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Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 3 - Alkanes and Cycloalkanes: Properties and Conformational AnalysisProblem 9a
Chapter 2, Problem 9a

Rank the following molecules from least water soluble to most water soluble. Explain your reasoning.
(a) H3C ― OH

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Step 1: Begin by identifying the functional group present in the molecule H₃C ― OH. This molecule contains a hydroxyl (-OH) group, which is polar and capable of forming hydrogen bonds with water molecules.
Step 2: Recall that water solubility is influenced by the ability of a molecule to interact with water through hydrogen bonding or dipole-dipole interactions. Molecules with polar functional groups, such as -OH, tend to be more water-soluble than nonpolar molecules.
Step 3: Consider the size of the molecule and the presence of nonpolar regions. In H₃C ― OH, the methyl group (H₃C-) is nonpolar, but the hydroxyl group dominates the solubility behavior due to its strong polarity and hydrogen bonding capability.
Step 4: Compare this molecule to other molecules in the problem (if provided). Rank them based on the number and type of polar functional groups, the size of the nonpolar regions, and their ability to form hydrogen bonds with water.
Step 5: Conclude that H₃C ― OH is likely to be moderately water-soluble due to the presence of the hydroxyl group, which enhances its interaction with water despite the small nonpolar methyl group.

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Key Concepts

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

Polarity

Polarity refers to the distribution of electrical charge over the atoms in a molecule. Molecules with polar bonds, such as those containing hydroxyl (–OH) groups, tend to interact favorably with water, a polar solvent. Understanding the polarity of the functional groups in the molecules is essential for predicting their solubility in water.
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Hydrogen Bonding

Hydrogen bonding occurs when a hydrogen atom covalently bonded to a highly electronegative atom, like oxygen, interacts with another electronegative atom. This type of bonding significantly increases the solubility of molecules in water, as it allows for strong interactions between the solute and solvent. Recognizing the presence and strength of hydrogen bonds in the molecules is crucial for ranking their solubility.
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Molecular Size and Structure

The size and structure of a molecule influence its solubility in water. Larger, more complex molecules may have hydrophobic regions that hinder solubility, while smaller, simpler molecules with polar functional groups are generally more soluble. Analyzing the molecular structure helps in understanding how these factors contribute to the overall solubility in water.
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Related Practice
Textbook Question

Choose the molecule in each pair you'd expect to have the higher boiling point. Explain your reasoning.

(a) eicosane ( C20H42) vs pentadecane

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Textbook Question

For each of the following line-angle drawings, 

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(b)

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Textbook Question

Rank the following molecules from least water soluble to most water soluble. Explain your reasoning.

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Choose the molecule in each pair you'd expect to have the higher boiling point. Explain your reasoning.

(b)

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Rank the following molecules from least water soluble to most water soluble. Explain your reasoning.

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Textbook Question

Draw the molecular orbital picture for propane. Your picture should clearly show the shape and hybridization of the carbons. Label all σ bonds.

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