Question

4. Using the CA-Cs bond axis as a reference, draw and label all relevant conformations (Newman projections) for octane. Also, identify the dihedral angle between the pertinent substituents on C4 and C5. Finally, draw a graph of relative energy vs. dihedral angle for octane. (24 points)

Answer 1

The structure of octane is When viewed through the C4-C5 axis with C4 on top and C5 on bottom, we can draw the following starting Newmann projection: н HTCH CH2CH3 H3CH2CH2CH We are starting with a dihedral angle of 180 between the alkyl groups on C4 and C5. Now, we will move 60 degrees clockwise on the back carbon to get all the major conformations as follows: 60 degree rotation of C5 clockwise HH HTCH,CHCH CH2CH2CH3 H2CH2CH2C Y H2CH2CH2C X H H B Dihedral angle :240) A Dihedral angle: 180 60 degree rotation of C5 clockwise HH 60 degree rotation of C5 clockwise HTH H2CH2CH2CH H2CH2CH2C D H2CH2CH2CQH CH2CH2CH3 Dihedral angle: 0) C Dihedral angle: 300 60 degree rotation of C5 clockwise 60 degree rotation of C5 clockwise Hснонс Н A H2CH2CH. C H H2CH2CH2C X H H2CH2CH2CVH E Dihedral angle:60) F (Dihedral angle: 120)

Now, we can quantify the interaction energies in each of the above structures to graph the energy vs the dihedral angle.

The approximate interaction energies of different interactions are

H-H eclipsed interaction: 4 kJ/mol

Alkyl-H eclipsed: 6.5 kJ/mol

Alkyl-alkyl eclipsed : 14 kJ/mol

Alkyl-alkyl gauche interaction: 4.5 kJ/mol

Now, structure A has no eclipsing interaction and no gauche interaction.

Hence, relative energy of structure A is 0 kJ/mol

Structure B has two alkyl-H eclipsed interaction and one H-H eclipsed interaction.

Hence, total relative energy is 6.5 + 6.5 + 4 = 17 kJ/mol

Structure C has only one alkyl-alkyl gauche interaction.

Hence, relative energy of C is 4.5 kJ/mol

Structure D has one alkyl-alkyl eclipsed and two H-H eclipsed interactions.

Hence, total relative energy of D is 14+4+4 = 22 kJ/mol

Structure E is equivalent to structure C. Hence, it has relative energy of 4.5 kJ/mol

Structure F is equivalent to structure B. Hence, it has relative energy of 17 kJ/mol.

Now, we can make the following table of relative energy vs dihedral angle to plot the graph

Dihedral angle	Relative enery(kJ/mol)
0	22
60	4.5
120	17
180	0
240	17
300	4.5

We get the following graph by plotting dihedral angle in x axis and relative energy in y axis.

Relative energy vs Dihedral angle of octane conformations Relative energy(kl/mol) 0 50 100 150 250 300 350 400 200 Dihedral angle