butane as it rotates around the single bond between C2 and C3.
Hint: One C—H bond eclipsed with another C—H bond contributes to 4.2 KJ/mol, one C—H bond eclipsed with a C—CH3 bond contributes 5.4 KJ/mol while One C—CH3 bond eclipsed with a C—CH3 bond contributes 13 KJ/mol to torsional energy. Similarly, Gauche (60° methyl group) will add 3.8 KJ/mol energy.
Draw potential energy vs dihedral angle graph for conformational analysis of propane as it rotates around the single bond between C1 and C2. Also, draw the Newman projections for each eclipsed and staggered conformation and show the dihedral angles.
Hint: One C—H bond eclipsed with another C—H bond contributes to 4.2 KJ/mol torsional energy. One C—H bond eclipsed with a C—CH3 bond contributes to 5.4 KJ/mol torsional energy.
Based on the given pair of conformations, predict which conformation is expected to be more favored at equilibrium.
Provide the Newman projection for the molecule given below. Project on the 3,4-bond and ensure that the dihedral angle between the chlorine and hydrogen atoms is 180°.
Consider the following pairs of conformations.
Identify which conformation has the lowest strain energy.
The chair conformation of bromocyclohexane with the bromo substituent in an axial position is about 0.43 kcal/mol more stable compared to the conformation with the bromo substituent in an equatorial position. Determine how much more stable the anti-conformation of 1-bromopropane is than its gauche conformation. (Rotate about C1-C2 bond)