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Working with Organic Compounds - Acid and Base Properties

written by: Harlan Bengtson • edited by: Lamar Stonecypher • updated: 12/20/2010

Acid base properties of organic compounds are the same as the properties of inorganic acids and bases. Organic acids and bases are all weak acids and bases, however, while there are strong and weak inorganic acids and bases. Organic acids and bases, thus don't exhibit the properties strongly.

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    What are Organic Compounds?

    There doesn't seem to be a universally agreed upon, simple definition of "organic compound." At one time the term meant any carbon containing compound that is part of living matter or matter that was once living. Prior to the early 1800s it was believed that organic compounds could only be synthesized in living organisms by action of a "life force" (vis vitalis) that is present only in such living organisms. Now there are many synthetic organic compounds that were never part of anything living. Most definitions of "organic compound" include the fact that it contains carbon and commonly has the carbon bonded to hydrogen and oxygen. Also, it must be noted that several carbon containing compounds are considered to be inorganic rather than organic compounds. This includes carbon monoxide, carbon dioxide, carbonates, cyanides, cyanates, carbides, and thyocyanates.

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    What are Acid - Base Properties?

    Acid and base properties of organic compounds are much the same as acid and base properties of inorganic compounds. Properties of acids (either organic acids or inorganic acids) include:

    • They have a pH less than 7.
    • They have a sour taste.
    • They produce hydrogen ions when dissolved in water.
    • They are corrosive to human tissue (i.e. they 'burn' your skin).
    • They react with bases to form a salt and water.

    Common properties of bases (either organic bases or inorganic bases) include:

    • They have a pH more than 7.
    • They have a 'soapy' feel.
    • They have a bitter taste.
    • They are corrosive to human tissue (i.e. they 'burn' your skin).
    • They react with acids to form a salt and water.

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    Organic Acids

    Organic acids are, in general, weak acids. That is, they do not ionize nearly completely in weak aqueous solution. Some inorganic acids (also known as mineral acids) are strong acids. That is, they ionize nearly completely in weak aqueous solution. Some common strong mineral acids are hydrochloric acid, sulfuric acid, and nitric acid. The acid properties listed above are 'stronger' or more noticeable for the strong mineral acids than for the weak organic acids. Strong acids (in aqueous solution) will have a pH much lower than 7, will produce many hydrogen ions, and will be very corrosive to the skin. Weak organic acids, on the other hand, will have a pH closer to 7 than a strong acid; will produce some hydrogen ions when dissolved in water, but not as many as a strong acid; and will not be as corrosive to the skin as a strong acid.

    carboxyl group Most common organic acids contain a carboxyl group, usually written as - COOH. The structure of the carboxyl group is shown in the diagram at the left. The hydrogen-oxygen bond in the OH is the weakest bond in the carboxyl group, so Hydrogen ions are released from carboxyl acetic acid containing organic acids when they are dissolved in water.

    One of the simpler organic acid is acetic acid (vinegar) with the formula: CH3COOH (or C2H4O2). An expanded version of the formula, showing the carboxyl group is shown in the diagram at the right. Some other common organic acids are:

    • propionic acid - CH3CH2COOH (or C3H6O2)
    • lactic acid - CH3CH(OH)COOH (or C3H6O3)
    • formic acid - HCOOH (or CH2O2)
    • oxalic acid - COOHCOOH (or H2C2O4)
    • citric acid - COOHCH2C(OH)COOHCH2COOH (or C6H8O7)

    The formulas for each of these organic acids are shown first in a format that emphasizes the functional groups (including COOH at least once in each of them) and then in the more compact, less descriptive format that you will often see.

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    Organic Bases

    Organic bases typically do not ionize nearly completely in weak aqueous solution, so they are classified as weak bases. This is in contrast to strong bases like sodium hydroxide or potassium hydroxide, which do ionize nearly completely in weak aqueous solution. Just as with the weak and strong acids, the weak organic bases don't exhibit the properties of bases as 'strongly' as strong bases.

    Organic bases exhibit their basic behavior by having an attraction for hydrogen ions, usually through one or more nitrogen atoms in the molecule. Some common organic bases are:

    • methyl amine - CH3NH2 (or CH5N)
    • pyridine - C5H5N
    • imidazole - C3H4N2
    • glycine - C2H5NO2

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    Summary

    Acid and base properties of organic compounds are much the same as the properties of inorganic acids and bases. There are both strong and weak inorganic acids and bases, however, while organic acids and bases are universally weak acids and weak bases. Thus the inorganic acids and bases don't exhibit the acid and base properties as 'strongly' as the strong mineral acids and inorganic bases.

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    References

    1. Virtual Text of Organic Chemistry 1999 An Interactive Textbook

    2. Yoder, C.H., Leber, P.A., Thomsen, M.W., The Bridge to Organic Chemistry: Concepts and Nomenclature, (2010), Hoboken, NJ, John Wiley & Sons.

    3. Hornby, M. and Peach, J., Foundations of Organic Chemistry (Oxford Chemistry Primers), (1993), Oxford, Oxford University Press.