Confused with MP and BP... help!

[192LT192]

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between cyclopentane and n-pentane.... which has higher BP and which has higher MP?

the answer is that the cyclopentane has both the higher MP and higher BP. why?????? cant the n-pentane fit tighter together and therefore be harder to boil and therefore have a higher BP? also shouldnt the n-pentane have a higher MP bc its easier to fit it into a lattice?

i clearly dont understand what youre supposed to be thinking about when solving higher mp/bp in orgo. can someone help with the logic behind this?

 

[Irrizistuble]

i would think cyclic compounds are pretty stable which makes it harder to break the bonds increasing the MP and BP

 

[ttran9229]

between cyclopentane and n-pentane.... which has higher BP and which has higher MP?

the answer is that the cyclopentane has both the higher MP and higher BP. why?????? cant the n-pentane fit tighter together and therefore be harder to boil and therefore have a higher BP? also shouldnt the n-pentane have a higher MP bc its easier to fit it into a lattice?

i clearly dont understand what youre supposed to be thinking about when solving higher mp/bp in orgo. can someone help with the logic behind this?

Cycloalkanes have higher contact surface area than the corresponding alkanes. This increases the effects of intermolecular forces which results in higher m.p. and b.p. Branching, on the other hand, reduces contact area resulting in lower m.p. and b.p.

To illustrate this, form a fist with both of your hands. Place both your fists together in such a way as to maximize the contact area. Now, do the same exercise with both of your hands open. You should be able to see a big difference.

 

[192LT192]

Cycloalkanes have higher contact surface area than the corresponding alkanes. This increases the effects of intermolecular forces which results in higher m.p. and b.p. Branching, on the other hand, reduces contact area resulting in lower m.p. and b.p.

To illustrate this, form a fist with both of your hands. Place both your fists together in such a way as to maximize the contact area. Now, do the same exercise with both of your hands open. You should be able to see a big difference.

thank you so much. this really helped me out!

 

[bbwreckerz04]

Cycloalkanes have higher contact surface area than the corresponding alkanes. This increases the effects of intermolecular forces which results in higher m.p. and b.p. Branching, on the other hand, reduces contact area resulting in lower m.p. and b.p.

To illustrate this, form a fist with both of your hands. Place both your fists together in such a way as to maximize the contact area. Now, do the same exercise with both of your hands open. You should be able to see a big difference.

Branching actually increases mp due to symmetry packing.

 

[ttran9229]

Branching actually increases mp due to symmetry packing.

I agree, but the degree of symmetrical packing depends on the molecule. For example, melting point for pentane and hexane is shown below.

Melting points
n-pentane −129.8 °C
isopentane −160 °C
neopentane −18 °C

n-hexane −95 °C
isohexane -153°C
neohexane -98.8 °C

For their iso- isomers, both molecules have lower m.p. when compared to their straight chain, but neopentane has higher while neohexane has lower m.p. when compared to their straight chains. As you have pointed out, "packing" does increase m.p., but it also depends on the degree of this phenomenon. This can be seen in the hexane isomers. Neohexane has a higher m.p. than isohexane, but lower m.p. than n-hexane. One can reason how well a molecule with stack on each other, but, in general, branching tends to lowers m.p. if "packing" is not signifcant.