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melting point confusion

Discussion in 'MCAT: Medical College Admissions Test' started by jhk43, Mar 21, 2004.

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  1. jhk43

    jhk43 Senior Member
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    EK says one thing, TPR says another.

    Regarding Melting point and branching:

    EK: Increase in branching, INCREASE in MP
    <doesnt explain why>

    TPR: Increase in Branching, DECREASE in MP
    "easier to stack straight chans than branched ones, thus easier to form solid (lower melting point)"

    Intuitively TPR makes sense...but whats the right answer?
     
  2. premyo2002

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    TPR says it right.. But be aware that size matters
     
  3. jhk43

    jhk43 Senior Member
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    lemme think.

    disruptions/kinks/trans unsaturations in the bilipid membrane makes it more fluid....that means...."worse" stacking (branching) makes it more liquid...meaning it lowers melting point.

    increased branching -> lower MP.

    gotcha
     
  4. ASDIC

    ASDIC The 9th Flotilla
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    when non-volatile solutes are present in solution, they disrupt the intermolecular forces (branching) between the solvent molecules.

    So when freezing occurs, the solvent molecules will tend to come together (volume decreases as temp decreases), however their interactions with other solvent molecules is blocked by the solute particles. As a result, the solvent will stay in its liquid phase for lower temperatures, until the temp is sufficent for the solvent molecules to overcome the solute particles.

    The intermolecular forces could be hydrogen bonds, van der waals, dipole interactions etc.
     
  5. willthatsall

    willthatsall Unretired
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    For the person who was asking this, the correct answer is as follows:

    As BRANCHING in the carbon chain INCREASES, the MELTING POINT of the compound INCREASES.

    I think the previous posters are confusing this with boiling point (branching up = boiling point down), but this is not the case with melting point. As the branching increases, the molecules form more compact 3-D structures and can fit more easily into a solid structure. It is a little counterintuitive until you think about it like this, but trust me.
     
  6. jhk43

    jhk43 Senior Member
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    wouldnt branching form LESS compact structures, thus, keeping it at a liquid? ie decreased melting point?

    Linear Stacks of molecules vs. T shaped molecules. Seems like linear stacks form better solids....dunno
     
  7. willthatsall

    willthatsall Unretired
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    No, more branching = more compact. less branching = longer chain = not as compact. I checked my organic text to make sure.
     
  8. fun8stuff

    fun8stuff *hiding from patients*
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    i was taught and have read many places that:

    more branching = cannot pack well, so melts at lower temp
    less branching = long chain are able to pack together, stronger
    intermolecular forces, melting temp is higher

    Real life example prooving point:
    Margarine, cooking oil, and butter. Margarine and that cooking oil stuff are made up of trans-fatty acids with trans double bonds. Now this isn't exactly branching but it is just one step down. The trans double bonds causes kinks and prevents it from packing tight, whereas butter made up of long chains on saturated hydrocarbons packs tightly. Thus butter is a solid at room temp with a higher melting point and cooking oil is a liquid at room temp with a lower melting temp.

    Say you are a plumber and have to stack pipe into your truck. Would you rather pre-assemble the pipe? That is, would you rather stack the pipe in long lengths of straight pipe or long lengths of pipe with other pipe branching off every which way?
     
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  9. fun8stuff

    fun8stuff *hiding from patients*
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  10. willthatsall

    willthatsall Unretired
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    I'm going to copy this straight out of my Organic Textbook:

    (Wade Organic Chemistry 4th Edition)

    Branching of the chain also affects an alkane's melting point. A branched alkane generally melts HIGHER than the n-alkane with the same number of carbon atoms. Branching of an alkane gives it a more compacet three-dimensional structure, which packs more easily into a solid structure and increases its melting point. The boiling points and melting points of three isomers of formula C6H14 are given below. The boiling points decrease and the melting points increase as the shape of the molecule becomes more highly branched and compact.

    The molecules that are illustrated are:

    (2-methyl pentane: bp = 60 mp = -154)
    (2,3-dimethyl butane: bp = 58 mp = -135)
    (2,2-dimethyl butane: bp = 50 mp = -98)



    Now, having said that, the Kaplan big book disagrees. It also says what most of you are saying, that MP and BP are affected in the same way as branching increases. I would tend to have more confidence in my textbook, especially since it gives examples of specific molecules. Anyone else know what's up? I might email my old Organic professor and see what he has to say about the discrepancy.
     
