Grossman, CHE 230 7.1 7. Acids and Bases. 7.1 Definitions. Brønsted acids are proton donors, and Brønsted bases are proton acceptors. Examples of Brønsted acids: HCl, HBr, H2SO4, HOH, HO+H2, N+H4, NH3, CH3CO2H, H—CH2COCH3, H—C≡CH, H—CH3. Examples of Brønsted bases: anything with a lone pair. Brønsted acids are generally neutral or cationic, and Brønsted bases are generally neutral or anionic. Some compounds (H2O, NH3, acetone) can be either acids or bases. Lewis acids are electron acceptors, and Lewis bases are electron donors. The set of Lewis and Brønsted bases are essentially identical, but many Lewis acids exist that are not proton donors. E.g., BF3, C+Me3, and many metal salts such as ZnCl2, TiCl4, etc. We can also consider the proton, H+, a Lewis acid itself. Some books say that all Brønsted acids are Lewis acids also, but we will use the term Lewis acid to denote only those acids to which a bond can be made without breaking another bond. By this definition, H—Cl is not a Lewis acid, because making a bond to H requires breaking the H—Cl bond. Lewis acids are either neutral or cationic. When we say “acid”, we will mean “Brønsted acid”. If we mean “Lewis acid”, we will say “Lewis acid”. This is the commonly accepted practice among organic chemists. When an acid HX gives up a proton, the species that is left behind (X–) is called the conjugate base of HX. Likewise, when a base B¨ accepts a proton, the product +BH is called the conjugate acid of B¨. 7.2 Curved arrows. What happens to the electrons in an acid–base reaction? The base uses its pair of electrons to make a new bond to H. In the process, the bond between H and the acidic atom breaks, and the pair of electrons from that bond becomes a lone pair in the conjugate base. We use curved arrows (the same as the ones we use in drawing resonance structures) to show whither the electrons in the starting materials moved to reach the products. The electron pair from NH3 moves to form a bond between N and H; the electrons in the bond between O and H move to become a lone pair on O. O HH3CONH3OHH3CONH3Grossman, CHE 230 7.2 We used curved arrows to keep track of electrons in the course of a reaction. You must learn how to use curved arrows correctly. They provide the basic way of communicating in organic chemistry. Any dufus can keep track of nuclei, but only the adept can keep track of electrons. Curved arrows are how we do it. 7.3 Acid–Base Equilibria. Kas and pKas. When an acid is dissolved in H2O, it transfers its proton to the solvent to some extent. E.g., CH3CO2H + H2O → CH3CO2– + H3O+ Note the change in the charges when the proton is transferred. The equilibrium constant for this reaction is: K = [CH3CO2–] [H3O+][CH3CO2H] [H2O]________________ Because H2O is the solvent, we define a new quantity, Ka: Ka = K[H2O] = [CH3CO2–] [H3O+] [CH3CO2H] ________________ = 1.8 × 10–5 (for acetic acid). The larger the value of Ka, the more likely an acid is to ionize, and hence the stronger that acid is. Because values for Ka vary by up to 60 orders of magnitude, we usually refer to them with a logarithmic (common log, or log10) scale: pKa = –log(Ka) pKa(acetic acid) = –log(1.8 × 10–5) = 4.7 The smaller the value of pKa, the stronger that acid is. Common mineral acids such as HCl have pKa around –10. The pKa (H2O) = 15, while pKa (H3N) = 35, and pKa (CH4) ≈ 50. It is important to see the relationship between the strength of an acid/base and the strength of its conjugate base/acid. If an acid is very strong, then when it gives up its proton, the conjugate base will be very unwilling to take that proton back, so it will be a weak conjugate base. strong acid weak conjugate base weak acid strong conjugate baseGrossman, CHE 230 7.3 strong base weak conjugate acid weak base strong conjugate acid We arbitrarily define “strong acids” as acids with pKa < 0. Moderately weak acids have 20 > pKa > 0. Very weak acids have 35 > pKa > 20. Extremely weak acids have pKa > 35. You should be able to look at an acid–base equilibrium and determine whether the equilibrium lies to the right or left anbd by how much, given the pKas of the two acids. E.g., consider: CH3CO2H + NH3 → CH3CO2– + NH4+ The pKa of acetic acid is 4.7, and the pKa of NH4+ is 10. (Note that even though NH3 can act as an acid, in this reaction it is acting as a base, so its pKa is irrelevant.) The equilibrium lies to the right because it lies on the side of the weaker acid. The equilibrium constant is Keq = 10(10 – 4.7). (Keq must be greater than 0 for a reaction that lies towards products. If the reaction had been written in the reverse way, then Keq = 10–(10 – 4.7).) 7.4 Factors affecting pKas. I do not want you to memorize a large number of pKas. I do want you to be able to look at two compounds and tell me which compound is more acidic. There are several factors that affect acidity, and we will examine each in turn: electronegativity, size, charge, inductive effects, hybridization, resonance, steric effects. The general rule for determining acidity is: look at the stability (energy) of the conjugate base. The lower in energy the conjugate base, the stronger the acid. This is because acidity is determined by the difference in free energy between the acid and the conjugate base. In most organic acids, the conjugate base is much higher in energy than the acid, so factors that affect energy generally have a much stronger influence on the conjugate base than they do on the acid. (Giving $10 to a poor man will have much more effect than giving $10 to a rich man.) 7.4.1 Comparing different elements 7.4.1.1 Size When comparing two acids in which the protons to be given up are directly attached to two elements in the same column of the periodic table, the heavier element is more acidic than the lighter one, all other things being equal. HI (–10) > HCl (–7) >> HF (3). This trend is contrary to electronegativity. The trend is due to the increasingly poor overlap between the tiny H(1s) orbital and the orbital of the acidicGrossman, CHE 230 7.4 element as you go down the periodic table. Electronegativity is not the sole determining factor in acidity!!! Size matters! 7.4.1.2 Electronegativity When comparing two acids in which the protons to be given up are directly attached to two elements in the same row of the
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