Functional Groups, Orbitals, and Geometry Resonance Structures Bond Polarity - Part I A bond is polar when the charge is not equally shared between the two atoms. The more electronegative atom will have a partial negative charge (δ-). The arrow shows the dipole moment. Here we show partial charges. Acids and Bases-Definitions Arrhenius acid: A substance which dissolves in water to produce H+. Brønsted-Lowry acid: a proton donor H+ is a proton. Lewis acid: an electron pair acceptor Arrhenius Acids and Bases Arrhenius acid: which dissolves produce H+. Arrhenius base: which dissolves produce OH-. A substance in water to A substance in water to Limited to aqueous solutions. Does not explain a reaction such as NH3(g) + HCl(g) NH4Cl(s) Brønsted-Lowry Acids and Bases B-L acids are proton donors. B-L bases are proton acceptors. The emphasis is on the transfer of the H+. This links acids and bases. A B-L acid HB has a conjugate base: HB H+ + B:This is the equation for HB acting as an acid. Brønsted-Lowry Acids and Bases HB H+ + B: This is the equation for HB acting as an acid. B:- is the conjugate base. B:- +H2O HB + OH This is the equation for B- acting as a base in water. B:- + HA HB + A This is the equation for B- acting as a base with an acid other than water. Be able to write these types of equations for any B-L acid or base. Brønsted-Lowry Acids and Bases Ammonia acting as an acid: NH3 NH2- + H+ Ammonia acting as a base: NH3(aq) + H2O NH4+(aq) + OH(aq) What is the conjugate acid and what is the conjugate base of ammonia? Is ammonia a conjugate acid or base? Acid Strength and pKa HB H+ + B:- Ka = acid dissociation constant Ka = [H+][B-] [HB] pKa = -log Ka The more completely an acid dissociates in water, the stronger it is. The stronger the acid, the larger its Ka and the smaller its pKa. Comparing Acid Strengths Which is the stronger acid, ammonia or water? There are two ways to find an answer: The quantitative way: compare pKa values. The qualitative way: compare the stabilities of the conjugate bases. Comparing Acid Strengths The quantitative way: compare pKa values. NH3 NH2- + H+ pKa = 36 H2O(l) H+(aq) + OH-(aq) pKa = 15.7 Water is the stronger acid. Comparing Acid Strengths The qualitative way: compare stabilities of the conjugate bases. NH3 NH2- + H+ H2O(l) H+(aq) + OH-(aq) The more stable the conjugate base is in water, the stronger the acid. The amide ion is such a strong base it cannot exist in water, therefore ammonia is the weaker acid. Comparing Acid Strengths You will find it very helpful in studying organic chemistry to have a good idea of the relative strengths of some of the more common compounds acting as acids. Please become VERY familiar with Table 1-5. Comparing Acid Strengths by Comparing Structures How does the structure of a compound affect its acid/base properties? Look at the stability of the conjugate base. The more stable the conjugate base, the stronger its acid. Electronegativity Size/polarizability Resonance Stabilization Induction Hybrid orbital containing electrons Comparing Acid Strengths by Comparing the Stabilities of the Conjugate Bases Electronegativity (e.n.) A more electronegative atom holds negative charge more easily. Many bases are anions. The more stable the anion, the weaker the base: e.n.(C) < e.n.(N)<e.n.(O)<e.n.(F) Base strength: CH3->NH2->OH->F Acid strength: CH4<NH3<H2O<HF Comparing Acid Strengths by Comparing the Stabilities of the Conjugate Bases Size A larger anion is more stable: Size/stability: F- < Cl- < Br- < I Acid strength: HF < HCl < HBr < HI Base strength: F- > Cl- > Br- > I- Comparing Acid Strengths by Comparing the Stabilities of the Conjugate Bases Resonance Stabilization An anion stabilized by resonance has a stronger conjugate acid. Comparing Acid Strengths by Comparing Structures Induction Look at nearby atoms. Electronegative atoms “pull” electron density away (induction). This can stabilize a negative charge. (Note: they must be very close to the negative charge to be effective.) Trichloroacetic acid is stronger than acetic acid. more stable Comparing Acid Strengths by Comparing Structures Hybrid orbital containing electrons Acetylene (H-C≡C-H), believe it or not, can act as an acid with certain really strong bases. H-C≡C-H + B:- H-C≡C:- + HB The sp orbital is short (50% s character) and stabilizes the anion by holding the electrons closer to the nucleus. Lewis Bases and Acids Lewis looked at acid/base behavior from the viewpoint of the bonds that are formed instead of the transfer of a proton. Lewis Bases and Acids Lewis bases have nonbonding electrons that can be donated to form new bonds. Lewis bases are nucleophiles (lovers of nuclei +++). Lewis acids accept these electrons. Lewis acids are electrophiles (lovers of electrons ---). Two Bases Worth Knowing NaH and NaNH2 sodium hydride sodium amide sodium methoxide sodium ethoxide Given the reactants, be able to write the products of any acid/base reaction! Identifying Bases NaH and NaNH2 Amines Hydroxide ion, OHAlkoxide ions, e.g. CH3OAlcohols Water Identifying Acids Inorganic (the seven strong acids) Carboxylic acids Phenols Alcohols Water These are pretty much in order from strongest to weakest.