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15.2: Physical Properties of Alcohols; Hydrogen Bonding

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    Comparison of the physical properties of alcohols with those of hydrocarbons of comparable molecular weight shows several striking differences, especially for those with just a few carbons. Alcohols are substantially less volatile, have higher melting points, and greater water solubility than the corresponding hydrocarbons (see Table 15-1), although the differences become progressively smaller as molecular weight increases.

    Table 15-1: Comparison of Physical Properties of Alcohols and Hydrocarbons

    Roberts and Caserio Screenshot 15-1-1.png

    The reason for these differences in physical properties is related to the high polarity of the hydroxyl group which, when substituted on a hydrocarbon chain, confers a measure of polar character to the molecule. As a result, there is a significant attraction of one molecule for another that is particularly pronounced in the solid and liquid states. This polar character leads to association of alcohol molecules through the rather positive hydrogen of one hydroxyl group with a correspondingly negative oxygen of another hydroxyl group:

    Roberts and Caserio Screenshot 15-1-2.png

    This type of association is called “hydrogen bonding,” and, although the strengths of such bonds are much less than those of most conventional chemical bonds, they are still significant (about \(5\) to \(10 \: \text{kcal}\) per bond). Clearly then, the reason alcohols have higher boiling points than corresponding alkyl halides, ethers, or hydrocarbons is because, for the molecules to vaporize, additional energy is required to break the hydrogen bonds. Alternatively, association through hydrogen bonds may be regarded as effectively raising the molecular weight, thereby reducing volatility (also see Section 1-3).

    The water solubility of the lower-molecular-weight alcohols is pronounced and is understood readily as the result of hydrogen bonding with water molecules:

    Roberts and Caserio Screenshot 15-1-3.png
    Figure 15-1). Indeed, the physical properties of higher-molecular-weight alcohols are very similar to those of the corresponding hydrocarbons (Table 15-1). The importance of hydrogen bonding in the solvation of ions was discussed in Section 8-7F.
    Roberts and Caserio Screenshot 15-1-4.png
    Figure 15-1: Dependence of melting points, boiling points, and water solubilities of straight-chain primary alcohols \(\ce{H} \ce{-(CH_2)}_n \ce{-OH}\) on \(n\). The arrows on the solubility graph indicate that the scale is on the right ordinate.

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."

    This page titled 15.2: Physical Properties of Alcohols; Hydrogen Bonding is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.