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Chapter 3

Chapter 3. An Introduction to Organic Reactions and Their Mechanism Acids and Bases. 3.4 Acid Strength. Strong acids: completely ionized or completely dissociated Forward reaction is predominated Most of HA is dissociated Conjugated base is weak and has low attraction for proton

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Chapter 3

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  1. Chapter 3 An Introduction to Organic Reactions and Their Mechanism Acids and Bases

  2. 3.4 Acid Strength • Strong acids: completely ionized or completely dissociated • Forward reaction is predominated • Most of HA is dissociated • Conjugated base is weak and has low attraction for proton HA(aq) + H2O(l)  H3O+(aq) + A-(aq)

  3. Acids Strength • Weak acids: partially ionized or dissolved • Reverse reactions is predominated • Most of HA is undissociate • Conjugated base is strong and has high attraction for proton HC2H3O4(aq) + H2O(l) H3O+(aq) +C2H3O4-(aq)

  4. Acid Strength

  5. 3.5 The Strength of Acids and Bases: Ka and pKa • The acidity constant, Ka • a quantitative measure of the strength of an acid in solution. • It is known as the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions • Larger Ka, stronger the acid

  6. Acidity and pKa • Logarithm scale of Ka with the base of 10 • pKa = - log Ka • The larger pKa, the smaller extent of dissociation • Weaker acids

  7. Relative Strength of selected acids and their conjugated bases

  8. Examples • An acid (HA) has Ka = 10-7, what is its pKa? • Another acid (HB) has Ka = 5, what is its pKa? • Which is the stronger acid?

  9. Predicting the Strength of Bases • The strength of acids will decide the how weak or strong its conjugated bases • Stronger the acids, the weaker will be its conjugated base • After losing a proton, acid will become a conjugated base • When a base accepts a proton, the resulting chemical is called the conjugate acid of that original base

  10. Acid and its conjugated base

  11. Examples • The pKa of anilinium ion (C6H5N+H3) is equal to 4.6. On the basis of this fact, decide whether aniline (C6H5NH2) is a stronger or weak base than methylamine

  12. 3.6 Predicting the Outcome of Acid-Base Reactions • Acid-base reactions always favor the formation of the weaker acid and weaker base • Equilibrium favor the formation of the most stable (lowest potential energy) species • Stronger acid + stronger base  weaker base + weaker acid

  13. Example • Consider the mixing of an aqueous solution of acetic acid, CH3CO2H (see Table 3.1), and NaOH. What acid base reaction, if any would take place • H2SO4(aq) + NH3(aq)  NH4+(aq) + HSO4-(aq)

  14. 3.6 Water Solubility as the Result of Salt Formation • Recall: Longer chain of carbon (4 or more) will be less soluble • Higher molecular weight of carboxylic compounds do not dissolve in water • Instead, they dissolved in a strong base to form a water-soluble salt

  15. Water Solubility • We can predict that an amine will react with aqueous hydrochloric acid

  16. Water Solubility • Methylamine and most amines of low molecular weight are very soluble in water • Formed a water-soluble salt from hydrochloric acid

  17. 3.7 Relationships between structure and Acidity • The strength of a Bronsted-Lowry acid depends on the extend to which a proton can be separated from it and transferred to a base. • Removing H+  breaking a bond • Making the conjugated base more electronegative • Accepting H+  forming a bond • Making the conjugated acid less electronegative

  18. 3.7 Relationships between structure and Acidity • Bond strength to proton decreases as we move down to column • Decreasing effectiveness of orbital overlap between the hydrogen 1s orbital and the orbitals of successively larger element in the column • Less effective the orbital overlap, the weaker bond, stronger acid • Acidity increases from left to right

  19. 3.7 Relationships between structure and Acidity

  20. 3.7 Relationships between structure and Acidity

  21. 3.7 The Effect of Hybridization

  22. 3.7 The Effect of Hybridization • Electrons of 2s orbtials have lower energy than those of 2p orbitals because these electrons are much closer to the nucleus • Having more s orbitals means that electrons of the anion will be lower in energy and more stable

  23. 3.7 The Effect of Hybridization • sp C – sH hybridization • Contain 50% s character • 1s + 1p • More electronegative spC

  24. 3.7 The Effect of Hybridization • sp2C – sH hybridization • Contains 33.3% s character • 1s + 2p • Less electronegative than spC

  25. 3.7 The Effect of Hybridization • sp3C-sH hybridization • Contains 25% s character • 1s + 3p • Least electronegative C

  26. 3.7 The Effect of Hybridization

  27. 3.7 The Effect of Hybridization • Relative basicity of the carbanions • Ethynide ion is the weakest base • The more electronegative carbon, the more stable the anion

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