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Amazing Organic Chemistry Idea

Discussion in 'MCAT Discussions' started by VandyDerm, Jun 1, 2008.

  1. VandyDerm

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    This is a great question to keep things in perspective....but it could save time with all u geniuses.

    THE POINT OF THIS THREAD IS THE FOLLOWING: WE MAY GET QUESTIONS ON WHAT EFFECT DOES CHANGING THE MOLECULE HAVE ON ITS PHYSICAL PROPERTIES SO WHY NOT ACCOUNT FOR ALL OF THEM RIGHT HERE ON SDN!! Some are more obvious then others, some are slightly stated in the book....but lets get the facts straight.

    Lets ignore functional groups for now.

    So say you have a starting compound with pentane. What effect does doing the following have an effect on Boiling point, melting point, heat of hydrogenation, heat of combustion, COMPARED to Pentane if you change pentane.

    1.) Example: Extend length by -Ch3 group (i,e,- hexane)- it would increase BP and Increase MP, but even more because more nonpolar group, heat of combustion and hydrogenation probably increases with another carbon methyl group.

    2.) Branching, i.e. - 2-methylbutane- compared to pentane, both have 5 carbons, but branching would increase MP because now it is more easily packed, and decrease BP because more condensed, not sure about solubility, but I assume Heat of hydorgenation and combustion are less because molecule is more compact.

    etc.... do you catch my where im going..so what would UNSATURATION, i.e.- pentene do on those factors..

    Now if we account for adding functional groups....does the order of acidity chart parallel that of higher BP, higher MP, higher heat of hydr and comb....for example..does the acidity chart ORDER of compounds if you know what i mean (carboxylic acids are most acidi followed by diketone compounds, followed by water, then alcohols, then acid chloride, then aldehyde, then ketone, then anhydride, then ester, then amide, then prob like ethers, etcc...weak bases) so is the MOST acidic also the one with hte highest mp, bp, heat of hydr, comb....

    Other properties (chemical) like bond energy and bond length are probably more obvious with the hybridization ideas.
     
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  3. gogopogo

    gogopogo Canadian stereotype, eh?
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    Hey, good thread idea, however I am confused?
    Doesn't increased branching of a molecule reduce melting point while potentially increasing boiling point (assuming a permanent dipole is created).
     
  4. Brooks Hatlin

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    My innovative flash-card design is much more amazing. Sorry, not sharing until matriculation, however. It's that amazing.

    ya rly.
     
  5. phospho

    phospho SDN Lifetime Donor
    Lifetime Donor Classifieds Approved 10+ Year Member

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    only 4 posts and you got yourself banned? now that's a record...


    :thumbdown:
     
  6. Fireballfx

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    Branching molecules reduces the area over which VDW forces can act. Effectively, the "looser" packing translates into a lower MP and BP.
     
  7. foodfood

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    i thought more branching does decrease melting point b/c the packing is interrupted, but when you add more carbons you INCREASE the boiling point because the molecular weight increases....feel free to correct me if I am wrong!!
     
  8. supafield

    supafield Dream Big
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    Adding more carbons (theoretically more area to form bonds) will increase the BP.... but the primary factor according to TPR is branching, which lowers the BP not because the molecule becomes more condensed as the original poster stated.... it's because the molecules if you could think of them as puzzle pieces don't fit nearly as nicely as they did when it was straight chained pentane... which in turn limits the amount of dipole dipole interactions. Weaker bonds require less energy to break said bonds and BP decreases.

    MP decreases as well due to branching (doesn't form a crystalline structure as easily so need lower temperatures to freeze the molecule)

    The effects of size can be observed by the preferred phase of a hydrocarbon at room temperature

    1-3 hydrocarbon molecules usually are gases
    4-16 liquid
    16+ waxy solid...
     
  9. LikeClockWork

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    Probably a repeat troll that just has their IP address flagged. :rolleyes:
     
  10. VandyDerm

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    EXAMKRACKERS EVEN SAYS....branching should INCREASE MP---its like a weird exception because with branching now, its easier to pack, now that the molecule is condensed, so when u compare 2-methylbutane vs pentane, the branching allowes easier packing, thus less cooling required to pack it into a crystalline structure, THUS---> higher MP
     
  11. supafield

    supafield Dream Big
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    I really don't think this is right...
    I searched the melting pts of both substance....
    2-methylbutane has a MP of (-160 C)
    Pentane has a MP of (-130 C)


    I did Butane (-138 C) and Isobutane (-159.6 C)

    Unless examkrackers knows something some of these studies didn't Branching will decrease MP because it's harder to have it form a nice organized lattice with random shapes than it is for a straight hydrocarbon...

    they may have been mentioning an exception? I'm not sure... but that's the general rule for branching of hydrocarbons
     
    #10 supafield, Jun 2, 2008
    Last edited: Jun 2, 2008
  12. Kaustikos

    Kaustikos Archerize It
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    I will input some advice about this. ExamKrackers HAS been wrong before and I would not put my faith in their books. I've noted two exceptions thus far in their materials that would question their validity.

    One of them - Not all adrenal secretions are steroid derived. So, if you see information leading you the other way (numerous sites online/books) then I would put my faith in that.

    So, for the case of branching - decreased melting point. Branched alkanes have a decreased melting point for reasons stated before.

    Although, for carbon length - think of gasoline/gas in general. propane vs octane. (just as a mnemonic/help remember)
     
  13. sicboy188

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    i would agree with you that some of their generalizations are a little shakey, but EK prefaced their qualification of adrenal medulla and cortex secretions, considering them as really two separate organs that make two different types of hormones. its pretty clear in their endocrine discussion.
     
  14. VandyDerm

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    So basically, 2methylbutane should have a lower MP than pentane and 2,2, dimethyl propane should have an even LOWER MP than 2-methylbutane...if this is the CASE, what are the effects of unsaturation, increasing the length as well as the trends of functional groups (i.e. - carboxylic> BP Water > alcohol.etc...
     
  15. unsung

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    dude, I wouldn't try to memorize my way through stuff like this, because there are always exceptions. For example, basicity of amines (ammonia, vs. primary, vs 2ndary, vs tertiary, etc.). Also, melting points- it doesn't have as nice of a pattern as boiling points. Etc. etc. the exceptions will screw you up.

    It's better just to understand LD forces, periodic trends, relationship between acidity & stability of conjugate base, etc. Anyway, there's just too many "factoids" to memorize, if you want to go that route.
     

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