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Alkane Bracnhing Discrepancy

Discussion in 'MCAT Study Question Q&A' started by Doctor D, Jun 2, 2008.

  1. Doctor D

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    In examkrackers they clearly say that branching of an alkane results in a lower BP and a higher MP. On SDN a post by QofQuimica states that branching raises both BP and MP. Which one is correct.
     
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  3. BerkReviewTeach

    BerkReviewTeach Company Rep & Bad Singer
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    Be careful with the general rule here.

    For MP, assuming the number of carbons remains constant, as branching increases, it becomes more difficult for molecules to pack into a lattice, which reduces the intermolecular forces. This results in less energy being required to melt the solid. It's similar to the concept behind increased fluidity of a cell membrane as the fats in the bilayer become saturated. They pack better, which in turn causes stronger forces (interactions), so they become more solid like (loses fluidity). Better packing = higher MP.

    However, if branching is caused by the addition of carbons to the chain, then the MW of the compound increases, and thus the MP increases. There is a hidden problem here that wasn't true in the first case.

    It is quite possible that both sources you reference are correct, but that it happens to be slightly different cases they are considering.
     
  4. minhaj

    minhaj Awesomeness Incarnate
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    The boiling point of Alkanes depends on the vanderwaals forces. As the surface area increases, so does the boiling point because the adjacent molecules can better interact resulting in greater vanderwaal forces. Due to this reason, straight chains have a greater boiling point than branched chain.

    Melting point is a little tricky. Melting point depends on how well a solid is packed into its rigid form. If branching enables the molecule to be arranged in a tightly packed solid form, then the melting point increases. However, if the branching hinders the tight packaging of the molecule, then this would lower the melting point. In other words, branched chain alkanes can have higher or lower melting points depending on the packaging of the molecules into a solid.

    Hope that was helpful

    Edit: I assumed that the straight chain and branched chain have the same number of carbons.
     
    #3 minhaj, Jun 2, 2008
    Last edited: Jun 2, 2008
  5. Doctor D

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    I think you did a good job of explaining it. A branched alkane will have less surface area on a molecule because the "inner" portions of that molecule are blocked by the branches. A branched molecule will not interact with nearby molecules as a non-branched molecule so there are less forces holding it together so the vapor pressure increases accordingly (BP down). :thumbup:
     
    #4 Doctor D, Jun 2, 2008
    Last edited: Jun 2, 2008
  6. minhaj

    minhaj Awesomeness Incarnate
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    Precisely!:thumbup:
     

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