## Relationship between Standard Molar Entropy and Molecular Complexity (9.71)

Michelle_Nguyen_3F
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Joined: Wed Sep 21, 2016 2:59 pm

### Relationship between Standard Molar Entropy and Molecular Complexity (9.71)

In the Course Reader, on p. 36, it says that "More complex molecules have higher $S^{\circ}$ due to a greater number of vibrational and rotational motions (atoms have a greater number of possible positions" which I understand. However,one of the homework questions, 9.71 asked "Which substance would you expect to have the highest molar entropy in the liquid phase, benzene, methanol (CH3OH), or 1-propanol (CH3CH2CH2OH)?" Both benzene and 1-proanol have the same number of atoms, but in the solutions manual, it says that benzene has a more rigid planar ring structure. In determining the higher molar entropy of a substance, how do we classify a structure as rigid or flexible? Thank you!!

Chem_Mod
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### Re: Relationship between Standard Molar Entropy and Molecular Complexity (9.71)

Comparing the molecular structures, you can see that benzene is unique for having double bonds between the carbons (sp 2 hybridized), so for each carbon there are 3 of sp2 hybrized orbitals interacting with the surrounding 2 carbons and 1 hydrogen. Since sp 2 hybridized orbitals are bonding, that leaves a single p orbital on each carbon to interact through pi-pi overlap. Now, would this pi-pi overlap interaction make the molecule more rigid or no?

Michelle_Nguyen_3F
Posts: 40
Joined: Wed Sep 21, 2016 2:59 pm

### Re: Relationship between Standard Molar Entropy and Molecular Complexity (9.71)

Chem_Mod wrote:Comparing the molecular structures, you can see that benzene is unique for having double bonds between the carbons (sp 2 hybridized), so for each carbon there are 3 of sp2 hybrized orbitals interacting with the surrounding 2 carbons and 1 hydrogen. Since sp 2 hybridized orbitals are bonding, that leaves a single p orbital on each carbon to interact through pi-pi overlap. Now, would this pi-pi overlap interaction make the molecule more rigid or no?

Yes, it would. Thank you!

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