BP and MP

[marymatthews]

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I just want to get this straight, because every book says something different...
Branching decreases boiling point, but increases melting point?
Higher MW increases BP, but decreases MP?

 

[casper22]

I want to say this is true... however would love to see other response...

 

[gigawatt]

The rules for MP and BP are the same, with one caveat.

Higher MP and BP:
1. more hydrogen bonding
2. higher molecular weight
3. less branching

Caveat: the effect of branching on MP can go either way depending on how it enables or prevents close packing of molecules...so this is unlikely to be tested.

 

[2PacClone23]

By extension, it also raises FP too right?

 

[mm12785]

Branching decreases boiling point, but INCREASES melting point because branched molecules can fit into a crystal.

 

[marymatthews]

Ok so is what a i posted right or wrong?

 

[drpj]

Basically, branching DECREASES BP but INCREASES MP.

Higher MW increases both.

 

[gigawatt]

The trends for MP and BP are the same. In most cases, branching disrupts the crystal packing and decreases melting point. The caveat is cases where branching provides symmetry or better packing, which is when melting point increases.

http://en.wikipedia.org/wiki/Pentane#Physical_properties

Everything is spelled out here using the n-pentane, isopentane, and neopentane series as an example.

 

[marymatthews]

i am more confused than before
i am getting different answers?

 

[UndergradGuy7]

The trends for MP and BP are the same. In most cases, branching disrupts the crystal packing and decreases melting point. The caveat is cases where branching provides symmetry or better packing, which is when melting point increases.

http://en.wikipedia.org/wiki/Pentane#Physical_properties

Everything is spelled out here using the n-pentane, isopentane, and neopentane series as an example.

How is that the same... you just said melting point increases and boiling point decreases. Thats not the same trend?

Anyway correct answer..
Branching decreases BP, increases MP

Source:
http://masterorganicchemistry.wordpress.com/2010/07/09/chemical-tetris/
(its orgohacks so you know its gotta be right, and Chad says it too)

 

[Avery07]

Higher MW increases both.

Like undergrad guy says, branching decreases BP and increases MP.

Or.. you can say branching sandwiches.

 

[healthcaree]

kaplan says:


increasing chain length: increase BP, MP, density

increasing branching: decrease BP, MP, density




i used this theory and never got any bp/mp question wrong.. ??


"Greater branching reduces the surface area of a molecule, decreasing the weak intermolecular attractive forces (van der Waals forces). Hence, the molecules are held together less tightly, effectively LOWERING THE BOILING POINT.

In addition, branched molecules are more difficult to pack into a tight, three-dimensional structure. This difficulty is reflected in the LOWER MELTING POINTS of BRANCHED alkanes."


--- from Kaplan Blue Book


ummmm. bp and mp trends are the same..? just like gigawatt said?

 

[rejuvenatedsoul]

lol i typed into google "branching decreases melting point" and it changed it to "branching increases melting point" 😛

 

[Adammo]

lol i love how many of u guys watch chad's videos, i recognized like 5 quotes straight from him 🙂

 

[UndergradGuy7]

kaplan says:


increasing chain length: increase BP, MP, density

increasing branching: decrease BP, MP, density


--- from Kaplan Blue Book


ummmm. bp and mp trends are the same..? just like gigawatt said?

It has been discussed here that the kaplan book was wrong on this.

 

[marymatthews]

ok so what is the OFFICIAL right answer?

 

[healthcaree]

just went through my ochem text book from school, but it does not contain any info about branching.. it just talks about van der Waals radius..

 

[rnbaxter]

I tutor organic chemistry, that being said.

1)The stronger the intermolecular forces the higher the boiling point.

2)The larger the surface area the higher the boiling point. Therefore, branching decreases the boiling point.

3) The stronger the intermolecular forces the higher the melting point.

4) The larger the surface area the higher the melting point.

5) Increasing the symmetry of the molecule increases the melting point.

Between 4 and 5 is where you guys are lost I think. Here are some molecules:

A)Propane MP:-190 C
B)Butane MP: -138 C
C)2-methylbutane MP: -160 C
D) (CH3)4C MP: -17 C

A and B demonstrate increasing SA increases MP
B and C demonstrate branching decreasing the SA which decreases MP
C and D demonstrate high level of symmetry which increases MP

Your welcome in advance!

 

[jwnichols21]

Bump. I am still confused on this topic; is there not an overall consensus that we can make regarding this issue? Is Kaplan BB wrong?

 

[jwnichols21]

My Kaplan teacher sent me an explanation pretty much confirming what Kaplan BB says with evidence, so I guess Chad and Destroyer are incorrect.

 

[EmpathMan]

Bump.

I've been trying to search around and google for the consensus on this. Did it ever get resolved? I'm still finding very mixed opinions.

 

[njohns24]

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