1 / 51

Comprehensive Guide to Acids and Bases in Organic Chemistry

Learn about the reaction patterns, Brønsted-Lowry definitions, acid-base equilibria, molecular structure influences on acidity, and the concepts of Lewis acids and bases in organic chemistry. Gain insights on conjugate acids and bases, resonance effects, and the role of pi electrons in forming carbocations. Improve your understanding of acid-base strengths, hybridization, and equilibrium reactions in the context of organic compounds.

gdoris
Download Presentation

Comprehensive Guide to Acids and Bases in Organic Chemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 1. Continue Acids andBases

  2. Organic Acids and Organic Bases • The reaction patterns of organic compounds often are acid-base combinations • The transfer of a proton from a strong Brønsted acid to a Brønsted base, for example, is a very fast process and will always occur along with other reactions

  3. H3O+(aq) H+(aq) + H2O(I) H2O KOH K+ + -OH(aq) Arrhenius Acids and Bases • acid: substance that produces H3O+ ions aqueous solution hydronium ion • base: substance that produces OH- ions in aqueous solution

  4. Brønsted-Lowry Definitions • Acid: a proton donor • Base: a proton acceptor

  5. Conjugate Acids & Bases • Brønsted-Lowry does not require water, only H+ transfer • acid-base reaction:aproton (H+)transfer reaction

  6. Conjugate Acids & Bases • curved arrows show the flow of electrons in an acid-base reaction recall resonance - also uses curved arrows

  7. Conjugate Acids & Bases • molecules may have 2 or more sites that can accept a H+ • e.g. carboxylic acids, esters, and amides • protonation favored where the charge is more delocalized which oxygen is protonated?

  8. Conjugate Acids & Bases • resonance octets greater contribution even with plus charge on O

  9. H+ on the carbonyl Conjugate Acids & Bases • H+ on the hydroxyl can “write” contributing structures create & separate charge

  10. Pi Electrons As Basic Sites • pi electrons - base - donate a pair of e-s Result: formation of a carbocation, C has 6-e-s, +1charge

  11. Acids & Base Strengths • acid strength - expressed by equilibrium constant dissociation (ionization) of acetic acid pKa = -log Ka Ka ranges from 1015 for the strongest acids to very small values (10-60) for the weakest

  12. 3.18 HF

  13. pKa 4.76 pKa 9.24 Acid-Base Equilibria • Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base

  14. CO2 + H2O Acid-Base Equilibria • acetic acid + sodium bicarbonate (omit Na+ ) pKa= 4.76 6.36

  15. Molecular Structure and Acidity • relative acidities: the more A:(-) stable, greater the acidity of H-A Ways to stabilize A:(-) the negative charge ON a more electronegative atom ON larger atom RESONANCE delocalized STABILIZED by inductive effect IN an orbital with more s character

  16. C o n j u g a t e b a s e A c i d Molecular Structure and Acidity within a period - the greater the electronegativity of A:(-) the more A:(-) isstablized  the stronger the acid pKa16 pKa38 pKa51

  17. Molecular Structure and Acidity • Size of A:(-)Within a column - • the larger the atom bearing the (-), the greater its stability

  18. Molecular Structure and Acidity • Resonance delocalized of charge in A- • Compare alcohol and carboxylic acid acidity resonance stabilization

  19. Molecular Structure and Acidity • inductive effect, electron-withdrawing covalent bonds transmit electronegativity (polarizing) effects push or pull shared e(-)s of adjacent atoms decreases w/ distance

  20. Molecular Structure and Acidity • inductive effect: butanoic and chlorobutanoic acids

  21. Molecular Structure and Acidity • Hybridization • greater the % s character with (-) the more stable the anion

  22. Organic Acids

  23. Conjugate Bases:

  24. Carboxylic Acids:

  25. Organic Bases • Have an atom with a lone pair of electrons that can bond to H+ • Nitrogen-containing compounds derived from ammonia are the most common organic bases • Oxygen-containing compounds can react as bases when with a strong acid or as acids with strong bases

