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Which plays a bigger role in determining molecules with higher boiling point? molecular weight or intermolecular forces? Or it depends on the situation?
boiling points of molecule
- Thread starter AntonFreeman
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it depends on the situation. if the molecule A has only a slightly lesser mass than B but is capable of hydrogen bonding, A will have a considerably higher BP. in general, know that H-bonding is the most important factor in determining boiling points. after that, i'd have to look at each problem specifically.
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As you move up the basic alkanes, i.e. hexane, heptane, octane, etc, each methyl substituent addition increase the boiling point about 25 or 30 degrees C.
However, propanol, which can participate in hydrogen bonding, has about the same boiling point as heptane 98C while the equivalently sized butane has a bp of about 0C.
With dipole dipole interactions its harder to predict. Dipole dipole forces are intermediate as intermolecular force between London dispersion and hydrogen bonding. I would choose the dipole dipole substance as higher boiling over a completely nonpolar substance in a question, but diethyl ether and pentane are actually about the same in bp (about 35C), though diethyl ether has a slight dipole moment, while acetone, which is lighter than either, is much higher boiling. Even though the oxygen in diethyl ether is sp3 hybridized, the dipole moment is extremely low compared to acetone. Some polar substances are only so slightly polar that it doesn't effect bp, though this does not apply to aldehydes and ketones.
In a related topic, the MCAT often will see whether you know that saturated fats have a higher mp and bp than unsaturated fats of the same size because the kinks due to cis or trans double bonds lead unsaturated fatty acid chains to have less access to one another for London dispersion force.
However, propanol, which can participate in hydrogen bonding, has about the same boiling point as heptane 98C while the equivalently sized butane has a bp of about 0C.
With dipole dipole interactions its harder to predict. Dipole dipole forces are intermediate as intermolecular force between London dispersion and hydrogen bonding. I would choose the dipole dipole substance as higher boiling over a completely nonpolar substance in a question, but diethyl ether and pentane are actually about the same in bp (about 35C), though diethyl ether has a slight dipole moment, while acetone, which is lighter than either, is much higher boiling. Even though the oxygen in diethyl ether is sp3 hybridized, the dipole moment is extremely low compared to acetone. Some polar substances are only so slightly polar that it doesn't effect bp, though this does not apply to aldehydes and ketones.
In a related topic, the MCAT often will see whether you know that saturated fats have a higher mp and bp than unsaturated fats of the same size because the kinks due to cis or trans double bonds lead unsaturated fatty acid chains to have less access to one another for London dispersion force.
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