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Formal Charge Effect > Electronegativity Effect Chemistry 231L Group Activity #3 – 15 points (Please turn in pages 2 – 5, one paper per group of 2) Introduction: Acid/Base summary I) How do you compare acidity between two species? 1) If you know the pKa value, then lower pKa = stronger acid 2) If you don’t know the pKa value, then you can look at the structures of the molecules to help you decide II) How do you compare basicity between two species? 1) If you have pKa values available, look at the pKa values of the conjugate acids. Lower pKa = stronger acid; its conjugate base will be the weaker base 2) If pKa values are not available, then look at the structure and try to ask the following questions: which of the species is better off as a base; which species can accommodate the negative charge better? III) How do you compare acidity between two species when pKa values aren’t available? 1) Electronegativity and Size a) As you go down the periodic table, size is more important than electronegativity; e.g. iodide anion is larger than fluoride anion. Iodide anion can accommodate its negative charge better (wants to keep the electrons for itself) than fluoride, therefore iodide anion is a weaker base than fluoride anion. Weak bases gives strong conjugate acids, thus HF is a weaker acid than HI. – – b) As you go across the periodic table, electronegativity is more important then size; e.g. F vs. CH3 ; fluorine is more electronegative than carbon, therefore it can accommodate the negative charge – – better than carbon. Thus F is a weaker base than CH3 ; Weak bases gives strong conjugated acids, thus, HF is a stronger acid than CH4. 2) Hybridization a) Negative charge located in an sp orbital will be more accommodated than negative charge located in sp3 orbital; because higher s character will give more positive charge stability (it is closer to the nucleus). Therefore, negative charge located in an sp orbital will produce a weaker base than negative charge located in an sp3 orbital; e.g. a terminal alkyne hydrogen is more acidic than hydrogen bonded to an sp3 carbon. 3) Inductive Effect = long range electronegativity effect (the pull of nearby atoms on the electrons) 4) Resonance Effect The more resonance structures a conjugate base can offer, the more stable the base is; therefore, a weaker base and its corresponding conjugate acid will be stronger (BUT BE CAREFUL!). IV) How do you determine which way the equilibrium will go in an acid-base reaction? 1) Identify the acid on the reactant side and on the product side 2) Look up pKa and compare their acidity (or use the techniques above if pKa values aren’t available) 3) The equilibrium will go away from the stronger acid Formal Charge Effect > Electronegativity Effect > Resonance Effect > Inductive Effect Chemistry 231L Group Activity # 3, 15 points Names: ______________________________ Acid and Base Chemistry and Organic Molecules _____________________________ I) Determining Acid strength using pKa values a) Which acid is stronger? HCl or CH3COOH? Why? b) Which acid is stronger? Water or ethanol? Why? II) Determining Base strength using pKa values (See Table 2.3, p. 46, in McMurray) Remimber: strong acids produce weak conjugate bases. a) Which is the stronger base? Fluoride ion or hydroxide ion? Why? – – – b) Rank the following in order of increasing basicity: Cl CH3COO OH c) Write down the method that you use to help you determine base strength; compare and contrast this with your method for finding acid strength. III) Determining the major species present in acid/base equilibrium and deciding if a reaction will proceed as it is written. Identify the acid and base on the left side of the equation and the conjugate acid and base on the right side of the equation for the following equilibriums: Acid = The H+ donor Base = the H+ acceptor Conjugate base = the anion resulted from deprotonation of an acid Conjugate acid = the cation resulted from protonation of an base – – a) CH3CH3 + NH2 CH3CH2 + NH3 – – b) CH3CCH + NH2 CH3CC + NH3 Reaction Acid Base Conjugate Acid Conjugate Base a b a) Which way is the equilibrium favored in reaction a? forward backward b) Which way is the equilibrium favoring in reaction b? forward backward IV) Acid Strength: Electronegativity Effect (See Figure 2.2, p. 30 in McMurray text) a) Rank each group of acids from least acidic to most acidic. Explain your reasoning based on electronegativity effects. i) HF, HCl, HBr, and HI ii) H2O, H2S, H2Se iii) CH4, NH3, H2O, HF b) Does your ranking match with the pKa values? If not, explain. V) Acid Strength: Hybridization a) Rank these acids from least acidic to most acidic. Explain your reasoning based on hybridization effects. CH3CH3, CH2CH2, CHCH b) Does your ranking match with the pKa values? If not, explain. c) Write the conjugate base of the three molecules in (a) and rank their basicity. d) Does this ranking match what you would expect based on hybridization effects? (Hint: consider % s character on the carbon bearing the negative charge.) VI) Acid Strength: Inductive Effect (long range electronegativity effect) Inductive effect: the effect of an electronegative atom is felt by neighboring atoms, AND can be felt more than one atom away. It is like the ripple effect that is caused when you throw a brick into a still pond. The waves are intense near the impact site, but get smaller farther away. H2 C F H3C C H2 a) Draw the three possible conjugate bases of 1-fluropropane (CH3CH2CH2F, shown above). Hint: They result + from removal of the H from C1, C2, and C3, respectively b) Which of the three conjugate base is most stabilized by the inductive effect? In other words, which of the three bases is more stable (weakest)? c) Circle the most acidic group of H’s on 1-fluropropane? d) Which of the three conjugate bases is the strongest base? (Strong bases do not want to give up H+) VII) Acid Strength: Resonance Effect a) Which of the following is the stronger acid, CH3CH2OH or CH3COOH. Why? (Hint: draw all possible resonance structures for each conjugate base.) b) Does your answer above reflect the pKa values for these compounds? If not, why not? VIII) Hierarchy of Effects on Acid Strength Strongest = Formal Charge effect = neutral happier than negative Very Strong = Electronegativity = lone pair on F happier than lone pair on O > N > C Strong = Resonance effect = the more “delocalized” an electron, the happier it is Weak = Inductive effect = long range electronegativity effect a) For each compound below, circle the most acidic H (or H’s if there is a tie) F H O N H H O O Cl O H I II III b) In compound I, explain why the electronegativity effect is stronger than the inductive effect. c) In compound II explain why the formal charge effect is stronger than the electronegativity effect. d) In compound III, to which position could you move the Cl so that it would have an equivalent inductive effect on both of the OH groups? (Draw this new structure which will be an isomer of compound III, and will have two equivalent most acidic H’s) .
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