ACID-BASE THEORY EVALUATION OF ACIDS

REVIEW OF REVIEW OF MAJOR THEORIES

3 Arrhenius acids and bases: An Arhennius acid yields a proton in solution. An Arhennius base yields a hydroxide ion in solution. Bronstead-Lowry acids and bases: A Brønsted-Lowry acid is a proton donor. A Brønsted-Lowry base is a proton acceptor. Lewis acids and bases: A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor. Acids and Bases Three acid-base systems:

Arrhenius Acids and Bases Acids dissociate in water to give H 3 O + ions. Bases dissociate in water to give OH - ions. K w = [H 3 O + ][OH - ] = 1.0 x at 24°C pH = -log [H 3 O + ] Strong acids and bases are 100% dissociated. =>

HA + B - A - + BH acid base conjugate base conjugate acid BRONSTED - LOWRY THEORY +H + -H + acid = proton donor base = proton acceptor Everything is defined in terms of the proton, the only recognized type of acid in B-L theory. HA + B: A: - + BH +

pKa’s AND ACID STRENGTH pK a - An alternative to K a to describe acid strength.

Acid and Base Strength Acid dissociation constant, K a Base dissociation constant, K b For conjugate pairs, (K a )(K b ) = K w Spontaneous acid-base reactions proceed from stronger to weaker.

pK a = - log K a K a = [H 3 O + ] [A - ] [HA] HA + H 2 O H 3 O + + A - DEFINITION OF pK a Compare the definition of pH (A concise way to state the strength of an acid.) pH = - log[H + ]

pK a = - log K a COMPARISON OF pK a and K a VALUES The smaller the value of the pK a the stronger the acid. pK a strong acidsweak acids KaKa We will use pK a to describe the strengths of acids. It is a single number, without exponents.

Spontaneous or not? pK a 4.74pK b 3.36pK b 9.26pK a pK a = -log Ka larger pKa = weaker acid acidbase acid base ?

LEWIS ACID - BASE THEORY B: electron-pair donor nucleophiles BASE A ACID electron-pair acceptor electrophiles more general than Bronsted theory Subtances other than proton donor and acceptors are recognized as acids and bases. =>

electron pair acceptor electron pair donor acidbase How are the bonds made and broken? The “curved arrow” formalism.

The arrow illustrates the flow of the electrons. This formalism is useful in describing reactivity and bonding. The “curved arrow formalism” is an extremely important concept in organic chemistry.

an elimination reaction The “curved arrow formalism” is an extremely important concept in organic chemistry. organic product

Organic compounds can behave as acids in acid-base reactions.

In organic chemistry two or more acid base reactions can compete.

a substitiution reaction an elimination reaction

Identify the acid and base in the following reactions. acid base acid base

Identify the conjugate acid and conjugate base in the following reactions. conjugate acid conjugatebase acid base

Identify the acid and base in the following reactions. acid base acid base

electron pair acceptor electron pair donor acidbase? which are the major species present at equilibrium? which is the stronger acid? which is the stronger base? conjugate acid conjugate base electron pair acceptor electron pair donor

acidacid which is the stronger acid? pK a = basebase K a = 10 7 K a =

basebase which is the stronger base? pK a =

basebase the stronger acid gives the weaker base the weaker acid gives the stronger base pK a

Strong acids give weak bases. Weak acids give strong bases. -7 pK a 4.74 ~50 aciditybasicity

pK a = (b) Identify the weakest acid. (c) Identify the strongest base. (a) Identify the acids and bases. acidbasebaseacid Consider the following equilibrium. (d) Does this equilibrium lie to the right or left?

Acids are electron pair acceptors. Bases are electron pair donors. K a = 10 -pKa Strong acids give weak bases. Weak acids give strong bases. An acid base equilibrium will always favor the weak acid and weak base.