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Branched and Unbranched Isomers - Boiling point differences. Watch

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    Hi,

    Can someone please explain why branched isomers have lower boiling points than unbranched isomers?

    Various websites indicate that this is because unbranched molecules have a larger surface area, so the molecules can pack closer together and therefore the VDW forces are stronger.

    Surely branched molecules have a larger surface area than unbranched molecules, because the bonding is more 'spread out?'

    Can someone please explain what is wrong with my understanding?

    Thanks!
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    They can only form vdW when the molecules are close to each other.

    Compare butane with methylpropane.

    In butane, all four carbon atoms can get close.

    In the branched one, one of the carbons on each molecule can never get near the other molecule.

    Butane can form more vdW than methylpropane.

    Also, I'd avoid using the term vdW (which refers to more than one class of IMF). Use London forces et.al.
    • Thread Starter
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    Thanks for your reply.

    Why can one of the Carbon's on each molecule never get near the other molecule?

    Thanks!
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    Ask to borrow a molimod kit at school/college for a couple of minutes and you'll see for yourself.
 
 
 
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