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How is alkane mp affected by branching?

Discussion in 'MCAT: Medical College Admissions Test' started by AHH78, Apr 9, 2004.

  1. AHH78

    AHH78 Junior Member

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    I could be getting this mixed up, so could someone please clear up the confusion. I know that increased branching is supposed to weaken Vanderwaals forces. It would make sense that this would decrease the mp since there are weaker intermolecular forces and hence not as high temp should be needed to break these weaker forces in going from solid to liquid. I swear EK says that it decreases bp and increases mp. Can someone confirm, does branching decrease or increase mp?
     
  2. CoverMe

    CoverMe Registered Republican
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    branching decreases mp.

    Melting points and boiling points are determined by the intermolecular forces of attraction between molecules in the solid and liquid states, respectively. For nonpolar molecules like alkanes, the main intermolecular forces of attraction are London dispersion forces. London dispersion forces increase with increasing number of carbon atoms and increase with the area of contact between neighbouring molecules. With respect to the latter, the area of contact depends on the shape, decreasing as branching increases in the molecule. Therefore, a straight-chain alkane has a higher boiling point than a branched alkane with the same number of carbons.
    For polar compounds such as alcohols, amines, and carbonyl compounds, the important intermolecular forces are dipole-dipole attractions and hydrogen bonding. These generally lead to higher boiling points than for nonpolar molecules.

    check out http://www.chembio.uoguelph.ca/educmat/chm19104/org-faq.htm
     
  3. AHH78

    AHH78 Junior Member

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    Thanks, that makes sense.
     
  4. willthatsall

    willthatsall Unretired
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