4 4-1 Organic Chemistry William H. Brown & Christopher S. Foote

4 4-2 AcidsandBases Chapter 4

4 4-3 Brønsted-Lowry Definitions  Acid:  Acid: a proton donor  Base:  Base: a proton acceptor

4 4-4 Conjugate Acids & bases  Conjugate base  Conjugate base: the species formed from an acid when it donates a proton to a base  Conjugate acid:  Conjugate acid: the species formed from a base when it accepts a proton from an acid

4 4-5 Conjugate Acids & Bases  Which is the favored site of protonation?

4 4-6 Conjugate Acids & Bases  Which is the favored site of protonation?

4 4-7 Strong Acids and Bases  Strong acid:  Strong acid: completely ionized in aqueous solution. Examples are HCl, HBr, HI, HNO 3, HClO 4, and H 2 SO 4  Strong base:  Strong base: completely ionized in aqueous solution. Examples are LiOH, NaOH, KOH, Ca(OH) 2, and Ba(OH) 2

4 4-8 Acids & Base Strengths  Acetic acid is a weak acid it is incompletely ionized in aqueous solution

4 4-9 Weak Acids and Bases  The equation for the ionization of a weak acid, HA, in water and the acid ionization constant, K a, for this equilibrium are

Weak Acids and Bases

Weak Acids and Bases  Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base

Structure and Acidity A. Electronegativity of the atom bearing the negative charge within a period, the greater the electronegativity of the atom bearing the negative charge, the more strongly its electrons are held, the more stable the anion A -, and the greater the acidity of the acid HA

Structure and Acidity B. Size of the atom bearing the negative charge within a column of the Periodic Table, acidity is related to the size of the the atom bearing the negative charge atomic size increases from top to bottom of a column the larger the atom bearing the negative charge, the greater its stability, and the greater the acidity of HA

Structure and Acidity C. Resonance Delocalization of Charge in A - for compounds with the same functional group, the more stable the anion A -, the stronger the acid HA compare an alcohol and a carboxylic acid ionization of the O-H bond of an alcohol gives an anion for which there is no resonance stabilization

Structure and Acidity ionization of a carboxylic acid gives a resonance- stabilized anion a carboxylic acid is a stronger acid than an alcohol

Structure and Acidity D. Electron-withdrawing inductive effect the polarization of electron density of a covalent bond due to the electronegativity of an adjacent covalent bond

Structure and Acidity stabilization of an anion by the inductive effect falls off rapidly with increasing distance of the electronegative atom from the site of the negative charge

Structure and Acidity E. Hybridization for anions differing only in the hybridization of the charged atom, the greater the % of s character to the hybrid orbital of the charged atom, the more stable the anion, and the greater the acidity of HA

Lewis Acids and Bases  Lewis acid:  Lewis acid: any molecule of ion that can form a new covalent bond by accepting a pair of elections  Lewis base:  Lewis base: any molecule of ion that can form a new covalent bond by donating a pair of elections

Lewis Acids and Bases

Prob 4.8 For each conjugate acid-base pair, identify the first species as an acid or base and the second as its conjugate base or acid.

Prob 4.9 Complete a net ionic equation for each proton-transfer reaction. Label the original acid and its conjugate base.

Prob 4.10 Complete a net ionic equation for each proton-transfer reaction. Label the original acid and its conjugate base.

Prob 4.11 Write a structural formula for the conjugate acid formed by treating each molecule or ion with HCl.

Prob 4.13 Account for the greater stability of the anion derived from acetone compared with the anion derived from ethane.

Prob 4.16 Arrange the compounds in each set in order of increasing acid strength.

Prob 4.17 Arrange the compounds in each set in order of increasing base strength.

Prob 4.20 Write an equation for the reaction of acetic acid, pK a 4.76, with each base. Which equilibria lie toward the left? Which lie toward the right?

Prob 4.21 Write an equation for the reaction of ethanol, CH 3 CH 2 OH, pK a 15.9 with each base. Which equilibria lie toward the left? Which lie toward the right?

Prob 4.22 Benzoic acid, C 6 H 5 COOH, pK a 4.19, is only slightly soluble in water but its salt, C 6 H 5 COO - Na +, is quite soluble in water. In which solutions will benzoic acid dissolve more readily than in water?

Prob Methylphenol, CH 3 C 6 H 4 OH, pK a 10.26, is only slightly soluble in water, but its sodium salt, CH 3 C 6 H 4 O - Na +, is quite soluble in water. In which solutions will 4- methylphenol dissolve?

Prob 4.24 For an acid-base reaction, one way to determine the predominant species at equilibrium is to say that the reaction arrows points to the acid with the higher value of pK a. Explain why this rule works.

Prob 4.25 Will acetylene react with sodium hydride according to the following equation to form a salt and hydrogen?

Prob 4.26 Using the values of pK a given in Table 4.1, calculate the equilibrium constant, K eq, for this acid-base reaction.

Prob 4.28 Complete the equation for each Lewis acid-base reaction.

Prob 4.29 For each reaction, label the Lewis acid and the Lewis base, and use curved arrows to show the flow of electrons in each reaction.

Prob ,4-Pentanedione is a considerably stronger acid than acetone. Write a structural formula for each conjugate base, and account for the greater stability of the conjugate base from 2,4-pentanedione.

Prob 4.32 In which equation does the sec-butyl cation react as a Lewis acid? In which equation does it react as a Brønsted-Lowry acid? Write Lewis structures for the reactants and products, and show by the use of curved arrows how each reaction occurs.

Prob 4.34 Write equations for the reaction of each compound with H 2 SO 4, a strong protic acid.

Prob 4.36 Write a structural formula for the conjugate base formed when each compound is treated with one mole of a base stronger than the compound’s conjugate base.

Prob 4.39 For each pair of molecules or ions, select the stronger base and write its Lewis structure.

Prob 4.43 When imidazole is dissolved in water, proton transfer to it gives a cation. Is the cation better represented by A or B?

Acids and Bases End Chapter 4