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Acid/Base in Organic Chemistry

Acid/Base in Organic Chemistry. Brønsted-Lowry definition Acids donate a proton Bases accept a proton Recall from General Chemistry this classic example. Conjugate Acids and Bases. Brønsted-Lowry definition A conjugate acid results when a base accepts a proton

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Acid/Base in Organic Chemistry

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  1. Acid/Base in Organic Chemistry • Brønsted-Lowry definition • Acids donate a proton • Bases accept a proton • Recall from General Chemistry this classic example

  2. Conjugate Acids and Bases • Brønsted-Lowry definition • A conjugate acid results when a base accepts a proton • A conjugate base results when an acid gives up a proton • Recall the ionization of HCl in water: • Practice: Label the acid, base, and the conjugates in the reaction below

  3. Acids and Bases • Draw reasonable products for the following acid/base reaction, and label the conjugates.

  4. Curved Arrows in Reactions • Recall Curved Arrow movement in Resonance: relocation of pairs of electrons • Consider the following generic acid/base reaction • Here is the INTERMOLECULAR curved arrow movement, leading to forming/breaking bonds • The curved arrows show the reaction mechanism

  5. Arrow pushing in Acid Base Reactions • Starts with base’s electron pair (“:”) forming bond with H • To maintain Octet, H atom then returns electrons to O. • The acid lose its H+ whilst gains electron pair to form negative charge

  6. Practice: Arrow Pushing • Arrow pushing starts with base’s electron pair forming bond with H atom; meanwhile H atom returns electrons to O atom. • Provide products and curved arrows for the following acid base reaction

  7. Practice: Arrow pushing in Reactions • Identify the acid/base steps in the following mechanism

  8. Strength of Acids • Recall from General Chemistry, how do “strong” acids/bases differ from “weak” acids/bases? • The strength of an acid or base is helpful to predict how reactions will progress (where the equilibrium lies, toward the reactants or products) • We will learn to do Quantitative strength analysis – using pKa values to compare the strengths of acids • We will learn to do Qualitative strength analysis – comparing the general stability of structures.

  9. Strength of Acidity: Ka , pKa Review from General Chemistry Ka and pKa • Ka : the equilibrium constant for the reaction equilibrium between an acid and WATER • Large Ka indicates Strong acids, and vice versa. • Small pKa indicates strong acid • Larger pKa indicates anion A-less stable than acid HA

  10. Questions: • Which is stronger, HCl or H2SO4? • Which is stronger acid, water or ammonia?

  11. Strength of Base Stronger the acid prefer dissociation of proton, thus less likely the conjugate base to accept proton Higher pKa, the stronger base for the conjugate base. Amine is more basic than ROH

  12. Structural Factors affecting Acidity • The more effectively a conjugate base can stabilize its negative charge (anion), the stronger the acid • Factors affecting the stability of a negative formal charge? • The type of Atom that carries the charge • Resonance • Induction • The type of Orbital where the charge resides • These factors can be remembered with the acronym, ARIO • Check http://www.youtube.com/watch?v=RDMO8eV_r8w

  13. ARIO: Electronegativity • ARIO - The type of atom that carries the charge • Within a row, more electronegative atoms are better at stabilizing “-” charge. Example: HF > HOH > H2N-H • Propanol more acidic than butane • Within a column, larger atoms have weaker X-H bond thus stronger acid. Example: HO-H < HS-H, HI > HBr methanethiol (CH3SH) more acidic than methanol (CH3OH)

  14. ARIO: Resonance • ARIO - Resonance can greatly stabilize a formal negative charge by spreading it out into partial charges (Ch. 2) Practice: Predict the relative acidity of ethanol vs. acetic acid

  15. ARIO: Induction • Induction: Electronegative atom draws electron density away from neighboring atoms, stabilizing the proximal negative charge. Example: Relative strength • Trifluoroacetic acid > Trichloroacetic acid > Acetic acid.

  16. Resonance vs. Induction • Does induction also play a role in explaining why acetic acid is stronger than ethanol? Resonance in most cases is stronger than induction in stabilizing the negative charge.

  17. Induction in action • Explain the pKa differences below

  18. ARIO: Orbital effect • The type of orbital affect the stability of a formal “-” charge • Negative charge is better stabilized when closer to the nucleus. • Within the same shell, s orbital is smaller than p orbital. More s character (less p character) in hybrid orbitals gives smaller hybrid orbitals, better stabilize negative charge • Stability on negative formal charge: sp > sp2 > sp3 25% s 33% s 50% s

  19. Example: Hybridization affects acidity

  20. Application of Acidity in reactivity • If a strong base were to react with the following molecule, which proton would most likely to react? Which proton is the most acidic? Given pKa: Csp3-H (~50), =Csp2-H (44), ≡Csp-H (25)

  21. How to apply ARIO • When assessing the acidity of protons, we generally use ARIO as our order of priority • The type of atomthat carries the charge • Resonance • Induction • The type of orbital where the charge resides • Compare ethanol and propylene. Which has a more stable conjugate base? WHY? pKa = 16 pKa = 43

  22. ARIO has limitations • ARIO is a good general guideline for comparing similar structures. But it sometimes fails Using ARIO, acetylene _________ acidic than ammonia. • The following reaction will favor (reactant, product)

  23. Practice: Comparison of Acidity • For each of the molecules below, rank the labeled Hydrogen atoms in order of increasing pKa value Increasing pKa = decreasing acidity strength ARIO: e, d, a, b, c

  24. Positive charge on Acidity (ario) • Sometimes acids have positive charge: H3O+ and NH4+ • Then the conjugate bases will be neutral. H2O and NH3 The stability of the acids (charged) depends on the stability of the positive charge (the ario+) • The more stable for the acid, the lower acidity • Resonance stabilizes cation • Lower electronegativity (a), lower induction (i), lesser s in orbital (o) stabilize cation. This is opposite to ARIO for anion (base).

