Chapter 3: Acids and Bases in Organic Chemistry PDF

Chapter 3 Acids and Bases An Introduction to Organic Reactions and Their Mechanisms Chapter 3 In this chapter we will consider: ❖ Rules that show how to classify reactive groups within molecules from the standpoints of acids and bases as well as from electron...

Chapter 3 Acids and Bases An Introduction to Organic Reactions and Their Mechanisms Chapter 3 In this chapter we will consider: ❖ Rules that show how to classify reactive groups within molecules from the standpoints of acids and bases as well as from electron-rich and electron- poor domains ❖ The step-by-step processes of a chemical reaction and how to codify these processes into a few specific, easy-to-understand types Chapter 3 1. Acid–Base Reactions ❖ Many of the reactions that occur in organic chemistry are either acid–base reactions themselves or they involve an acid–base reaction at some stage ❖ Acid–base reactions are simple fundamental reactions that will enable you to see how chemists use curved arrows to represent mechanisms of reactions and how they depict the processes of bond breaking and bond making that occur as molecules react Chapter 3 1A. Brønsted–Lowry Acids and Bases ❖ Brønsted–Lowry acid–base reactions involve the transfer of protons ❖ A Brønsted–Lowry acid is a substance that can donate (or lose) a proton ❖ A Brønsted–Lowry base is a substance that can accept (or remove) a proton Chapter 3 ❖ Example Base Conjugate Acid (H+ acceptor) of H2O H O + H Cl H O H + Cl H H Acid Conjugate Base (H+ donor) of HCl Chapter 3 1B. Acids and Bases in Water ❖ Hydronium ion (H3O+) is the strongest acid that can exist in water to any significant extent: Any stronger acid will simply transfer its proton to a water molecule to form hydronium ions ❖ Hydroxide ion (HO-) is the strongest base that can exist in water to any significant extent: Any base stronger than hydroxide will remove a proton from water to form hydroxide ions Chapter 3 ❖ Total ionic reaction H O H + Cl + Na O H 2 H O + Na + Cl H H Spectator ions ❖ Net reaction H O H + O H 2 H O H H Chapter 3 2. How to Use Curved Arrows in Illustrating Reactions ❖ Curved arrows show the direction of electron flow in a reaction mechanism Draw the curved arrow so that it points from the source of an electron pair to the atom receiving the pair. (Curved arrows can also show the movement of single electrons) Always show the flow of electrons from a site of higher electron density to a site of lower electron density Chapter 3  Never use curved arrows to show the movement of atoms. Atoms are assumed to follow the flow of the electrons Make sure that the movement of electrons shown by the curved arrow does not violate the octet rule for elements in the second row of the periodic table Chapter 3 ❖ Examples Chapter 3 ❖ Examples NOT −O N H H C H + Chapter 3 ❖ Examples Chapter 3 3. Lewis Acids and Bases ❖ Lewis Acids are electron pair acceptors ❖ Lewis Bases are electron pair donors Lewis Base (e⊖ pair donor) − + Cl H + NH3 Cl + H NH3 Lewis Acid (e⊖ pair acceptor) Chapter 3  Lewis Base (e⊖ pair donor) −Cl Cl −  + Cl Al  + NH3 Cl Al NH3 − Cl Cl  Lewis Acid (empty p orbital of Aluminum allows AlCl3 acts as e⊖ pair acceptor) ❖ In Lewis acid–base theory, the attraction of oppositely charged species is fundamental to reactivity Chapter 3 4. Heterolysis of Bonds to Carbon: Carbocations and Carbanions ❖ Heterolysis heterolytic A B bond A + B cleavage ions A B A + B Chapter 3  Normally requires the bond to be polarized + − A B Usually occurs with assistance + − Y A B Y A + B Chapter 3 Chapter 3 ❖ Carbocations are electron deficient. They have only six electrons in their valence