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.

Ch.10 - Structure and Synthesis of Alcohols

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

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Wade 9th Edition

Ch.10 - Structure and Synthesis of Alcohols

Problem 6

Chapter 10, Problem 6

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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Related Practice

Textbook Question

Predict which member of each pair will be more acidic. Explain your answers.

d. 2,2-dichloropropan-1-ol or 2,2-difluoropropan-1-ol

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Textbook Question

Give a systematic (IUPAC) name for each diol

(d)

(e)

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Textbook Question

Without looking them up, rank the following compounds in decreasing order of acidity. These examples represent large classes of compounds that differ widely in acidity.

water, ethanol, 2-chloroethanol, tert-butyl alcohol, ammonia, sulfuric acid, hexane, hex-1-yne, acetic acid

Textbook Question

Predict which member of each pair will be more soluble in water. Explain the reasons for your answers.

(a) hexan-1-ol or cyclohexanol

(b) heptan-1-ol or 4-methylphenol

(d) hexan-2-ol or cyclooctane-1,4-diol

(e)

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Textbook Question

Give a systematic (IUPAC) name for each diol

(a) CH₃CH(OH)(CH₂)₄CH(OH)C(CH₃)₃

(b) HO-(CH2)₈-OH