Acids and Bases; Electrophiles and Nucleophiles 2-2 Acid and base strengths are measured by equilibrium constants. Brønsted-Lowry acid: a proton donor. Brønsted-Lowry base: a proton acceptor. In water:

Water itself is both an acid and a base: The pH of a solution is defined: In pure water, the pH is 7. In acidic solutions the pH is less than 7 and in basic solutions the pH is above 7. The behavior of a general acid, HA, in water can be described: The pK A is the pH at which the acid is 50% dissociated. If the pK A is less than 1, the acid is termed strong; if greater than 4, weak.

We can estimate relative acid and base strengths. The relative strength of a weak acid, HA, increases with: Increasing electronegativity of A (CH 4 < NH 3 < H 2 O < HF) Increasing size of A (HF < HCl < HBr < HI) Resonance in A -

Lewis acids and bases interact by sharing an electron pair. Lewis Acid: A species containing an atom that is at least two electrons short of a closed outer shell. Lewis Base: A species containing at least one lone pair of electrons.

A Lewis base shares its lone pair with a Lewis acid to form a new covalent bond:

The dissociation of a Brønsted acid is the reverse of the association of a Lewis acid and a Lewis base: Note that the curved arrow points to the departing anion.

Electrophiles and nucleophiles also interact through movement of an electron pair. Many processes in organic chemistry exhibit characteristics of acid-base reactions. The carbon atom in chloromethane is termed electrophillic. The oxygen in the hydroxide is termed nucleophillic. The reaction of chloromethane and hydroxide ion is an example of a nucleophillic substitution reaction.

The terms Lewis acid and nucleophile are synonymous. Nucleophilic substitution is a general reaction of the class of compounds called haloalkanes. Additional examples include: The C-X bond in these examples constitutes the functional group, or center of chemical reactivity for the haloalkane class of organic compounds.