shell, and because of this, carbocations are Lewis acids C + B C B carbocation anion (a Lewis acid) (a Lewis base) C + O H C O H H H carbocation water (a Lewis acid) (a Lewis base) Chapter 3 4A. Electrophiles and Nucleophiles ❖ Because carbocations are electron- seeking reagents, chemists call them electrophiles (meaning electron- loving) ❖ Electrophiles are reagents that seek electrons so as to achieve a stable shell of electrons like that of a noble gas Chapter 3 ❖ All Lewis acids are electrophiles. By accepting an electron pair from a Lewis base, a carbocation fills its valence shell C + B C B carbocation anion (a Lewis acid (a Lewis base) and electrophile) Chapter 3 ❖ Carbon atoms that are electron poor because of bond polarity, but are not carbocations, can also be electrophiles + − B + C O B C O Lewis base Lewis acid electrophile Chapter 3 ❖ Carbanions are Lewis bases ❖ A nucleophile is a Lewis base that seeks a positive center such as a positively charged carbon atom Chapter 3 5. The Strength of Brønsted-Lowry Acids and Bases: Ka and pKa ❖ In contrast to strong acids such as HCl and H2SO4, acetic acid is a much weaker acid O O H3C OH + H2O H3C O + H O H H At 25oC, in a 0.1 M acetic acid solution, only about 1% of the acetic acid molecules ionize Chapter 3 5A. The Acidity Constant, Ka O O H3C OH + H2O H3C O + H O H H ❖ Equilibrium constant (Keq) ⊖ ⊕ [CH3CO2 ] [H3O ] Keq = [CH3CO2H][H2O] Chapter 3 ❖ For dilute aqueous solutions, the concentration of water is essentially constant (~55.5M); and the Keq expression can be written in terms of the acidity constant (Ka) ⊖ ⊕ [CH3CO2 ] [H3O ] Ka = Keq [H2O] = [CH3CO2H] ❖ At 25°C, the acidity constant for acetic acid is 1.76 x 10−5 Chapter 3 ❖ For any weak acid dissolved in water HA + H2O H3O + A ⊕ ⊖ [H3O ] [A ] Ka = [HA] ❖ An acid with a large value of Ka a strong acid ❖ An acid with a small value of Ka a weak acid Chapter 3 5B. Acidity and pKa pKa = − log Ka ⊕ pH = − log [H3O ] ❖ For acetic acid the pKa is 4.75 pKa = − log [1.76 x 10−5] = − [− 4.75] = 4.75 Chapter 3 ❖ The larger the value of the pKa, the weaker the acid Increasing acid strength CH3CO2H CF3CO2H HCl pKa = 4.75 > pKa = 0 > pKa = −7 Weak Very acid strong acid Chapter 3 ❖ Relative Strength of Selected Acids & Their Conjugate Bases Increasing acid strength O H H H O Acid HCl Ph S OH H3C O H O HNO3 O H H p Ka -7 -6.5 -2.9 -2.5 -1.74 -1.4 O O Conjugate Cl Ph S O CH3OH H2O NO3 Base O Increasing base strength Chapter 3 ❖ (Cont'd) Increasing acid strength O O H O Acid HF Ph N H F3C OH Ph OH H3C OH H p Ka 0.18 3.2 4.21 4.63 4.75 O O O Conjugate F Ph NH2 Base F3C O Ph O H3C O Increasing base strength Chapter 3 ❖ (Cont'd) Increasing acid strength O O H H OH Acid H N H H3C N H H O H H H H p Ka 9.0 9.2 9.9 10.6 15.7 O O O Conjugate NH3 CH3NH2 HO Base Increasing base strength Chapter 3 ❖ (Cont'd) Increasing acid strength OH O Acid OH HC H H H H p Ka 16 18 19.2 25 35 O O Conjugate O HC C H Base Increasing base strength Chapter 3 ❖ (Cont'd) Increasing acid strength H H Acid H2N H H2C H3C C H H H p Ka 38 44 50 Conjugate NH2 H2C CH H3C CH2 Base Increasing base strength Chapter 3 5C. Predicting the Strength of Bases ❖ The stronger the acid, the weaker its conjugate base ❖ The larger the pKa of the conjugate acid, the stronger the base Increasing base strength ⊖ ⊖ ⊖ Cl CH3CO2 HO Very weak base Weak base Strong base pKa (HCl) pKa (CH3CO2H) pKa (H2O) = −7 = 4.75 = 15.7 Chapter 3 ❖ Example Base CH3OH H2O Conjugate H3C O H H O H Acid H H p Ka -2.5 -1.74 ⊕ ❖ Since CH3OH2 is a stronger acid than ⊕ H3O , H2O is a stronger base than CH3OH Chapter 3 6. How to Predict the Outcome of Acid-Base Reactions ❖ Acid–base reactions always favor