CHAPTER FOUR Alcohols and Alkyl Halides The Bronsted-Lowry definitions of acids and bases are widely used in organi emistry. As noted in the preceding equation, the conjugate acid of a substance is formed when it accepts a proton from a suitable donor. Conversely, the proton donor is converted to its conjugate base. A conjugate acid-base pair always differ by a single proton TPROBLEM 4.6 Write an equation for the reaction of ammonia (: NH,)with hydro- gen chloride(HCi). Use curved arrows to track electron movement, and identify the acid, base, conjugate acid, and conjugate base. In aqueous solution, an acid transfers a proton to water. Water acts as a Bronsted base H H Water Acid Conjugate acid of water The systematic name for the conjugate acid of water(H3o)is oxonium ion. Its com- mon name is hydronium ion The strength of an acid is measured by its acid dissociation constant or [H3O’I[A Table 4.2 lists a number of Bronsted acids and their acid dissociation constants Strong acids are characterized by Ka values that are greater than that for hydronium ion (H3O, Ka=55). Essentially every molecule of a strong acid transfers a proton to water in dilute aqueous solution. Weak acids have Ka values less than that of H3o; they ar incompletely ionized in dilute aqueous solution A convenient way to express acid strength is through the use of pka, defined as Thus, water, with Ka =1.8 X 10, has a pKa of 15.7; ammonia, with Ka l0, has a pKa of 36. The stronger the acid, the larger the value of its Ka and the smaller the value of pKa. Water is a very weak acid, but is a far stronger acid than ammo- nia. Table 4.2 includes pKa as well as Ka values for acids. Because both systems are widely used, you should practice converting Ka to pKa and vice versa PROBLEM 4.7 Hydrogen cyanide(HCn)has a pka of 9.1. What is its Ka? ls HCN a strong or a weak acid? An important part of the Bronsted-Lowry picture of acids and bases concerns the relative strengths of an acid and its conjugate base. The stronger the acid, the weaker the conjugate base, and vice versa. Ammonia(NH3) is the second weakest acid in Table 4.2. Its conjugate base, amide ion(H2N ) is therefore the second strongest base Hydroxide(Ho) is a moderately strong base, much stronger than the halide ions F Cl, Br and I, which are very weak bases. Fluoride is the strongest base of the halides but is 10-times less basic than hydroxide ion. Back Forward Main Menu Study Guide ToC Student OLC MHHE WebsiteThe Brønsted–Lowry definitions of acids and bases are widely used in organic chemistry. As noted in the preceding equation, the conjugate acid of a substance is formed when it accepts a proton from a suitable donor. Conversely, the proton donor is converted to its conjugate base. A conjugate acid–base pair always differ by a single proton. PROBLEM 4.6 Write an equation for the reaction of ammonia (:NH3) with hydro￾gen chloride (HCl). Use curved arrows to track electron movement, and identify the acid, base, conjugate acid, and conjugate base. In aqueous solution, an acid transfers a proton to water. Water acts as a Brønsted base. The systematic name for the conjugate acid of water (H3O) is oxonium ion. Its com￾mon name is hydronium ion. The strength of an acid is measured by its acid dissociation constant or ionization constant Ka. Ka  Table 4.2 lists a number of Brønsted acids and their acid dissociation constants. Strong acids are characterized by Ka values that are greater than that for hydronium ion (H3O, Ka  55). Essentially every molecule of a strong acid transfers a proton to water in dilute aqueous solution. Weak acids have Ka values less than that of H3O; they are incompletely ionized in dilute aqueous solution. A convenient way to express acid strength is through the use of pKa, defined as follows: pKa  log10 Ka Thus, water, with Ka  1.8  1016, has a pKa of 15.7; ammonia, with Ka 1036, has a pKa of 36. The stronger the acid, the larger the value of its Ka and the smaller the value of pKa. Water is a very weak acid, but is a far stronger acid than ammo￾nia. Table 4.2 includes pKa as well as Ka values for acids. Because both systems are widely used, you should practice converting Ka to pKa and vice versa. PROBLEM 4.7 Hydrogen cyanide (HCN) has a pKa of 9.1. What is its Ka? Is HCN a strong or a weak acid? An important part of the Brønsted–Lowry picture of acids and bases concerns the relative strengths of an acid and its conjugate base. The stronger the acid, the weaker the conjugate base, and vice versa. Ammonia (NH3) is the second weakest acid in Table 4.2. Its conjugate base, amide ion (H2N), is therefore the second strongest base. Hydroxide (HO) is a moderately strong base, much stronger than the halide ions F, Cl, Br, and I, which are very weak bases. Fluoride is the strongest base of the halides but is 1012 times less basic than hydroxide ion. [H3O][A] [HA] H H O Water (base) H A Acid A Conjugate base Conjugate acid of water H H O H 134 CHAPTER FOUR Alcohols and Alkyl Halides Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website