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The Effect of structure on Acid-Base properties

The Effect of structure on Acid-Base properties. What structural properties of a molecule cause it to behave as an acid or as a base ?

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The Effect of structure on Acid-Base properties

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  1. The Effect of structure on Acid-Base properties • What structural properties of a molecule cause it to behave as an acid or as a base? • There are two main factors that determine whether a molecule containing an H-X bond will behave as a Brønsted-Lowry acid: the strength of the bond and the polarity of the bond. • The more polar the bond, the less likely the molecule is to dissociate. The stronger the H-X bond the harder it is to break.

  2. Another important class of acids are the oxyacids, which characteristically contain the grouping H-O-X. • The acid strength increases with an increase in the number of oxygen atoms attached to the central atom. • For example, in the series containing chlorine and a varying number of oxygen atoms, HOCl is a weak acid, but the acid strength increases as more oxygen atoms are added.

  3. This happens because the very electronegative oxygen atoms are able to draw electrons away from the chlorine atom and weaken the O-H bond. • This means that a hydrogen is more likely to be “let go”. • This effect is also observed for hydrated metal ions. • The greater the charge on the metal ion, the more acidic the hydrated ion becomes.

  4. For acids containing the H-O-X grouping, the greater the electronegativity of X, the greater the acid strength.

  5. The Lewis Acid-Base Model • A Lewis acid is an electron-pair acceptor, and a Lewis base is an electron-pair donor. • In other words, a Lewis acid has an empty atomic orbital that is can use to accept (share) an electron pair from a molecule that a lone pair of electrons (Lewis base).

  6. Note that Brønsted-Lowry acid-base reactions (proton donor-proton acceptor reactions) are encompassed by the Lewis model but the reverse is not true. • The model below where the base accepts the proton from the acid is a Brønsted-Lowry reaction as well as a Lewis reaction. • This reaction is not considered a Brønsted-Lowry reaction, only a Lewis reaction.

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