Brønsted-Lowry Acids and Bases

Transcription

1 Brønsted-Lowry Acids and Bases In 1884, Svante A. Arrhenius (Sweden, ) defined an acid as a material that can release a proton or hydrogen ion ( + ). Cl in water. A base (then called an alkali) is a material that can donate a hydroxide ion ( ). Na in water. This concept led to a definition of acids and bases as reported independently in 1923, by Thomas M. Lowry (England, ) and Johannes Nicolas Brønsted (Denmark, ). According to this definition, an acid is a material that donates a proton and a base is a material that can accept a proton, the Brønsted-Lowry definitions mentioned above. 1 Cl(aq) + Cl (aq) (a q) Svante A. Arrhenius (Sweden, ) Johannes Nicolas Brønsted (Denmark, ) Thomas M. Lowry (England, )

2 Brønsted-Lowry Acids and Bases 2 According to Brønsted and Lowry, an acid-base reaction is defined in terms of a proton transfer. By this definition, the reaction of Cl in water is: Cl(aq) + Cl (aq) (a q) Water is the base in this reaction. Acid and base reactions in General chemistry are always done in water. When a Brønsted-Lowry acid such as Cl is placed in water, a proton is transferred to a water molecule (water is the base); a conjugate acid is formed (the hydronium ion 3 + ) as well as a conjugate base (the chloride ion). In neutral pure water (no acid is present), the hydrogen ion concentration is about 1.0 x 10-7 M (p of 7). An increase in the concentration of hydrogen ions above 1.0 x 10-7 M gives an acidic solution, with a p less than 7. If the p is >7, it is considered a basic solution.

3 Strong Brønsted-Lowry Acids and Bases 3 Cl(aq) + Cl (aq) (a q) The criterion for acid strength is the extent of ionization in water. Strong acids are extensively ionized in water. The mineral acids Cl, Br, I, 2 S 4, N 3, and Cl 4 are all strong acids. Strong bases are extensively ionized in water. Common strong bases include Na (soda lye), K (potash lye), Li, Cs, Mg() 2, Ca() 2, and Ba() 2. The relative strength of these acids and bases are defined by their reaction with water.

4 Strong Brønsted-Lowry Acids and Bases 4 Cl(aq) + Cl (aq) (aq) Equilibrium = 2 chemical reactions in competition In the case: reaction 1: Cl + 2 (Cl is the acid; 2 is the base) reaction 2: Cl ( 3 + is the acid; Cl is the base) To determine the position of the equilibrium (K), MUST consider BT reactions, not just reaction 1.

5 Weak Brønsted-Lowry Acids and Bases 5 Weak acids are solutes that react reversibly with water to form 3 + ions, and there are two categories. molecules containing an ionizable hydrogen atom such as nitrous acid (N 2 ) cations such as the ammonium ion (N 4+ ). Weak bases are defined as solutes that react with water molecules to acquire a + ion and leave an hydroxide ion ( ) behind. An example of a weak base is the reaction of ammonia + water to give the ammonium cation and hydroxide anion. Weak acids that will discussed in this book are acetic acid (C 3 C), boric acid( 3 B 3 ), butanoic acid (C 3 C 2 C 2 C), formic acid (C), hydrocyanic acid (CN), and uric acid (C 5 3 N 4 3 ), and the fatty acid known as stearic acid [C 2 (C 2 ) 15 C 2 C, an 18-carbon acid]. Weak bases include ammonia (N 3 ), and amines such as methylamine (C 3 N 2 ), triethylamine [(C 3 ) 3 N], and pyridine (C 5 5 N. The relative strength of these acids and bases are defined by their reaction with water.

6 We ave a PRBLEM!!!!!!!!! 6 Many organic acids and bases are insoluble in water, so the classical Brøns ønsted-lowry definition does not formally apply. A very weak acid can react with a particularly strong base, and a very weak base may react with a particularly strong acid. A solvent other than water is required to put organic acids or bases into solution, and this other solvent is typically another organic compound. Diethyl ether (C 3 C 2 C 2 C 3 ), hexane (C 3 C 2 C 2 C 2 C 2 C 3 ), and dichloromethane (C 2 Cl 2 ) are common solvents. A water-organic solvent mixture is sometimes required to make the acid-base components soluble.

7 Comparing an rganic Acid and Cl 7 Water reacts with the strong acid Cl or the much weaker organic acid formic acid (C). The reaction of formic acid and water generates the conjugate base known as the formate anion, and the conjugate acid, the hydronium ion. Formic acid is a weaker acid than Cl, so the equilibrium lies to the left for C when compared with the equilibrium for Cl. Note the (aq) term, which indicates solvation of that molecule or ion by the solvent, which is water. Note also that the term reaction is used for the two acid-base equilibrium processes Cl (aq) Cl 3 + (aq) (aq) C formic acid (aq) C (aq) (aq) formate anion

8 Comparing Two rganic Acids 8 Water might react as a base with the acid ethanol (C 3 C 2 ). If water is the base, ethanol, C or Cl can be the acid. In all cases, the oxygen of 2 removes a proton ( + ) from the acid, so all three acids are Brønsted-Lowry acids. Ethanol is known to be a much weaker than formic acid, so the equilibrium for this reaction lies almost entirely to the left. By comparing three different acids for ionization in water, an order of acid strength is established: Cl > C > ethanol. Cl (aq) Cl 3 + (aq) (aq) C formic acid (aq) C (aq) (aq) formate anion C 3 C 2 (aq) + 2 C 3 C 2 (aq) (aq) ethanol ethoxide

