Chapter 4: Acids and Bases

1. Chapter 4: Acids and Bases

2. Arrhenius Acids and Bases • In 1884, Svante Arrhenius proposed these definitions • acid: a substance that produces H3O+ ions in aqueous solution • base: a substance that produces OH- ions in aqueous solution Brønsted-Lowry Acids & Bases • Acid: a proton donor • Base: a proton acceptor • Brønsted-Lowry definitions do not require water as a reactant

3. Brønsted-Lowry Acids & Bases • we can use curved arrows to show the transfer of a proton from acetic acid to ammonia

4. Conjugate Acids & Bases • conjugate base:the species formed from an acid when it donates a proton to a base • conjugate acid: the species formed from a base when it accepts a proton from an acid A weak acid has always a strong conjugated base !!!

6. Brønsted-Lowry Acids & Bases • Note the following about the conjugate acid-base pairs in the table: 1. an acid can be positively charged, neutral, or negativelycharged; examples of each type are H3O+, H2CO3, and H2PO4- 2. a base can be negatively charged or neutral; examples are OH-, Cl-, and NH3 3. acids are classified a monoprotic, diprotic, or triprotic depending on the number of protons each may give up; examples are HCl, H2CO3, and H3PO4 4. several molecules and ions appear in both the acid and conjugate base columns; that is, each can function as either an acid or a base 5. there is an inverse relationship between the strength of an acid and the strength of its conjugate base • the stronger the acid, the weaker its conjugate base • HI, for example, is the strongest acid and its conjugate base, I-, is the weakest base in the table • CH3COOH (acetic acid) is a stronger acid that H2CO3 (carbonic acid); conversely, CH3COO- (acetate ion) is a weaker base that HCO3- (bicarbonate ion)

7. Acid and Base Strength • Strong acid: one that reacts completely or almost completely with water to form H3O+ ions • Strong base: one that reacts completely or almost completely with water to form OH- ions • Weak acid: a substance that dissociates only partially in water to produce H3O+ ions • Weak base: a substance that dissociates only partially in water to produce OH- ions

8. Acid-Base Reactions • acetic acid is incompletely ionized in aqueous solution • the equation for the ionization of a weak acid, HA, is pKa acid dissociation constant (pKa = -log10Ka)

12. Acid-Base Equilibrium • Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and the weaker base • equilibrium lies on the side of the weaker acid and the weaker base

13. Structure and Acidity • The most important factor in determining the relative acidity of an organic acid is the relative stability of the anion, A-, formed when the acid, HA, transfers a proton to a base • We consider three factors: • the electronegativity of the atom bonded to H in HA • resonance stabilization of A- • the inductive effect

14. Structure and Acidity • Electronegativity of the atom bearing the negative charge; within a period • the greater the electronegativity of the atom bearing the negative charge, the more strongly its electrons are held, the more stable the anion A- • and the greater the acidity of the acid HA

15. Structure and Acidity • Resonance delocalization of charge in A- • compare the acidity of an alcohol and a carboxylic acid • carboxylic acids are weak acids; values of pKa for most unsubstituted carboxylic acids fall within the range of 4 to 5 • alcohols are very weak acids; values of pKa for most alcohols fall within the range of 15 to 18

16. Structure and Acidity • the greater the resonance stabilization of the anion, the more acidic the compound • there is no resonance stabilization in an alkoxide anion • we can write two equivalent contributing structures for the carboxylate anion; the negative charge is spread evenly over the two oxygen atoms

17. Structure and Acidity • The inductive effect • the polarization of electron density transmitted through covalent bonds by a nearby atom of higher electronegativity

19. Lewis Acids and Bases • Lewis acid:any molecule or ion that can form a new covalent bond by accepting a pair of electrons • Lewis base:any molecule or ion that can form a new covalent bond by donating a pair of electrons

20. Lewis Acids and Bases • some organic Lewis bases and their relative strengths in proton-transfer reactions

21. Lewis Acids and Bases • Another type of Lewis acid we will encounter in later chapters is an organic cation in which a carbon is bonded to only three atoms and bears a positive formal charge • such carbon cations are called carbocations