  25. Practice: Positive charge on Acidity • ario+ • Rank the following acids in order of increasing strength Increasing acid strength= destabilized cation 3, 5, 4, 2, 1

  26. Using pKas to predict Equilibria • Using pKa values, you can predict which direction an acid/base equilibrium will favor • For acid base reaction (proton transfer), stronger acid react with base to form weaker acid (pKa value of reactant acid should be less than product acid) • Because for the above rxn Krxn = Ka(reactant)/Ka(product)

  27. Practice: Is the product favored in an acid base rxn? B. Will hydroxide ion react with ammonia to form water and amide ion?

  28. *Predicting Equilibrium Position • Is the reaction below, reactant or product favored? Which is stronger acid? ARIO on the anion: only Resonance makes difference. Between two resonance structures, sulfur anion is more stable than oxygen anion (ARIO)

  29. Weakest Acid for Protonation? pKa of the acid (reactant) < pKa of acid (product) pKa 16~18 ~20 38~40 Step 1. Write a generic equation for each of above: HA + ________  A- + _________(conj acid) Step 2. Find pKa’s for the conjugate acid A.

  30. Choosing a Base for Deprotonation pKa 16~18 Step 1. Write a generic equation for each above: A- + ________  HA + _________ Step 2. Find pKa’s for each acid Step 3. Look for acid in the table with lower pKa

  31. Solvent: Leveling Effect • Most reactions take place in solvent; solvent is not suppose to be involved in reaction • Reality: Most solvents are potentially ACID (mostly) or Base (rare) that may react with solute (acid or base). • Because water can act as an acid or a base, it has a leveling effect on strong acids and bases • Acids stronger than H3O+ can not be used in water as they protonate water • Bases stronger than OH- can not be used in water as they deprotonate water.

  32. Leveling Effect on Reaction • Is water a good solvent to allow acetate ion as reactant? pKa= 15.7 pKa= 4.75 • With water as the solvent, the CH3CO2–will react with the water, but the equilibrium greatly favors the reactant side (K = Kw/Ka << 1) or little reaction occurs. • so water is an appropriate solvent

  33. Leveling Effect for Acid Acids stronger than H3O+ cannot be used in water because their reaction with water. Example, molecular sulfuric acid completely reacts with solvent water: Thus no molecular sulfuric acid to be available to react with another reagent Solution: Use solvent of weaker basicity (higher pKb or lower pKa for the conjugate acid), or nonbasic aprotic solvent

  34. Leveling Effect for Base Bases stronger than OH– can not be used in water. • For example, water would not be an appropriate solvent for the following reaction. pKa ~25 ~38 The higher acidity of water (pKa = 15.7) allows reaction H2O + base  OH- + acid So water reacts with both reactant base and product base!!! Butyl lithium (LiC4H9) reacts with water.

  35. Steric Effect on pKa • ARIO predicts pKa of ethanol and tert-Butanol be similar • Bulky methyl groups obstructs ion-dipole attractions between solvent and butoxide anion (steric hinderance), thus not well solvated as the ethoxide

  36. Solvation Solvation is critically important in reactions: • The solvent is often needed to stabilize transition states, reaction intermediates, and/or products to allow a reaction to occur • Solvent Effect: pKa for acetic acid: 4.75 (water) 23.5 (CH3CN)

  37. Additional Practice Problems • Which side of the following generic reaction will be favored, and what will the ratio of products/reactants be? HA + B:  HB + A: pKa = 5 8

  38. Additional Practice Problems • Rank the following bases in order of increasing strength and rank their conjugates in order of increasing pKa. • A < C < E < D < B

  39. Importance of Counterions • Counterions (counter-ion): known as Spectator ions, always present, but not in net ionic equation • Phase transfer catalysis: Solvent helps stabilize the counterion to facilitate reaction. Purple Benzene Example: Oxidation of alkene by KMnO4 (ionic) • Water as solvent: immiscible between alkene and aqueous phase (KMnO4(aq)), poor reaction. • Crown ether added to water helps solvate K+ and speed up reaction

  40. PTC

  41. Lewis Acids and Bases • A Lewis acid acceptsand shares a pair of electrons • A Lewis base donatesand shares a pair of electrons • Brønsted-Lowry Acids are Lewis acids, but not vice versa. • Brønsted-Lowry Bases are Lewis bases, but not vice versa. Q: (water, HCl) loses electron pair, thus is (acid, base)

  42. Lewis Acids and Bases • Some Lewis acid/base reactions can not be classified using the Brønsted-Lowry definition A. Circle (BF3, water) is the Lewis acid. B. Water reacts with CO2 to form carbonic acid. Which is the Lewis base

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