the formation of the weaker acid and the weaker base ❖ Acid–base reactions are under equilibrium control ❖ Reactions under equilibrium control always favor the formation of the most stable (lowest potential energy) species Chapter 3  stronger weaker base base O O O R O H + Na OH R O Na + H H stronger weaker acid acid pKa ~3-5 pKa = 15.7 Chapter 3 6A. Water Solubility as the Result of Salt Formation ❖ Most carboxylic acids containing more than 5 carbons are insoluble in water ❖ However, due to their acidity, they are soluble in aq. NaOH O O O R O H + Na OH R O Na + H H (R>5 carbons) Soluble in water Insoluble (due to its polarity in water as a salt) Chapter 3 ❖ Similarly, amines with high molecular weights are insoluble in water ❖ However, due to their basicity, they are soluble in aqueous acids H R NH2 + H O H Cl R N H Cl + H2O H H Water Water Insoluble Soluble (salt) Chapter 3 Chapter 3 7. Relationships between Structure and Acidity H–F H–Cl H–Br H–I Bond Length (Å) 0.92 1.28 1.41 1.60 pKa 3.2 -7 -9 -10 Increasing acidity ❖ The strength of H–X bond H–F > H–Cl > H–Br > H–I The stronger the H–X bond, the weaker the acid. Chapter 3 ❖ Thus acidity increases as we descend a vertical column in a group in the Periodic Table − HF F − Increasing HCl Cl Increasing acidity HBr Br − basicity − HI I The stronger the acid, the weaker the conjugate base. Chapter 3  − + − + − + − + H3C—H H3N—H HO—H F—H Electro- 2.5 2.1 3.0 2.1 3.5 2.1 4.0 2.1 negativity pKa 48 38 15.7 3.2 The higher the electronegativity of an atom, the easier it will acquire a negative charge. Chapter 3 ❖ Thus acidity increases from left to right when we compare compounds in the same row of the Periodic Table Increasing acidity H3C–H H2N–H HO–H F–H CH NH OH F 3 2 Increasing basicity Chapter 3 Acidity increases within a given row (electronegativity effect) C N O F Acidity increases within a Hydride (H3C–H) (H2N–H) (HO–H) (F–H) (bond strength effect) pKa 48 38 15.7 3.2 S Cl given column (HS–H) (Cl–H) 7.0 -7 Se Br (HSe–H) (Br–H) 3.9 -9 I (I–H) -10 Chapter 3 7A. The Effect of Hybridization (50% (33.3% (25% s character) s character) s character) sp sp2 sp3 H H H H H C C H C C H C C H H H H H pKa = 25 pKa = 44 pKa = 50 ❖ Having more s character means that the electrons of the anion will, on the average, be lower in energy, and the anion will be more stable Chapter 3 ❖ Relative Acidity of the Hydrocarbons H H H H H C C H > C C > H C C H H H H H ❖ Relative Basicity of the Carbanions H H H C C H > C C > H C C H H H H Chapter 3 7B. Inductive Effects ❖ Inductive effects are electronic effects transmitted through bonds ❖ The inductive effect of a group can be electron donating or electron withdrawing ❖ Inductive effects weaken as the distance from the group increases Chapter 3  The C–C H3C CH3 bond is nonpolar. + + − H3C CH2 F 2 1 ❖ The positive charge that the fluorine imparts to C1 is greater than that imparted to C2 because the fluorine is closer to C1 Chapter 3 10. Acidity: Carboxylic Acids versus Alcohols O H3C OH CH3CH2 OH Acetic acid Ethanol pKa = 4.75 pKa = 16 DG° = 27 kJ/mol DG° = 90.8 kJ/mol Chapter 3  O O + H2O + H3O CH3 O H CH3 O acetic acid acetate CH3CH2 O H + H2O CH3CH2 O + H3O ethanol ethoxide Chapter 3 ❖ When comparing acidity of organic compounds, we compare the stability of their conjugate base. The more stable the conjugate base, the stronger the acid CH3COOH CH3CH2OH pKa 4.75 16 Chapter 3 10A. The Effect of Delocalization ❖ The conjugate base acetate is more stable (the anion is more delocalized) than ethoxide due to resonance stabilization O O O CH3 O CH3 O CH3 O Thus, acetic acid is a stronger acid than ethanol Chapter 3 10B. The Inductive Effect O < CH3 O