Conformational Analyses of Ethane and Butane

Conformational Analyses of Ethane and Butane

Introduction:

This exercise was designed to help me become familiar with using Chem3D and Excel. I was required to draw two hydrocarbons (ethane and butane) using Chem3D (a product of CambridgeSoft). I was then required to complete steric energy analyses for each. For the ethane, I was to rotate the C1 methyl group (the methyl group on the first carbon) in 15 degree increments, and calculate the energy interactions after each rotation. For the butane, I was to rotate around the C2-C3 bond (the bond between the 2^nd and 3^rd carbons), starting with the dihedral angle, and calculating the energy interactions after each rotation. The rotation for the butane also occurred in 15 degree increments.

I then entered my data into Microsoft Excel spreadsheets, completing data tables and graphs for each conformational analysis. For the ethane I recorded torsion, 1-4 van der Waals, and total energy. For the butane I recorded torsion, non-1-4 van der Waals, 1-4 van der Waals, and total energy. I chose to draw my Newman Projections using Microsoft Word, rather than ChemDraw (another product of CambridgeSoft), for purely aesthetic reasons.

Another purpose of this activity was to help illustrate the concept that as groups on a hydrocarbon move, due to rotation around a bond, and are in different placements in relationship to other groups on the same molecule, the amount of energy involved in what are called steric interactions fluctuates. There are several types of steric interactions, and as molecules increase in length and branching, and the substituted groups increase in size, the type and combination of the different steric interactions increases.

An appropriate metaphor might be that of an extended family choosing to live under one roof. When it is just the couple, then there is plenty of room, and each person can enjoy personal space. As children are added, there is less room for each person. If grandparents, aunts, uncles and cousins are thrown in, it becomes extremely crowded, and there is very little room for anyone to enjoy personal space, increasing the likelihood of discord, and the general irritation of rubbing shoulders in close quarters.

Ethane – C₂H₆

Lewis Structure:

Analysis of Energies:

Ethane, with only carbon-hydrogen bonds interacting with each other, displays simple sine curves when the torsion, 1-4 Van der Waals, and total energies are graphed. The maximum energies are the absolute maximum; the minimum energies are the absolute minimum, and there are no intermediate maxima or minima. In addition, the minima and maxima for torsion, 1-4 van der Waals, and total energy fall at the same rotation points.

The maxima occur when the methyl groups are aligned, while looking down the carbon-carbon bond. This is also referred to as an eclipsed formation (see Newman Projections). This occurs, because the energies of the carbon-hydrogen bonds are fighting against each other in several different ways (see graph of energies).

The minima occur when the methyl groups are rotated so that the hydrogens on the C1 methyl group sit between the hydrogens on the C2 methyl group, while looking down the carbon-carbon bond. This is also referred to as a staggered formation (see Newman Projections). In this conformer the bonds, and the electrons in the bonds have more space, so have less steric interaction (see graph of energies).

Physical Properties:

Colorless, odorless gas

Molecular Weight: 30.07

Density: 1.212 kg/cm³

Solubility in H₂O: 4.7 g/100 mL

Melting Point: -182.8°C

Boiling Point: -88.6°C

Eclipsed Formation: 60°, 180°, 300° (maximum energy)

Staggered Formation: 0°, 120°, 240°, 360°

(minimum energy)

n-Butane – C₄H₁₀

Lewis Structure:

Analysis of Energies:

Butane, with two more carbons than ethane, has much more steric interaction when the different groups are rotated around their bonds. When looking at the rotation around the C2-C3 bond, we are functionally looking at the interaction of a carbon-methane bond with another carbon methane bond, carbon-methane with carbon-hydrogen, and carbon-hydrogen with carbon-hydrogen. This complicates the steric interactions, moving the graph away from the sine curve exhibited by the ethane.

When starting the conformational analysis at the dihedral angle, the shape of the carbon-carbon bonds is roughly a “U” formation. This conformer exhibits the maximum energy point for 1-4 van der Waals, non-1-4 van der Waals, and total energy. This is not the maximum point for torsion, which appeared at 120° and 240°. This is different from ethane, which exhibits maxima and minima for all energies at the same points of rotation.

Also different from ethane are the presence of secondary minima and maxima for several of the types of energy examined. When looking at total energy, we see that butane has maxima at 0° and 360° (see graph of energies). Secondary maxima are found at 120° and 240° and secondary minima are found at 75° and 285° (see chart for maxima and minima of individual energies). This occurs, because the carbon-methane bonds interact differently with other carbon-methane bonds, than they do with carbon-hydrogen bonds. In addition, the carbon-hydrogen bonds interact differently with other carbon-hydrogen bonds than they do with carbon-methane bonds. Like ethane, maxima are observed in eclipsed conformers, and minima are observed at staggered conformers (see Newman projections).

In general, the bigger the groups involved in the bond interaction, the more steric energy is involved. This means that a carbon-methane bond interacting with another carbon-methane bond is at a much higher energy level than a carbon-methane bond interacting with a carbon-hydrogen bond, which in turn is at a higher energy level than a carbon-hydrogen bond interacting with a carbon-hydrogen bond.

Line-Angle Structure:

Physical Properties:

Colorless gas

Molecular Weight: 58.08

Density: 2.52 g/L (g)

Solubility in H₂O: 6.1 mg/100 mL

Melting Point: -138°C

Boiling Point: -0.5°C

Highly flammable

Inhalation can be fatal

CH₃

Eclipsed Formation: Dihedral Angle

(maximum total energy)

Gauche Formation: 75°

(secondary minimum)

Eclipsed Formation: 120°

(secondary maximum)

Staggered Formation: 180°

(minimum total energy)

Eclipsed Formation: 240°

(secondary maximum)

Gauche Formation: 285°

(secondary minimum)

Energy	Max	Min	2° Max	2° Min
Total	0, 360	180	120, 240	75, 285
Torsion	120, 240	180	0, 360	60, 300
1-4 van der Waals	0, 360	180	120, 240	60,300
Non 1-4 van der Waals	0, 360	90-270 (with some inflection)	N/A	N/A

* Torsion and 1-4 van der Waals exhibit the same maxima and minima. Total energy exhibits the same maxima, but different secondary minima from torsion and 1-4 van der Waals. Non 1-4 van der Waals exhibits a minimum with inflection, but no secondary maxima or minima.

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