This is a long, graphics intensive post. You probably don’t need to read it all now, but chances are, you’ll REALLY need to know the skill I describe here soon. Because it’s an extremely common question found on The First Midterm.
What this post teaches you how to do is to calculate the energies of different Newman Projections and then graph them.
Conceptually, it’s not hard, IF you attack it methodically. Ultimately, it’s no more difficult than identifying a few items on a menu and tallying them up, and graphing them. [Note: if you still find this hard after reading this, *comment!!!!* . I want to be able to address any lingering questions you might have about this!]
Mastering this skill requires 3 parts.
First, take the molecule in question and draw out the 6 key conformations (see here).
Part 1. Examine the “menu”.
You have to be given this information: you can’t figure it out by thinking, because these are experimentally measured results.
Remember how “like charges repel”? Electrons don’t like each others company! So when electron clouds bump into one another, this has an energy “cost”. These are called “steric interactions”. The larger the group (such as CH3 vs. H) the greater the costs of steric interactions.
There are 4 key “costs” in our example. Note that these are in kcal/mol. Converting into kJ/mol is kinda like converting $USD into Argentine pesos: multiply by 4.184.
- eclipsing CH3 CH3 – the most “costly” interaction at 2.9 kcal/mol
- eclipsing CH3 H – this costs 1.4 kcal/mol
- eclipsing H H – this costs 1.0 kcal/mol
- gauche (“awkward”) CH3 CH3 (they are separated by 60 degrees) – 0.9 kcal/mol.
2. Calculate The Tally
So now, make a table. Add up each interaction in each conformation. Take the interaction for “12:00” (dihedral angle of 0°). One eclipsed CH3 CH3 (1 x 2.9), and two eclipsed H H interactions (2 x 1.0) . Total: 4.9 kcal/mol.
3. The Graph.
Now make a graph of angle versus energy. Plot the dots first. Then draw curvy lines between them. This tells you what happens to the energy level as you rotate the central bond (like you’d rotate the hands of a clock) through 360 degrees.
I know this was a long post. But it’s such a crucial skill to be able to know for the first part of the course. Use this as a walk-through when you need to learn this skill.