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Dimethylamine vs. Trimethylamine: Structures & Boiling Points

Chapter 10, Problem PROBLEM 10-6

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QUESTION:

Dimethylamine, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\), has a molecular weight of 45 and a boiling point of \(7.4\ ^{\circ}\mathrm{C}\). Trimethylamine, \(\left(\mathrm{CH}_3\right)_3\mathrm{N}\), has a higher molecular weight (59) but a lower boiling point \(\left(3.5^{\circ} \mathrm{C}\right)\). Explain this apparent discrepancy.

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QUESTION:

Dimethylamine, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\), has a molecular weight of 45 and a boiling point of \(7.4\ ^{\circ}\mathrm{C}\). Trimethylamine, \(\left(\mathrm{CH}_3\right)_3\mathrm{N}\), has a higher molecular weight (59) but a lower boiling point \(\left(3.5^{\circ} \mathrm{C}\right)\). Explain this apparent discrepancy.

ANSWER:

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Boiling point

The boiling point illustrates a chemical entity’s physical property governed by its intermolecular interactions. It defines the temperature value (energy required in the form of heat) at which a compound changes its physical state from liquid to gas.

The strength of intermolecular interactions governs a species’ boiling point, and the strength follows the order:

Hydrogen-bond > Dipole-dipole interactions > London forces.

The intermolecular forces in a compound can be identified as:

(1) Hydrogen bond: Occurs when a molecule has a hydrogen bond donor (hydrogen attached to an electronegative atom) and a hydrogen bond acceptor (an electronegative atom like O, N, or F).

(2) Dipole-Dipole interactions: Force of attraction between polar molecules.

(3) London forces: Force of attraction between non-polar molecules.

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Dimethylamine vs. Trimethylamine: Structures & Boiling Points
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Uncover the science behind boiling points and molecular interactions in this video. Explore why Dimethylamine boils at a higher temperature than Trimethylamine, despite having a lower molecular weight, and gain insights into the role of molecular structures in determining physical properties.


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