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Conjugate Acid Base Pairs • Conjugate Base: Formed from an Acid When It Donates a Proton to a Base

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Acids and Bases Dr. Sapna Gupta Arrows in Organic Chemistry synthesis (yield) equilibrium resonance retrosynthesis (backward) transfer of two electrons transfer of one electron Dr. Sapna Gupta/Acids and Bases 2 Acids and Bases • A Brønsted acid donates a hydrogen cation (H+) • A Brønsted base accepts the H+ • “proton” is a synonym for H+ - loss of an electron from H leaving the bare nucleus— a proton • Full headed arrows indicate transfer of electrons. Example Dr. Sapna Gupta/Acids and Bases 3 Conjugate Acid Base Pairs • Conjugate base: formed from an acid when it donates a proton to a base. A strong acid gives a weak conjugate base and vice versa. • Conjugate acid: formed from a base when it accepts a proton from an acid. A strong base gives a weak conjugate acid and vice versa. conjugate acid-base pair conjugate acid-base pair - + HCl( aq) + H2 O( l) Cl ( aq) + H3 O ( aq) Hydrogen Water Chloride Hydronium chloride ion ion (acid) (base) (conjugate (conjugate base of HCl) acid of H 2O) conjugate acid-base pair conjugate acid-base pair - + CH3 COOH + NH3 CH3 COO + NH4 Acetic acid Ammonia Acetate Ammonium ion ion (acid) (base) (conjugate base (conjugate acid Dr. Sapna Gupta/Acidsacetic acid)and Bases of ammonia) 4 Examples of Acids and Bases • There are inorganic (mineral) and organic acids and bases. Dr. Sapna Gupta/Acids and Bases 5 Solved Problems 1) What is conjugate acid of NH3? 2) What are the conjugate bases in the reaction below? a) NH2 + 2 2 b) NH2 CO3 + HSO4 HCO3 + SO4 - c) NH2 d) NH4 a) HCO3 and HSO4 + 2 e) NH4 b) HSO4 and CO3 2 c) CO3 and OH 2 d) SO4 and HSO4 2 2 e) CO3 and SO4 3) For the reaction below which two 4) A strong acid leads to a substances which are both acids a) weak conjugate acid + + CH3NH3 + H2O CH3NH2 + H3O b) strong conjugate base c) weak conjugate base + a) H2O and H3O d) strong base + b) CH3NH3 and H2O e) pure water + c) CH3NH3 and CH3NH2 + + d) CH3NH3 and H3O e) CH3NH2 and H2O Dr. Sapna Gupta/Acids and Bases 6 Acid Strength and pKa • Acid strength is the tendency of an acid to donate a proton. • The more readily a compound donates a proton, the stronger an acid it is. • Acidity is measured by an equilibrium constant. • When a Brønsted-Lowry acid H—A is dissolved in water, an acid-base reaction occurs, and an equilibrium constant can be written for the reaction. • It is more convenient to use “pKa” values than Ka values. Dr. Sapna Gupta/Acids and Bases 7 pKa Table Dr. Sapna Gupta/Acids and Bases 8 Factors Determining Acid Strength 1. Electronegativity: Across the row, e.g. H2O vs CH4 (the more electronegative the element the more acidic it is) 2. Size of the anion: down the group, e.g. HF, HCl, HBr, HI (the larger the anion the more stable it is) 3. Number of oxygen, e.g. HNO2 vs HNO3( more oxygen cause more electronegativity hence easier for H to leave) 4. Inductive effects, e.g. CH3COOH vs CH2ClCOOH 5. Resonance stabilization of conjugate base, e.g. CH3COOH vs CH3OH 6. Acidity of hydrocarbons due to delocalization of e- in conjugate base, e.g.CH=CH, CH2=CH2, CH3CH3 7. Solvent effect: more polar solvents will support more bronstead- lowry type acids because of ions formed during reactions. Dr. Sapna Gupta/Acids and Bases 9 1) Electronegativity in the Row Within a row, the greater the electronegativity of the atom bearing the negative charge stabilizes the anion and the acid is stronger. Dr. Sapna Gupta/Acids and Bases 10 2) Size of Anion • Within a column of the Periodic Table, acidity is related to the size of the the atom bearing the negative charge. • Atomic size increases from top to bottom of a column. • The larger the atom bearing the charge, the greater its stability. – CH S – + CH3 S H + CH3 O 3 CH3 O H Methanethiol Methoxide Methanethiolate Methanol pKa 7.0 ion ion pKa 16 (stronger acid) (stronger base) (weaker base) (weaker acid) Dr. Sapna Gupta/Acids and Bases 11 3) Number of Oxygen Atoms and Electronegativity • In all Bronsted acids the proton that dissociates is bonded to oxygen. • Compare the Lewis structures of phosphorous acid (H3PO3) and phosphoric acid (H3PO4). Phosphoric acid is triprotic and all three protons are bonded to oxygen whereas phosphorous is diprotic as it has only 2 protons bonded to oxygen, the third one, bonded to P, does not ionize. The bond between oxygen and hydrogen is more polar than between hydrogen and phosphorous due to electronegativity difference. • For