Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry

Problem 5

Chapter 13, Problem 5

Rank the boiling points of the following molecules. Explain your ranking ( 1 = highest bp; 3 = lowest bp ).

Verified step by step guidance

Step 1: Identify the molecular structures of the given molecules. Look for functional groups, molecular weight, and the presence of polar bonds or hydrogen bonding capabilities.

Step 2: Analyze the intermolecular forces present in each molecule. Molecules with stronger intermolecular forces (e.g., hydrogen bonding, dipole-dipole interactions, or London dispersion forces) typically have higher boiling points.

Step 3: Consider the molecular size and surface area. Larger molecules or those with extended surface areas tend to have stronger London dispersion forces, which can increase boiling points.

Step 4: Evaluate the polarity of each molecule. Polar molecules generally have higher boiling points than nonpolar molecules due to dipole-dipole interactions.

Step 5: Rank the molecules based on the strength of their intermolecular forces, molecular size, and polarity. Assign 1 to the molecule with the highest boiling point and 3 to the molecule with the lowest boiling point.

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Key Concepts

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

Intermolecular Forces

Intermolecular forces are the attractive forces between molecules that influence physical properties like boiling points. The main types include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Molecules with stronger intermolecular forces generally have higher boiling points because more energy is required to separate them during the phase change from liquid to gas.

Molecular Weight

Molecular weight, or molar mass, refers to the mass of a molecule and can affect boiling points. Generally, larger molecules with greater molecular weights exhibit higher boiling points due to increased London dispersion forces, which arise from temporary dipoles in larger electron clouds. However, this effect can be overshadowed by the presence of stronger intermolecular forces like hydrogen bonding.

Polarity

Polarity describes the distribution of electrical charge over the atoms in a molecule. Polar molecules have a significant difference in electronegativity between bonded atoms, leading to dipole-dipole interactions that can elevate boiling points. In contrast, nonpolar molecules primarily experience weaker London dispersion forces, resulting in lower boiling points. Understanding the polarity of the molecules in question is crucial for accurately ranking their boiling points.