Organic Chemistry - Boiling Pt / Melting Point

[MDJerome]

So from what I understand:

1. Increasing branching decreases mp and bp
2. Increasing molecular weight increases mp and bp

If that's the case, then when does the whole idea of "branching sandwiches" occur? ie, melting point raised but boiling point lowered with branching. I seen this on Chad's videos but read (above) in the TPR book and I'm a little confused now. Anyone care to explain?

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[kehlsh]

Um...
Increasing branching decreases the bp while it increases the mp.

It's mainly because boiling point is related to intermolecular forces while melting point is related to "packing" ability of alkanes.

So if you increase branching you have less contact surface area for dispersion forces thus lowering the bp. However, your ability to pack together and form a crystalline structure is enhanced, thus raising melting point.

Hope it helped,

Kehlsh

 

[orgohacks]

So from what I understand:

1. Increasing branching decreases mp and bp
2. Increasing molecular weight increases mp and bp

If that's the case, then when does the whole idea of "branching sandwiches" occur? ie, melting point raised but boiling point lowered with branching. I seen this on Chad's videos but read (above) in the TPR book and I'm a little confused now. Anyone care to explain?

I wrote a post about this yesterday. Let me know if it makes sense.
http://masterorganicchemistry.wordpress.com/2010/07/09/chemical-tetris/

-James

 

[folktale]

Um...
Increasing branching decreases the bp while it increases the mp.

It's mainly because boiling point is related to intermolecular forces while melting point is related to "packing" ability of alkanes.

So if you increase branching you have less contact surface area for dispersion forces thus lowering the bp. However, your ability to pack together and form a crystalline structure is enhanced, thus raising melting point.

Hope it helped,

Kehlsh

don't you mean "increasing branching decreases mp and increases bp"

 

[kehlsh]

don't you mean "increasing branching decreases mp and increases bp"

no

 

[fizzgig]

I wrote a post about this yesterday. Let me know if it makes sense.
http://masterorganicchemistry.wordpress.com/2010/07/09/chemical-tetris/

-James

this is a good post. EK said pretty much the same thing, that the MP trends were more difficult to predict, but having a specific example was helpful.