  11. OnMyWayThere

    OnMyWayThere OMS-III
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    I'd trust Wade... hey, why does boilding point go down with branching?
     
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  12. willthatsall

    willthatsall Unretired
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    According to Wade:

    BP goes down with branching because the molecules have less surface area touching each other than if they were straight chain (ie fewer london forces/ van der waals forces holding them together = lower BP)
     
  13. jhk43

    jhk43 Senior Member
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    iso butane MP: -159.6 C Boiling point: -11.7 C
    butane MP: -138.4 C Boiling point: 0.5 C


    Branching lowers MP
     
  14. willthatsall

    willthatsall Unretired
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    But what about the compounds that I posted above? It sounds like there might not be a rule that works for all situations. Also, notice in the passage that I took from the Organic text that it uses the word "generally". I emailed my organic professor, I'll paste whatever he has to say about it.
     
  15. fun8stuff

    fun8stuff *hiding from patients*
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    Ahh!! I think I remember now something about this rule is different depending on the size of the molecule, but who knows. I am interested in what you find out from your prof. I will hunt mine down and ask him...
     
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  16. OnMyWayThere

    OnMyWayThere OMS-III
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    Thanks, it makes sense!
     
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  17. willthatsall

    willthatsall Unretired
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    Ok, my organic prof wrote me back with the explanation. Here it is:

    "This trend (speaking of increased branching = increased melting point) works within branched alkanes, but not if you include the straight chain (or normal) alkane..

    i.e.: MP C BP C
    hexane -95 69
    2-methylpentane -154 60
    2,4-dimethylbutane -135 58
    2,2-dimethylbutane -98 50

    So the trend seems to only work when you compare branched with other branched. The more compact the structure, the higher the mp, and the lower the BP reflecting packing/intermolecular forces. The mp difference fails when you include hexane in the mix."

    So there is the answer. Straight chain alkanes have the highest melting point, but once there is some branching, melting point will increase with any further branching. So I guess in a sense both sources are right, but neither explains the phenomenon fully.
     
  18. xerxes

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    if you look at that tpr quote, it is internally inconsistent--"easier to form solid (lower melting point)" should read "easier to form solid (higher melting point)."

    right argument, wrong conclusion. if it easily forms a solid, it will remain a solid up to high temps.
     
  19. fun8stuff

    fun8stuff *hiding from patients*
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    Okay. This is what I remember.
     
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  20. icewave667

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    this website says:

    Melting points of octane isomers are used as examples. The correlation of melting point and molecular symmetry is very well illustrated. Initially, melting points decrease with increasing branching but then become larger again as the symmetry increases. Clicking on the structures will open up a ball-and-stick model of the molecule.

    Tab.1
    Structure Melting point [°C]

    MouseZoom
    Fig. Oc*tane
    -57°C

    MouseZoom
    Fig. 2-Methyl*hep*tane
    -109°C

    MouseZoom
    Fig. 2,2-Di*methyl*hexane
    -121°C

    MouseZoom
    Fig. 2,2,3-Trimethylpen*tane
    >-112°C

    MouseZoom
    Fig. 2,2,3,3-Tetram*ethylbu*tane
    + 101°C


    http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/2/vlu/alkane/alkane_struktur.vlu/Page/vsc/en/ch/2/oc/stoffklassen/systematik_struktur/acyclische_verbindungen/gesaettigte_kohlenwasserstoffe/alkane/verzweigte_alkane.vscml.html
     
  21. icewave667

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  22. Marine to MD

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    Wow, there is some massive confusion and incredible amounts of misinformation in this thread.

    First and foremost: In general, as the number of carbons in a straight chain alkane increases, the melting point increases. There is a slight exception to this rule with the first few alkanes. A side note is that the curve of the trend is not smooth as is the boiling point curve.

    The melting point trend in branched alkanes is not as straight forward as is the boiling point trend. Melting is dependent upon the molecules' ability to pack into solid form. Basically, there is no universal trend for branched alkane melting points. In some cases, increased branching leads to a decrease in MP whereas in others it increases MP.
     
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  23. SN2ed

    Moderator Emeritus 7+ Year Member

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    Thread closed. Please do not bump a thread from 2004.
     
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