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6.6: Physical Properties of Amides

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    Learning Objectives

    • Compare the boiling points of amides with alcohols of similar molar mass.
    • Compare the solubilities in water of amides of five or fewer carbon atoms with the solubilities of comparable alkanes and alcohols in water.

    With the exception of methanamide (common name: formamide) (Screen Shot 2021-04-03 at 5.41.04 PM.png), which is a liquid, all simple amides are solids (Table \(\PageIndex{1}\)). The lower members of the series are soluble in water, with borderline solubility occurring in those that have five or six carbon atoms. Like the esters, solutions of amides in water usually are neutral—neither acidic nor basic.

    Table \(\PageIndex{1}\): Physical Constants of Some Unsubstituted Amides
    Condensed Structural Formula

    Name

    common or IUPAC

    		 Melting Point (°C) Boiling Point (°C) Solubility in Water
    Screen Shot 2021-01-25 at 2.09.19 PM.png

    formamide

    or methanamide

    		 2 193 soluble
    Screen Shot 2021-01-25 at 2.09.59 PM.png

    acetamide

    or ethanamide

    		 82 222 soluble
    Screen Shot 2021-01-25 at 2.11.16 PM.png

    propionamide

    or propanamide

    		 81 213 soluble
    Screen Shot 2021-01-25 at 2.11.40 PM.png

    butyramide

    or butanamide

    		 115 216 soluble
    Screen Shot 2021-01-25 at 7.14.29 PM.png benzamide 132 290 slightly soluble

    The amides generally have high boiling points and melting points. These characteristics and their solubility in water result from the polar nature of the amide group and hydrogen bonding (Figure \(\PageIndex{1}\)). (Similar hydrogen bonding plays a critical role in determining the structure and properties of proteins, deoxyribonucleic acid [DNA], ribonucleic acid [RNA], and other giant molecules so important to life processes.

    clipboard_eb25efb5af4b7538afcaa46b316f25f88.png
    Figure \(\PageIndex{1}\): Hydrogen Bonding in Amides. Amide molecules can engage in hydrogen bonding with water molecules (a). Those amides with a hydrogen atom on the nitrogen atom can also engage in hydrogen bonding (b). Both hydrogen bonding networks extend in all directions.

    Concept Review Exercises

    1. Which compound has the higher boiling point—pentanamide, Screen Shot 2021-02-15 at 12.25.54 PM.png , or propyl ethanoate, Screen Shot 2021-02-15 at 12.30.47 PM.png, ? Explain.
    2. Which compound is more soluble in water—propanamide Screen Shot 2021-02-15 at 12.33.06 PM.pngor 1-pentene, CH2=CH-CH2-CH2-CH3, ? Explain.

    Answers

    1. pentanamide because the nitrogen-to-hydrogen (N–H) and the carbon-to-oxygen double (C=O) bonds can engage in hydrogen bonding; propyl acetate cannot engage in hydrogen bonding
    2. propanamide because the N–H and C=O bonds can engage in hydrogen bonding with water; 1-pentene cannot engage in hydrogen bonding with water

    Key Takeaways

    • Most amides are solids at room temperature; the boiling points of amides are much higher than those of alcohols of similar molar mass.
    • Amides of six or fewer carbon atoms are soluble in water.

    Exercises

    1. Which compound has the higher boiling point—butanamide, Screen Shot 2021-02-15 at 12.35.28 PM.png, or

      ethyl ethanoate,Screen Shot 2021-02-15 at 12.37.26 PM.png, ? Explain.

    2. Which compound has the higher boiling point—butanamide or N,N-dimethylethanamide Screen Shot 2021-04-03 at 6.00.31 PM.png? Explain.

    3. Which compound is more soluble in water—ethanamide, Screen Shot 2021-01-25 at 2.09.59 PM.png, or 1-butene, CH2=CH-CH2-CH3, ? Explain.

    4. Which compound is more soluble in water— N-methylethanamide, Screen Shot 2021-02-15 at 12.42.52 PM.png,  or 2-methylbutane, Screen Shot 2021-04-03 at 5.57.43 PM.png, ? Explain.

    Answers

    1. Butanamide because the nitrogen-to-hydrogen (N–H) and the carbon-to-oxygen double (C=O) bonds can engage in hydrogen bonding; ethyl ethanoate cannot engage in hydrogen bonding

     

    1. Ethanamide because the N–H and C=O bonds can engage in hydrogen bonding with water; 1-butene cannot engage in hydrogen bonding with water

    6.6: Physical Properties of Amides is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.