  26. Organic Bases

  27. Lewis Acids and Bases • Lewis acid:molecule/ion that accepts a pair of electrons • Lewis base:molecule/ion that donates a pair of electrons • The Lewis definition leads to a general description of many reaction patterns but there is no quantitatve scale of strengths as in the Brønsted definition of pKaThe combination of a Lewis acid and a Lewis base can shown with a curved arrow from base to acid • The combination of a Lewis acid and a Lewis base can shown with a curved arrow from base to acid forms a new covalent bond

  28. Lewis Acids and the Curved Arrow Formalism • The Lewis definition of acidity includes metal cations, such as Mg2+ • They accept a pair of electrons when they form a bond to a base • Group 3A elements, such as BF3 and AlCl3, are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases • Transition-metal compounds, such as TiCl4, FeCl3, ZnCl2, and SnCl4, are Lewis acids

  29. Lewis Acids and Bases examples carbocations very strong Lewis acid

  30. Lewis Acids and the Curved Arrow Formalism • Organic compounds that undergo addition reactions with Lewis bases (discussed later) are called electrophiles and therefore Lewis Acids

  31. Illustration of Curved Arrows in Following Lewis Acid-Base Reactions

  32. Some Lewis Acids:

  33. Lewis Bases • Most oxygen- and nitrogen-containing organic compounds are Lewis bases because they have lone pairs of electrons • Some compounds can act as either acids or bases, depending on the reaction

  34. Some Lewis Bases

  35. Lewis Acid-Base Reactions

  36. Imidazole:

  37. Drawing Chemical Structures • Condensed structures: C-H and C-C and single bonds aren't shown but understood • If C has 3 H’s bonded to it, write CH3 • If C has 2 H’s bonded to it, write CH2; and so on. • Horizontal bonds between carbons aren't shown in condensed structures - the CH3, CH2, and CH units are assumed to be connected horizontally by single bonds, but vertical bonds are added for clarity

  38. 2-methylbutane Structures

  39. Skeletal Structures • Minimum amount of visual information but unambiguous • C’s not shown, but assumed to be at each intersection of two lines (bonds) and at end of each line • H’s bonded to C’s aren't shown – whatever number is needed will be there to fill out the four bonds to each carbon. • All atoms other than C and H are shown

  40. Practice: How many H’s on each carbon?

  41. Solution:

  42. Problem: How many H’s on each carbon?

  43. Molecular Models • We often need to visualize the shape or connections of a molecule in three dimensions • Molecular models are three dimensional objects, on a human scale, that represent the aspects of interest of the molecule’s structure (computer models also are possible) • Drawings on paper and screens are limited in what they can present to you

  44. Molecular Models • Framework models (ball-and-stick) are essential for seeing the relationships within and between molecules – you should own a set • Space-filling models are better for examining the crowding within a molecule Framework Space-filling

  45. Summary • Organic molecules often have polar covalent bonds as a result of unsymmetrical electron sharing caused by differences in the electronegativity of atoms • The polarity of a molecule is measured by its dipole moment, . • (+) and () indicate formal charges on atoms in molecules to keep track of valence electrons around an atom • Some substances must be shown as a resonance hybrid of two or more resonance forms that differ by the location of electrons.

  46. Summary • A Brønsted(–Lowry) acid donates a proton • A Brønsted(–Lowry) base accepts a proton • The strength Brønsted acid is related to the -1 times the logarithm of the acidity constant, pKa. Weaker acids have higher pKa’s • A Lewis acid has an empty orbital that can accept an electron pair • A Lewis base can donate an unshared electron pair

  47. Summary • In condensed structures C-C and C-H are implied • Skeletal structures show bonds and not C or H (C is shown as a junction of two lines) – other atoms are shown • Molecular models are useful for representing structures for study

  48. Chapter 1-2, Questions

More Related