Topic 2: Stereochemistry Part 1: Conformation of Molecules

Question 1

Is axial or equatorial higher in energy and why

Answer 1

Axial
Unfavourable gauche interactions
Repulsive 1,3 diaxial interactions

Question 2

Why would an eclipsed form of ethane be higher in energy

Answer 2

Steric clash between hydrogen atoms
Electron repulsion in C-H bonds
No hyperconjugation causing a stabilisation interaction as C-H bonds not 180° (antiperiplanar)

Question 3

A barrier of … kJmol^-1 allows 1 rotation per second at … °C

Answer 3

73
25

Question 4

Every … kJmol^-1 changes the rate at … °C by a factor of ~10

Answer 4

6
25

Question 5

To see separate peaks in an NMR spectrum for different conformations, the rate of interconversion must be > … s^-1 with energy barrier … kJmol^-1 and temperature … °C

Answer 5

1000
-55
25

Question 6

If the energy barrier is > … kJmol^-1, conformation interconvert slowly enough to exist as different compounds

Answer 6

100

Question 7

Rotamers

Answer 7

Single rotational isomers

Question 8

Effect of increased straight on cyclical structures

Answer 8

Raises energy
Decreases stability

Question 9

Are saturated cyclic systems planar

Answer 9

No with one exception - 6 membered rings

Question 10

Why isn’t a five-members ring completely planar ?
What happens to the ring instead ?

Answer 10

Unfavourable eclipsing C-H bonds in planar position
One C goes above plane to give envelope form