branched alkane melting point

[justdominate]

From Chad and the DAT Destroyer, we learn that more branched alkanes have higher melting points.
However, Mike from Bootcamp shows us that this is only true when you exclude straight chain alkanes because straight chain alkanes have higher melting point than any branched alkanes (for same number of carbons)

Should I just go with the more branched alkanes have higher melting point rule for the DAT?

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[DAT Destroyer]

From Chad and the DAT Destroyer, we learn that more branched alkanes have higher melting points.
However, Mike from Bootcamp shows us that this is only true when you exclude straight chain alkanes because straight chain alkanes have higher melting point than any branched alkanes (for same number of carbons)

Should I just go with the more branched alkanes have higher melting point rule for the DAT?

Let me settle this, as it is a commonly asked question. Let us consider an alkane. Branching on alkanes are sometimes difficult to predict, but I will give you a great general rule, and you will be set. If..and only if.....the branching produces a HIGHLY SYMMETRICAL molecule do we see abnormally high melting points. When a highly symmetrical molecule is presented, it allows the molecule to pack closely into a crystal. For example....octane melts at minus 54 C.....now,,,,its highly symmetrical isomer 2,2,3,3-tetramethylbutane melts at......are you sitting down ? 101 C !! You heard me ! I hope that you now see what high symmetry can do. If the symmetry is not high, this does not hold true. If you see a question like this,,,,and like the one you saw in DESTROYER,,,,,,look for a molecule with high symmetry for the choice with the highest melting point ! I hope this clears up a concept that; I always see teachers fumbling.

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Dr. Romano

 

[justdominate]

Let me settle this, as it is a commonly asked question. Let us consider an alkane. Branching on alkanes are sometimes difficult to predict, but I will give you a great general rule, and you will be set. If..and only if.....the branching produces a HIGHLY SYMMETRICAL molecule do we see abnormally high melting points. When a highly symmetrical molecule is presented, it allows the molecule to pack closely into a crystal. For example....octane melts at minus 54 C.....now,,,,its highly symmetrical isomer 2,2,3,3-tetramethylbutane melts at......are you sitting down ? 101 C !! You heard me ! I hope that you now see what high symmetry can do. If the symmetry is not high, this does not hold true. If you see a question like this,,,,and like the one you saw in DESTROYER,,,,,,look for a molecule with high symmetry for the choice with the highest melting point ! I hope this clears up a concept that; I always see teachers fumbling.

Join my study group, I post many new questions daily.


Dr. Romano

Thank you for your reply Dr. Romano. The reason why I ask this question is because I saw a list of melting point of hexane and its branched versions and hexane had a slightly higher melting point (very slightly -95C to -98C) than the highest branched 6 carbon (2,2-dimethylbutane). The question I got wrong on the destroyer was regarding the melting point of pentane and 2,2-dimethylpropane. Is the melting point of dimethylpropane higher than pentane because it is highly symmetrical whereas the 2,2-dimethylbutane is less symmetrical?

or am I getting too nitpicky and just go with the rule of higher branched alkanes have higher melting point than less branched for the DAT