McMurry (kap.3,7)

Question 1

Newman projection with (3C) atomer - ex. propane.

3 interactions in eclipsed form (2x H-H bonds and 1x CH3-H bond)

Which of these have highest torsional strain?

Answer 1

The CH3-H bond have the highest (6Kj/mol)

Then the H-H have the lowest with (4Kj/mol).

In total =14 Kj/mol. in eclipsed form.

Question 2

For larger alkanes, the conformational situation becomes more complex, because not all staggered have the same energy, and not all eclipsed have the same energy. (C4)

What is the lowest energy arrangement then called? (most stable?)

Answer 2

Anti conformation - the two methyl groups are farthest apart from each other.

Question 3

What is the gauche conformation? (C4)

Answer 3

It is higher in energy (less stable) then the anti conformation, even though it has no eclipsing conformations. (the hydrogen atoms of the methyl groups are near one another in the gauche conformation which causes “steric strain”.

Question 4

What is steric strain?

Answer 4

The repulsive interaction occurred when atoms are forced closer together then their atomic radii. The result of trying to force two atoms to occupy the same space.

For example when the methyl of another methyl is closer to each other (not eclipsed), but besides each other in the gauche form, which causes steric strain.