The barrier to C-C bond rotation in bromoethane is 15 kJ/mol. What energy can you assign...

The barrier to C-C bond rotation in bromoethane is 15 kJ/mol. What energy can you assign to an H-Br eclipsing interaction?
Use strain energy increments from the table below in your calculation.

(Specify units, kJ/mol; Calculate to nearest 0.1 energy units)

		Energy cost
Interaction	Cause	(kJ/mol)	(kcal/mol)
H ↔ H eclipsed	Torsional strain	4.0	1.0
H ↔ CH₃ eclipsed	Mostly torsional strain	6.0	1.4
CH₃ ↔ CH₃ eclipsed	Torsional and steric strain	11.0	2.6
CH₃ ↔ CH₃ gauche	Steric strain	3.8	0.9

Expert Solution

The barrier to C-C bond rotation is the difference between relative energies of the most stable conformer and the least stable conformer of the molecule.

Now, for bromoethane, we can draw the most stable (staggered) and least stable(eclipsed) conformer as follows

Since the difference between the most stable and least stable conformer of bromoethane is given to be 15 kJ/mol, we can write

Hence, the energy assigned to the H-Br eclipsing interaction is about 7.0 kJ/mol.

queen_honey_blossom answered 2 years ago

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