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Chemistry LibreTexts

7.2.6: How to Spot a Base

  • Page ID
    52366
  • There is an equally simple method for figuring out which compounds are potential bases. Let us take a look at some common bases. The first bases that most people encounter are the metal hydroxides such as NaOH, KOH, and Mg(OH)2. The metal ions are generated when these compounds dissolve in water, but they typically do not play any role in acid–base reactions.131 The base in these compounds is the hydroxide (OH). Another common class of bases is molecules that contain nitrogen, like NH3. There many kinds of nitrogenous bases, some of which play a critical role in biological systems. For example, the bases in nucleic acids (DNA and RNA) are basic because they contain nitrogen. Let us not forget that water is also basic and can accept a proton.

    So what is the common structural feature in bases? Well, if an acid is the species with a proton to donate, then the base must be able to accept a proton. This means that the base must have somewhere for the proton to attach—it must contain a non-bonded (lone) pair of electrons for the proton to interact and bond with. If we look at our examples so far, we find that all the bases have the necessary non-bonded pair of electrons. Most common bases have either an oxygen or a nitrogen (with lone pairs of electrons) acting as the basic center. Once you learn how to spot the basic center, you can predict the outcome of a vast range of reactions rather than just memorizing them. It is often the case that if you can identify the acidic and basic sites in the starting materials, you can predict the product and ignore the rest of the molecule.

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    In general, nitrogen is a better proton acceptor than oxygen, because it is more basic. Ammonia (NH3) is more basic than water (H2O), and organic compounds with nitrogen in them are typically more basic than the corresponding compounds containing structurally-analogous oxygens (see the figure). If we compare the trend in basicity for a range of simple compounds across the periodic table, we see that basicity decreases from NH3 > H2O > HF. This effect parallels the increase in electronegativity across the row. The ability to of an electron pair to bond with and accept a proton depends on how tightly that electron pair is held in by the donor atom. In fluorine, the most electronegative atom, the electrons are held so tightly and so close to the atom’s nucleus that they are not available to bond with a proton. Oxygen holds onto its electron pairs a little less tightly, and so is more likely than fluorine to donate a lone pair to a proton. Nitrogen, however, is even less electronegative and therefore has a more available lone pair, making most nitrogen compounds basic.132

    Questions to Answer

    • Why did we not include CH4 or neon in this analysis?
    • Do you think compounds with ammonium (NH4+) are basic? Why or why not?
    • Can you draw the structure of a basic compound that has not yet been mentioned in the text?
    • Draw out the reactions of CH3NH2 and CH3OH with water. Label the conjugate acid and base pairs.
    • Which reaction is most likely to occur? Why?
    • How would you design an experiment to figure out whether a compound is an acid or a base (or both)?
    • What experimental evidence would you accept to determine if you had an acid or a base or both?

    References

    131 Although some highly-charged metal ions react with water, we will not consider these reactions at the moment. Group I and II cations are stable in water.

    132 There are some nitrogenous compounds that are not basic because the lone pair is already being used for some other purpose. If you continue to study organic chemistry, you will learn about these ideas in more detail.

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