Dimethylamine, (CH 3 ) 2 NH, has a molecular weight of 45 and a boiling point of 7.4 °C. Trimethylamine, (CH 3 ) 3 N, has a higher molecular weight (59) but a lower boiling point (3.5 °C). Explain this apparent discrepancy.

Welcome back everyone to another video. The molecular weights of propylene and tri methylamine are both 59 atomic mass units. Despite having the same molecular weight propylene has a boiling point of 48.6 °C. While triethyl has a lower boiling point of 3.5 °C. Explain this large difference in boiling points. Let's draw the two structures. We can start with propylene and to draw the structure, we are just going to draw a propel chain 1 to 3 carbon atoms bonded to an amino group. Now, we're also considering tri methylamine and that would be tertiary amine with three muscle groups attached to nitrogen. Now, we noticed that propine has a boiling point of 48.6 °C. While when it comes to trim methylamine, it has a lower boiling point of 3.5 °C, we can classify propine as a primary amine and tri methylamine as a tertiary amine. And in general, whenever we are comparing tertiary amines to secondary ones or primary ones, they will always have a lower boiling point. And now let's find out the reason essentially, we noticed that propine has hydrogen atoms bonded to nitrogen, meaning it is capable of hydrogen bonding. So we can say that hydrogen bonding is possible, hydrogen bonding is possible whenever we have we have hydrogen atoms bond its nitrogen oxygen or fluorine, right, we need to have an electron atom. So, intermolecular hydrogen bonding is possible for a primary and a secondary. I mean because their hydrogen atoms bond its nitrogen for tertiary means we can say that hydrogen bonding is not possible. So that the predominant force is actually our dipole dipole force. Let's remember that Hagen bonding as an intermolecular force is stronger. So, hygiene bonding is a stronger force than dipole dipole interactions. Therefore, we can say that because hygiene bonding is a stronger intermolecular force, a stronger intermolecular force will also cause a higher boiling point. Right. Let's recall that the stronger the intermolecular forces, the higher the boiling point. So they are directly proportional. And therefore this is our answer. We can conclude our findings as follows. We can say that propine has a higher boiling point because of its capability of warming hydrogen bonds. While tri melamine has a lower boiling point, since it cannot form hydrogen bonds. And as we know, hydrogen bonding is a strong instru molecule force which is stronger than di di interactions increasing the boiling point significantly. Thank you for watching.

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1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
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10. Addition Reactions3h 19m
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12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
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17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

26. Amines

Amine Alkylation

Organic Chemistry

26. Amines

Amine Alkylation

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Problem 6

Textbook Question

Dimethylamine, (CH₃)₂NH, has a molecular weight of 45 and a boiling point of 7.4 °C. Trimethylamine, (CH₃)₃N, has a higher molecular weight (59) but a lower boiling point (3.5 °C). Explain this apparent discrepancy.

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Identify the key structural difference between dimethylamine ((CH3)2NH) and trimethylamine ((CH3)3N). Dimethylamine contains a hydrogen atom bonded to the nitrogen, while trimethylamine has three methyl groups attached to the nitrogen, leaving no hydrogen directly bonded to it.

Explain the role of hydrogen bonding in boiling points. Molecules with N-H, O-H, or F-H bonds can form hydrogen bonds, which are strong intermolecular forces. These forces require more energy to overcome, leading to higher boiling points.

Analyze dimethylamine's ability to form hydrogen bonds. The N-H bond in dimethylamine allows it to form hydrogen bonds with other dimethylamine molecules, increasing its boiling point.

Discuss why trimethylamine cannot form hydrogen bonds. Since trimethylamine lacks an N-H bond, it cannot participate in hydrogen bonding. Its intermolecular forces are limited to weaker van der Waals forces and dipole-dipole interactions.

Conclude that despite trimethylamine's higher molecular weight, the absence of hydrogen bonding results in a lower boiling point compared to dimethylamine, which can form hydrogen bonds.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. In dimethylamine, the presence of a lone pair on the nitrogen allows for hydrogen bonds to form with other dimethylamine molecules, leading to a higher boiling point due to the additional energy required to break these interactions.

Steric Hindrance

Steric hindrance refers to the repulsion between bulky groups in a molecule that can affect its reactivity and physical properties. In trimethylamine, the three methyl groups create significant steric hindrance around the nitrogen atom, which limits the ability of the molecule to form hydrogen bonds compared to dimethylamine, resulting in a lower boiling point despite its higher molecular weight.

Molecular Weight vs. Boiling Point

While molecular weight can influence boiling point, it is not the sole determinant. The boiling point of a substance is affected by intermolecular forces, such as hydrogen bonding and van der Waals forces. In this case, dimethylamine's stronger hydrogen bonding outweighs the effect of its lower molecular weight compared to trimethylamine, leading to a higher boiling point for dimethylamine.

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