22. Stereochemistry

23. Stereostructures

25. Stereoisomers Conformation of acyclic compounds:

26. Stereoisomers Conformation of cyclic compounds:

27. Stereoisomers Conformation of cyclic compounds:

28. Stereoisomers Conformation of cyclic substituted compounds:

29. Stereoisomers Conformation of cyclic substituted compounds:

30. Geometric Isomers Cis/Trans isomers in compounds with double bonds

31. Stereoisomers • Enantiomers:nonsuperposable mirror images • as an example of a molecule that exists as a pair of enantiomers, consider 2-butanol Optical Isomers: chiral center chiral center

32. Enantiomers • one way to see that the mirror image of 2-butanol is not superposable on the original is to rotate the mirror image

33. Enantiomers • now try to fit one molecule on top of the other so that all groups and bonds match exactly • the original and mirror image are not superposable • they are different molecules with different properties • they are enantiomers (nonsuperposable mirror images)

34. Enantiomers • Objects that are not superposable on their mirror images are chiral (from the Greek: cheir, hand) • they show handedness • The most common cause of enantiomerism in organic molecules is the presence of a carbon with four different groups bonded to it • a carbon with four different groups bonded to it is called a chiral center • Enantiomers are optical active compounds.

35. Optical Activity • Ordinary light: light waves vibrating in all planes perpendicular to its direction of propagation • Plane-polarized light: light waves vibrating only in parallel planes • Polarimeter: an instrument for measuring the ability of a compound to rotate the plane of plane-polarized light • Optically active: showing that a compound rotates the plane of plane-polarized light Schematic diagram of a polarimeter

36. Optical Activity • Dextrorotatory (+): clockwise rotation of the plane of plane-polarized light • Levorotatory (-): counterclockwise rotation of the plane of plane-polarized light • Specific rotation: the observed rotation of an optically active substance at a concentration of 1 g/100 mL in a sample tube 10 cm long; for a pure liquid, concentration is in g/mL (density)

37. Configuration of Enantiomers R,S Convention -> Arrangement of groups around a chiral atom

38. Rules for Priority

39. Problem: assign an R or S configuration to each stereocenter S R S R S

40. Isomers with several chiral centers • For a molecule with 1 stereocenter, 21 = 2 stereoisomers are possible • For a molecule with 2 stereocenters, a maximum of 22 = 4 stereoisomers are possible • For a molecule with nstereocenters, a maximum of 2nstereoisomers are possible

41. Isomers with several chiral centers • 2,3,4-Trihydroxybutanal • two stereocenters; 22 = 4 stereoisomers are possible

42. Enantiomers & Diastereomers

43. Meso Compounds • Meso compound: an achiral compound possessing two or more stereocenters • tartaric acid • two stereocenters; 2n = 4, but only three stereoisomers exist

45. Chirality in the Biological World • Except for inorganic salts and a few low-molecular-weight organic substances, the molecules in living systems, both plant and animal, are chiral • although these molecules can exist as a number of stereoisomers, almost invariably only one stereoisomer is found in nature • instances do occur in which more than one stereoisomer is found, but these rarely exist together in the same biological system • It’s a chiral world!

46. Chirality in Biomolecules • Enzymes (protein bio-catalysts) all have many stereocenters • an example is chymotrypsin, an enzyme in the intestines of animals that catalyzes the digestion of proteins • chymotrypsin has 251 stereocenters • the maximum number of stereoisomers possible is 2251! • only one of these stereoisomers is produced and used by any given organism • because enzymes are chiral substances, most either produce or react with only substances that match their stereochemical requirements

47. Chirality in the Biological World • Schematic diagram of the surface of an enzyme capable of distinguishing between enantiomers

48. Chirality in Biomolecules • because interactions between molecules in living systems take place in a chiral environment, a molecule and its enantiomer or one of its diastereomers elicit different physiological responses • as we have seen, (S)-ibuprofen is active as a pain and fever reliever, whereas its R enantiomer is inactive • the S enantiomer of naproxen is the active pain reliever, whereas its R enantiomer is a liver toxin!

49. Racemic mixture:an equimolar mixture of two enantiomers • because a racemic mixture contains equal numbers of dextrorotatory and levorotatory molecules, its specific activity is zero • Resolution:the separation of a racemic mixture into its enantiomers

50. Enzymes as resolving agents