Why Is There Less Energy Difference for Iodobutane Compared to n-Butane?

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The difference in energy between gauche and anti of n-butane is 3.6kJ/mol. For the molecule iodobutane, I found the differnece in energy between the gauche and anti to be about 0.62KJ/mol...
What accounts for this difference?

and also, when you're asked to find the energy barriers to the rotation from anti to gauche and you have a graph of energy vs dihedral angle, you just find the difference in energy between anti and gauche right??
 

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jnimagine said:
The difference in energy between gauche and anti of n-butane is 3.6kJ/mol. For the molecule iodobutane, I found the differnece in energy between the gauche and anti to be about 0.62KJ/mol...
What accounts for this difference?

and also, when you're asked to find the energy barriers to the rotation from anti to gauche and you have a graph of energy vs dihedral angle, you just find the difference in energy between anti and gauche right??

Well what role does Iodine play here? You need to propose an explanation. Your second portion sounds correct if I am interpreting it correctly.
 


GCT said:
Well what role does Iodine play here? You need to propose an explanation. Your second portion sounds correct if I am interpreting it correctly.

ok to clarify I'm comparing the energy difference between gauche and anti of n-butane and 1-iodopropane not butane...- -;; what was i thinking...
anyways, I just think that there'd be more energy difference when there is Iodine because it's a bulkier group... hence more steric strain... but it turns out it's the opposite...! > . <
very confusedddd
 


You need to also consider that the bond length between carbon and iodine is about 2 angstroms while the bond length between carbons in butane is about 1.54 angstroms.

BTW... are you sure that iodine is bulkier than a methyl group?
 


chemisttree said:
You need to also consider that the bond length between carbon and iodine is about 2 angstroms while the bond length between carbons in butane is about 1.54 angstroms.

BTW... are you sure that iodine is bulkier than a methyl group?

so since they're farther apart, there's less energy..??
oh... I just thought iodine was bulkier... cuz.. it has a higher atomic number... nope... maybe not... lol ohh if methyl groupd is bulkier, it makes sense that it has a higher energy difference...
please clarify for me..! > . <
 


Atomic diameters (covalent):

Carbon - 1.52 A
Hydrogen - 0.72 A (and there's three of 'em) Only about half of this diameter adds to the total diameter of the methyl group.

Iodine - 2.66 A. So the methyl group is about 1.8 A in diameter which is smaller than iodine at about 2.7 A.

Oops! My reference shows the difference between gauche and anti in n-butane to be about 0.6 kcal/mol, not 3.6 kcal/mol. 3.4 kcal/mol is the energy difference between eclipsed and gauche... and it's kcal/mole not kJ/mole.
 
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chemisttree said:
Atomic diameters (covalent):

Carbon - 1.52 A
Hydrogen - 0.72 A (and there's three of 'em) Only about half of this diameter adds to the total diameter of the methyl group.

Iodine - 2.66 A. So the methyl group is about 1.8 A in diameter which is smaller than iodine at about 2.7 A.

Oops! My reference shows the difference between gauche and anti in n-butane to be about 0.6 kcal/mol, not 3.6 kcal/mol. 3.4 kcal/mol is the energy difference between eclipsed and gauche... and it's kcal/mole not kJ/mole.

no... the question stated that the difference between gauche and anti of n-butane is 3.6kJ/mol... not kcal/mol...
 


jnimagine said:
no... the question stated that the difference between gauche and anti of n-butane is 3.6kJ/mol... not kcal/mol...

OK, my bad. I should have looked at that more carefully.
 


chemisttree said:
OK, my bad. I should have looked at that more carefully.


so the reason why there's less energy difference for 1-iodopropane... is that I is a less bulkier group, and the bond length with C is longer??