Week 3 Practice Worksheet: Alkanes

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Week 3 – Acyclic Conformation Worksheet

Models and Drawing Structures

Organic molecules are three-dimensional structures. It can be difficult to work with the 2D representations between these structures if you can’t visualize them.

Possible methods for visualizing these molecules:

• Use MolView to build structures that you can rotate and zoom in on o A video tutorial for using MolView

• Buy a hand held model kit: one suggested kit on Amazon • Build models with marshmallows and pretzels

2D Representations of 3D Molecules

• Remember, the carbon atoms in alkanes have _____________ geometry, so they can NOT be planar.

Wedge-and Dash Structures: This is a method of representing the three-dimensional structure in which:

Solid lines represent bonds in the plane of the image

Wedges represent bonds pointing towards you

dashed lines represent bonds pointing away from the you.

• Try drawing a wedge-and-dash structure for propane and butane:

H3C CH3 HH

HH

H

H

propane butane

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2D Representations of 3D Molecules

Newman Projections: This is a method of representing the three-dimensional structure in which you draw the molecule as you (the dragonfly) would see it you look directly down a chemical bond from front to back.

The dot represents the front atom

The bigger circle represents the next carbon

• Try drawing a Newman projection for these structures. You can build them first if it helps.

H H3C

HH

H

H H

H CH3 H

HH

H H3C

H CH3

H

H

H H3C H

CH3

HH

H H3C

H CH3

H

H3CH2C

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Different Conformations

Newman projections can portray different rotations (conformations) of the molecules.

Some of these conformations have ‘names’.

Staggered: substituents on the neighboring carbons are 60° apart

Eclipsed: substituents on neighboring carbons are 0° apart (aligned)

Anti: Two substituents are 180° apart from each other

Gauche: a substituent that is 60° apart from a substituent on the back carbon

Several conformations of butane are shown below.

• Label each structure as staggered or eclipsed. • Label any structure that is anti staggered or gauche staggered.

CH3

H H CH3

HH H

H CH3 H

CH3H H

H

H

CH3

CH3H

H

H3C

H

CH3

HH H

HH3C H

H3C H

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Strain in Different Conformations

Steric strain occurs when atoms are brought too close together. There is an associated cost in energy due to overlapping electron clouds.

• Circle sources of steric strain in these molecules. • Calculate the increased energy due to steric strain. Each steric interaction increases the

energy by ~1 kcal/mole.

Torsional strain occurs when atoms separated by three bonds are placed in an eclipsed conformation instead of the more stable staggered conformation.

• Locate sources of torsional strain in these molecules.

C

H H

H

H

H H

H

H

C H

H

H

H

H

H H

C

H H

H

H

H H

H

H

H

H

H H

H H

H H H

H H

H

H HH

H H

H

H

H H

H H

H

H H

H H H

H H

H

H H

H

HH H

H

C H

H

H

H

H

H H

C

H H

H

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Energy Diagram of Rotation in Ethane At room temperature, the carbon-carbon bond of ethane is spinning rapidly. The following graph shows the potential energy of ethane in different conformations.

• Explain the difference in energy between the two conformations of ethane.

• Why are there only two conformations of ethane with different energy?

2.8 kcal/mol

H

H H H

HH H

H H H

HH H

H H H

HH

Po te nt ia lE ne rg y

Degrees of Rotation

HH

HHHH

HH

HHHH

HH

HHHH

HH

HHHH

0 60 120 180 240 300 360

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Energy Diagram of Rotation in 2-methylbutane

• Match the following conformations with the correct energy on the following graph.

0 60 120 180 240 300

Po te

nt ia

l E ne

rg y

(k ca

l/m ol

)

Degree of Rotation

1

2

3

4

5

6

1

2

3

4

H

H CH3

CH3

CH3H H

H CH3 CH3

H3C H H

H CH3 CH3

CH3

H

H

H CH3H3C

CH3

H