Answer:
21 KJ/mol
Explanation:
For this question, we have to start with the linear structure of 2-methylbutane. With the linear structure, we can start to propose all the Newman projections keep it in mind that the point of view is between carbons 2 and 3 (see figure 1).
Additionally, we have several energy values for each interaction present in the Newman structures:
-) Methyl-methyl gauche: 3.8 KJ/mol
-) Methyl-H eclipse: 6.0 KJ/mol
-) Methyl-methyl eclipse: 11.0 KJ/mol
-) H-H eclipse: 4.0 KJ/mol
Now, we can calculate the energy for each molecule.
Molecule A
In this molecule, we have 2 Methyl-methyl gauche interactions only, so:
(3.8x2) = 7.6 KJ/mol
Molecule B
In this molecule, we have a Methyl-methyl eclipse interaction a Methyl-H eclipse interaction and an H-H eclipse interaction, so:
(11)+(6)+(4) = 21 KJ/mol
Molecule C
In this molecule, we have 1 Methyl-methyl gauche interaction only, so:
3.8 KJ/mol
Molecule D
In this molecule, we have three Methyl-H eclipse interaction, so:
(6*3) = 18 KJ/mol
Molecule E
In this molecule, we have 1 Methyl-methyl gauche interaction only, so:
3.8 KJ/mol
Molecule F
In this molecule, we have a Methyl-methyl eclipse interaction a Methyl-H eclipse interaction and an H-H eclipse interaction, so:
(11)+(6)+(4) = 21 KJ/mol
The structures with higher energies would be less stable. In this case, structures B and F with an energy value of 21 KJ/mol (see figure 2).
I hope it helps!
