The boiling points of dimethylamine ((CH₃)₂NH) and trimethylamine ((CH₃)₃N) can be understood by considering both molecular weight and the types of intermolecular forces present in each compound.
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Molecular Weight: While it's true that trimethylamine has a higher molecular weight (59 g/mol) compared to dimethylamine (45 g/mol), boiling point is influenced not only by weight but also by the strength of intermolecular forces.
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Intermolecular Forces:
- Dimethylamine can form hydrogen bonds due to the presence of a nitrogen atom bonded to hydrogen. Hydrogen bonding typically occurs when hydrogen is bonded to highly electronegative elements (such as nitrogen, oxygen, or fluorine), allowing for stronger interactions between molecules.
- Trimethylamine, on the other hand, lacks a hydrogen atom directly bonded to the nitrogen that could facilitate hydrogen bonding. Instead, all three methyl groups (CH₃) create steric hindrance, which prevents close approach between molecules and reduces effective intermolecular interactions.
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Boiling Point: The boiling point is the temperature at which the vapor pressure of a liquid equals the atmospheric pressure. In the case of dimethylamine, the hydrogen bonds allow for a greater amount of energy (in the form of heat) to be required to separate the molecules during boiling, leading to a higher boiling point. Trimethylamine, lacking significant hydrogen bonding, requires less energy to overcome the weaker van der Waals forces present, resulting in a lower boiling point.
In summary, despite having a higher molecular weight, the lower boiling point of trimethylamine can be attributed to its inability to form hydrogen bonds, resulting in weaker intermolecular forces compared to dimethylamine, which can form strong hydrogen bonds.
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