Pentane vs. 2,2 Dimethylpropane

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Pentane vs. 2,2 Dimethylpropane

Postby EllenRenskoff-1C » Fri May 18, 2018 5:18 pm

During lecture today (Friday), Professor Lavelle talked about the melting point of pentane vs. that of 2,2 dimethylpropane. He said that the molecule that is less tightly packed has a higher melting point. I am confused about why this would be, because intuitively the molecule that is more tightly packed would be harder to melt/ break apart. Why is this the case?

Andrew Evans - 1G
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Re: Pentane vs. 2,2 Dimethylpropane

Postby Andrew Evans - 1G » Fri May 18, 2018 5:58 pm

Hi Ellen,
I didn't hear him say that molecules that are less tightly packed have higher melting points. In fact, I think your intuitions are correct. Since pentane is a long and thin chain of carbons, they can lie right up against one another with a small distance r, and therefore pentane will have stronger dispersion attractions and therefore have a higher melting point. Meanwhile the 2,2 dimethylpropane, with its near spherical structure is unable to get that close to the other molecule, and therefore has a large distance, weak dispersion interaction, and lower melting point.

Basically, you're right, your ears were wrong. ;)
-Andrew Evans
Section 1G

Susu Le 1F
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Re: Pentane vs. 2,2 Dimethylpropane

Postby Susu Le 1F » Fri May 18, 2018 6:03 pm

I think by tightly packed he meant that the atoms in the molecule are more tightly packed, which means the atoms are closer together. So the structure is more tightly packed, and not the molecules are packed together. Therefore, pentane is less tightly packed because it’s atoms are in a straight line.

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Re: Pentane vs. 2,2 Dimethylpropane

Postby AnthonyDis1A » Sun May 20, 2018 9:10 am

Molecular structures with greater surface area tend to have higher melting points.

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