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Chem 3A Final
Chem 3A Final
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Topic G

Newman Projection of Butane (and Gauche Conformation)

1.Staggered And Eclipsed Conformations Of Butane

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since all the bonds in butane are sigma bonds, they all rotate freely and both drawings are acceptable ways to draw butane

4.“Steric Interactions” In The syn- Conformation of Butane

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when electron clouds surrounding the hydrogen on the CH3 groups are brought close together, the electron-electron repulsion is destabilizing and their natural inclination is to “push away” from one another(repel), raising the energy.

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5.The CH3-CH3 Eclipsing Interaction In Butane “Costs” About 3 kcal/mol

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6.Two More Eclipsed Conformations Of Butane

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7.The Gauche Conformation of Butane – Not All Staggered Conformations Are Equal!

the next two staggered conformations of butane with a dihedral angle between methyl groups of 60 and 300 degrees respectively
these two conformations have a torsional strain of about +0.9 kcal/mol relative to the (lowest) anti conformation
This is because the two most electronegative groups are not that far apart. They are actual quite close together, creating the gauche conformation which is signifigantly higher than the anti conformation while still being lower than the syn conformation
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fun fact : gauche means awkward so think that it is the awkward in-between from anti to syn

8.Graphing Out The Conformations of Butane

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How To Draw The Cyclohexane Chair Conformation

Step 1: Draw in all Axial lines

Find the foot rest(downward point)→ draw an axial line straight down
Find head rest(upward point)→ draw an axial line straight up
Draw the rest of the axial lines up and down alternating from the lines you have already drawn

Step 2: Draw in all Equatorial lines

when the axial group is down → draw the equatorial group somewhat moving up
when the axial group is up → draw the equatorial group somewhat moving down
** hint: draw the equatorial groups parallel to the corresponding lines that have already been drawn in the notes(check notes G for further clarification)
All chairs should be drawn in this fashion, so if you have a chair that your not used to looking a certain way, follow these same rules and the results will be accurate

Cyclohexane Chair Conformation Stability: Which One Is Lower Energy?

1. A-Values Are A Useful Measure of Bulkiness

The greater the A-value, the bulkier the group is on a cyclohexane, meaning that it will be favored in the equatorial position

2. A-Values Are Additive 3. Example: Determining The Most Stable Conformation Of cis– And trans- 1,2-Dimethylcyclohexane

Since we can add up the A-values to determine the most high energy chair conformation, it is safe to assume the fewer axial groups a chair has, the lower energy and more favorable the chair will be.

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4. To Determine Chair Conformation Stability, Add Up The A-Values For Each Axial Substituent. The Lower The Number, The More Stable It is.

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