9 Acids in rganic Solvents Consider the reaction of formic acid using diethyl ether as a solvent rather than water. Note the (ether) term in the reaction, indicating solvation by ether rather than by water. The organic solvent diethyl ether is also the base, and the basic atom in diethyl ether is the oxygen. The reacts with the weak acid (formic acid) to give the formate anion (the conjugate base). The conjugate acid is the protonated form of ether (an oxonium ion. The equilibrium lies to the left in the reaction because that formic acid is a weak acid in ether, but the definition of strong or weak acid relating to dissociation in water no longer applies. The equilibrium for formic acid lies further to the left in ether when compared to the same reaction in water, so formic acid is a weaker acid in ether than in water. The strength of the acid depends on the base it reacts with, which is critical for establishing where the equilibrium lies, to the left or to the right. C(ether) + C (ether) + (ether) formic acid diether ether formate anion an oxonium ion 9

10 10 Acids in Different rganic Solvents If Cl is placed in anhydrous (no water) methanol, where methanol is the solvent, the oxygen of the methanol is the only available base for reaction with the acid Cl. The term (ether) is used for solvation in the formic acid reaction to show the analogy to the water reaction, but the (methanol) designation is omitted in this case. Although it is understood that all components are solvated by the solvent methanol, the solvation term is usually omitted in rganic chemistry and it will be omitted throughout the book. An acid reacts with a base, regardless of solvent, but the extent of ionization changes with the solvent and with the base. In this case, Cl is a stronger acid than formic acid but ether is a stronger base than methanol. Determine which is the better acid-base reaction by determining the equilibrium constant, K a. C(ether) + C (ether) + (ether) formic acid diether ether formate anion an oxonium ion C 3 + Cl C 3 2 Cl methanol an oxonium ion

11 Equilibrium Constants Cl (aq) Cl 3 + (aq) (aq) 11 The equilibrium constant, K a (acidity constant) in given by the following generalized equation when the reaction is done in water. [ + ] [B ] K a = and for Cl in water K a = [B] [ 3 + ] [Cl ] [Cl] The equation needs a base: water K a = [conjugate acid] [conjugate base] [acid] [base] so, K a = [ 3 + ] [Cl ] [Cl] [base] The base in the Cl reaction is actually the water from the solvent. Where is the term for water in the first set of equations? In General chemistry the water is removed from the equation to simplify calculations. owever, when water is removed from the equilibrium constant equation this also removes the base from the acid base equation.

12 Equilibrium Constants and pk a 12 The equilibrium constant, K a (acidity constant) in given by the following generalized equation when the reaction is done in water. [ + ] [B ] K a = and for Cl in water K a = [B] [ 3 + ] [Cl ] [Cl] The equation needs a base: water K a = [conjugate acid] [conjugate base] [acid] [base] so, K a = [ 3 + ] [Cl ] [Cl] [base] K! a!!!+ A- + :B B- + :A acid base!+ conjugate acid! conjugate base In this reaction, K a = and pk a = log K a Note that pk a and K a are inversely proportional and K a = 10 pka

13 Put the Base Back Into Acid-Base Reactions 13 The equilibrium constants for the formic acid reaction with ether as well as the methanol and Cl reaction must use the K a equations: [C ] [ ] K a = and K a = [C] [ ] [Cl ] [ C 3 2 ] [C 3 ] [ Cl ] Water is not used for many reactions found in rganic chemistry, and it is important to change the focus from acids that ionize in water to the concept that acids react with bases in order to ionize. It is important to remember that both the acid and the base must be identified in a reaction based on their reactivity.

14 There are Two Acid-Base Definitions. ow Are They Related? 14 A Brønsted-Lowry acid is defined as a proton donor and a Brønsted-Lowry base is a proton acceptor. A Lewis acid accepts an electron pair from a Lewis base, which donates an electron pair. When Cl reacts with water, the Brønsted-Lowry definition states that the proton (+) is donated to water, forming the hydronium ion. In this reaction, the oxygen atom of the water accepts the proton, but to accept the proton, the oxygen must donate two electrons to the and the accept two electrons from the oxygen - so the two definitions merge. The Lewis definition is more general because it focuses on transfer of electrons, and those atoms that are electron rich and electron donors, and those atoms that are electron deficient, and accept electrons. This definition is more important for tracking chemical reactions.

15 What appens When Water Accepts a Proton to Form the ydronium Ion? hydronium ion 15 The Lewis definition of a base is an electron donor and a Lewis base donates electrons to an atom other than hydrogen. The base donates two electrons to a proton. The definitions merge. A Brønsted-Lowry base accepts a proton by donating two electrons to that proton. A covalent bond requires two electrons. Therefore, bases may be defined as two electron donors to an electron deficient center. If the base donates electrons to a proton, it is a Brønsted-Lowry base. If the base donates electrons to an atom other than hydrogen, it is formally a Lewis base. A base is a two-electron donor.

16 Electron Flow and the Curved Arrow Formalism 16 hydronium ion All bases donate two electrons, and the electron flow is from the base to the acid, not from the acid to the base. An acid does not donate the proton, but rather the proton is attacked by the base to form a new bond to the proton, as shown for water and + to give the hydronium ion. A blue curved arrow is used to indicate the flow of electrons, and the electron flow is always from a source of high electron density to a point of low electron density. The direction of the arrow is always from the base to the proton of the acid, as illustrated.