An Introduction to Organic Reactions and Their Mechanism Acids and Bases

3.4 Acid Strength Strong acids: completely ionized or completely dissociated Forward reaction is predominated Most of HA is dissociated Conjugated base is weak and has low attraction for proton HA(aq) + H 2 O(l)  H 3 O + (aq) + A - (aq)

Acids Strength Weak acids: partially ionized or dissolved Reverse reactions is predominated Most of HA is undissociate Conjugated base is strong and has high attraction for proton HC 2 H 3 O 4 (aq) + H 2 O(l) H 3 O + (aq) +C 2 H 3 O 4 - (aq)

Acid Strength

3.5 The Strength of Acids and Bases: Ka and pKa The acidity constant, Ka a quantitative measure of the strength of an acid in solution. It is known as the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions Larger K a, stronger the acid

Acidity and pKa Logarithm scale of Ka with the base of 10 pKa = - log Ka The larger pKa, the smaller extent of dissociation Weaker acids

Relative Strength of selected acids and their conjugated bases

Examples An acid (HA) has Ka = 10-7, what is its pKa? Another acid (HB) has Ka = 5, what is its pKa? Which is the stronger acid?

Predicting the Strength of Bases The strength of acids will decide the how weak or strong its conjugated bases Stronger the acids, the weaker will be its conjugated base After losing a proton, acid will become a conjugated base When a base accepts a proton, the resulting chemical is called the conjugate acid of that original base

Acid and its conjugated base

Examples The pKa of anilinium ion (C 6 H 5 N + H 3 ) is equal to 4.6. On the basis of this fact, decide whether aniline (C 6 H 5 NH 2 ) is a stronger or weak base than methylamine

3.6 Predicting the Outcome of Acid- Base Reactions Acid-base reactions always favor the formation of the weaker acid and weaker base Equilibrium favor the formation of the most stable (lowest potential energy) species Stronger acid + stronger base  weaker base + weaker acid

Example Consider the mixing of an aqueous solution of acetic acid, CH3CO2H (see Table 3.1), and NaOH. What acid base reaction, if any would take place H2SO4(aq) + NH3(aq)  NH4+(aq) + HSO4-(aq)

3.6 Water Solubility as the Result of Salt Formation Recall: Longer chain of carbon (4 or more) will be less soluble Higher molecular weight of carboxylic compounds do not dissolve in water Instead, they dissolved in a strong base to form a water- soluble salt

Water Solubility We can predict that an amine will react with aqueous hydrochloric acid

Water Solubility Methylamine and most amines of low molecular weight are very soluble in water Formed a water-soluble salt from hydrochloric acid

3.7 Relationships between structure and Acidity The strength of a Bronsted-Lowry acid depends on the extend to which a proton can be separated from it and transferred to a base. Removing H+  breaking a bond Making the conjugated base more electronegative Accepting H+  forming a bond Making the conjugated acid less electronegative

3.7 Relationships between structure and Acidity Bond strength to proton decreases as we move down to column Decreasing effectiveness of orbital overlap between the hydrogen 1s orbital and the orbitals of successively larger element in the column Less effective the orbital overlap, the weaker bond, stronger acid Acidity increases from left to right

3.7 Relationships between structure and Acidity

3.7 The Effect of Hybridization

Electrons of 2s orbtials have lower energy than those of 2p orbitals because these electrons are much closer to the nucleus Having more s orbitals means that electrons of the anion will be lower in energy and more stable

3.7 The Effect of Hybridization sp C – sH hybridization Contain 50% s character 1s + 1p More electronegative spC

3.7 The Effect of Hybridization sp2C – sH hybridization Contains 33.3% s character 1s + 2p Less electronegative than spC

3.7 The Effect of Hybridization sp3C-sH hybridization Contains 25% s character 1s + 3p Least electronegative C

3.7 The Effect of Hybridization

Relative basicity of the carbanions Ethynide ion is the weakest base The more electronegative carbon, the more stable the anion