mineral acids more oxygen atoms means they are the more acidic e.g. HNO3 > HNO2 and H2SO4 > H2SO3 • For organic acids CH3COOH > CH3OH Dr. Sapna Gupta/Acids and Bases 12 4) Inductive Effect • Electronic effects that are transmitted through space and through the bonds of a molecule due to the electronegativity of an adjacent covalent bond. • Stabilization by the inductive effect falls off rapidly with increasing distance of the electronegative atom from the site of negative charge in the conjugate base. • We also see the operation of the inductive effect in the acidity of alchohols and acids and also the halogen substituted alcohols and carboxylic acids. H F H C-CH2 O-H F C-CH2 O-H H F Ethanol 2,2,2-Trifluoroethanol pKa 15.9 pKa 12.4 O O Cl O O Cl OH OH OH OH Cl Butanoic 4-Chlorobutanoic 3-Chlorobutanoic 2-Chlorobutanoic acid acid acid acid pKa 4.82 pKa 4.52 pKa 3.98 pKa 2.83 Dr. Sapna Gupta/Acids and Bases 13 5) Resonance Resonance delocalization of charge in A-. The more stable the anion, the more the position of equilibrium is shifted to the right. • Compare the acidity alcohols and carboxylic acids. • Ionization of the O-H bond of an alcohol gives an anion for which there is no resonance stabilization. + + - + CH3CH2O-H H2 O CH3 CH2O H3 O pKa = 15.9 An alcohol An alkoxide ion • Ionization of a carboxylic acid gives a resonance-stabilized anion. • The pKa of acetic acid is 4.76 O O O + + CH3 C + H2 O CH3 C CH3 C H3 O O H O O equivalent contributing structures; the carboxylate anion is stabilized by delocalization of the negative charge. • Carboxylic acids are stronger acids than alcohols as a result of the resonance stabilization of the carboxylate anion. Dr. Sapna Gupta/Acids and Bases 14 6) Hybridization • For anions differing only in the hybridization of the charged atom, the greater the % s character to the hybrid orbital of the charged atom, the more stable the anion, therefore more acidic. • Consider the acidity of alkanes, alkenes, and alkynes (given for comparison are the acidities of water and ammonia). More acidic More basic Weak Conjugate Acid Base pKa Water HO- H HO– 15.7 Alkyne HC C H HC C– 25 – Ammonia H2 N-H H2 N 38 – Alkene CH2 = CH- H CH2 = CH 44 – Alkane CH3 CH2 -H CH3 CH2 51 acidity Increasing Dr. Sapna Gupta/Acids and Bases 15 7) Effect of Solvent • Acidity values in gas phase are generally very low • It is difficult to separate the product ions without solvent molecules to stabilize them • Acetic acid has pKa of 130 in the gas phase • A protic solvent is one in which hydrogen is attached to a highly electronegative atom such as oxygen or nitrogen e.g. water • Solvation of both acetic acid and acetate ion occurs in water although the acetate is more stabilized by this solvation Dr. Sapna Gupta/Acids and Bases 16 Strength of Bases • Bases are opposite of acids in strength; a strong acid will give a weak conjugate base and a weak acid gives a strong conjugate base. - • E.g. HCl is a strong acid so Cl is a weak conjugate base; methanol (CH3OH) is - a weak acid so methoxide (CH3O ) is a strong conjugate base. • It is best to remember acidity rules to remember bases also. Dr. Sapna Gupta/Acids and Bases 17 Lewis Acids and Bases • Lewis acid: A molecule/ion that can accept a pair of electrons. • Lewis base: A molecule/ion that can donate a pair of electrons. • There is no pKa scale for these acids and bases. Their effectiveness is determined by how well they donate or accept electrons. • Examples of Lewis Acids • 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 • In case of organic compounds, any carbocation (carbon with a positive charge) would be a Lewis acid. Dr. Sapna Gupta/Acids and Bases 18 Organic Compounds as Lewis Bases • Any organic compound containing an atom with a lone pair (O,N) can act as a base Dr. Sapna Gupta/Acids and Bases 19 Solved Problems Which of the following is a Lewis base? Iodine trichloride, ICl3, will react with a - a) BCl3 chloride ion to form ICl4 . Which b) Cu2+ species, is the Lewis base this reaction? - - c) SH a) ICl4 2+ d) Mn b) ICl3 + - e) NH4 c) Cl d) the solvent Which one of the following is a Lewis Which of these species will act as a acid but not a Brønsted-Lowry acid? Lewis acid? 3+ a) Fe a) NH3 + + b) H3O b) NH4 - c) HSO4 c) H2O d) NH3 d) BF3 e) F Dr. Sapna Gupta/Acids and Bases 20 Lewis Acid Base Reactions - + A + :B A B new covalent bond Examples Lewis Lewis formed in this Lewis • For writing Lewis acid acid base acid-base reaction base reactions look for : electron poor and H H :Br : + : electron rich sites.
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