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3.1 Acids and Bases Brnsted-Lowry definitions: Acids donate a proton. Bases accept a proton.

Recall:

Conjugate base

Conjugate acid

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.1 Acids and Bases Use curved arrows to draw reasonable products for the following acid/base reaction, and label the conjugates.

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.2 Curved Arrows in Reactions In most reactions, PAIRS of electrons move, and we can show their movement with curved arrows. Consider the following generic acid/base reaction.

How are the curved arrows here different from the ones we use to represent resonance in Chapter 2? The curved arrows show the reaction MECHANISM. Learn to draw mechanisms!Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.2 Curved Arrows in Reactions Consider a specific acid/base example.

You could say the base attacks the acid. The acid cannot lose its proton without a base taking it. All acid/base reactions occur in one step. The mechanism shows two arrows indicating that two pairs of electrons move simultaneously.Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.2 Curved Arrows in Reactions Identify all acids in the following mechanism.

Practice with SKILLBUILDER 3.1.Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.3 Quantifying Acidity and Basicity Acidity Knowing the strength of an acid or base allows us to predict whether, and how, reactions will progress. We will review QUANTITATIVE acid strength analysis, using pKa We will review QUALITATIVE strength analysis, by comparing the chemical structures.

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.3 Quantifying Acidity and Basicity Acidity Quantitative strength analysisusing Ka and pKa values.Ka is the equilibrium constant for the reaction between an acid and WATER:

How does acid strength affect the value of Ka? Ka values range from 10-50 to 1010. The log of Ka is pKa, which ranges from -10 to 50.Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.3 Quantifying AcidityACID pKa 9 7 2.9 ACID pKa 15.7 16 18

See also Table 3.1. Each pKa unit represents an order of magnitude (power of 10). Which is stronger, HCl or H2SO4, and by exactly HOW MUCH? H2SO4 is 102 or 100 times stronger than HCl.

1.74

19.2

Practice with SKILLBUILDER 3.2.

4.75Copyright 2012 John Wiley & Sons, Inc.

50 3 -11Klein, Organic Chemistry 1e

3.3 Quantifying Basicity We also use pKa values to compare the strengths of bases: The stronger the acid the weaker its conjugate base. WHY?

Practice with SKILLBUILDER 3.3.

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.3 Using pKas to Analyze Equilibria Knowing the pKa values allows you to predict which direction an acid/base equilibrium will favor:

Why is the equilibrium arrow bigger on top than on bottom? Subtracting the pKa values (15.7 - 4.75 11) shows there will be 1011 more products than reactants.Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity Qualitative analysis compares structures to determine which is a stronger acid. The more effectively a conjugate base can stabilize its negative charge, the stronger the acid. Which is a stronger acid? WHY?or

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity What factors affect the stability of a negative formal charge? The type of atom that carries the charge Resonance Induction The type of orbital where the charge resides

These factors can be remembered with the acronym, ARIO.

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity The Type of Atom ARIOThe type of atom that carries the charge: More electronegative atoms stabilize negative charge better. Compare the acidity of the two compounds below:

Look up the pKa values for similar compounds in Table 3.1 to verify your prediction.

Practice with SKILLBUILDER 3.5.Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity Resonance ARIOResonance can stabilize a formal negative charge by spreading it out into two or more partial charges. Compare the acidity of the two compounds below by comparing the stabilities of their conjugate bases. How does resonance play a role?

Look up the pKa values in Table 3.1 to verify your prediction. Practice with SKILLBUILDER 3.6.Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity Induction ARIOInduction can also stabilize a formal negative charge by spreading it out. How is induction different from resonance? Compare the acidity of the two compounds below by comparing the stabilities of their conjugate bases. How does induction play a role?

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity Induction Does induction also play a role in explaining why acetic acid is stronger than ethanol?

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity Induction Explain the pKa differences below.

Practice with SKILLBUILDER 3.7.Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity Orbital ARIO The type of orbital also can affect the stability of a formal negative charge. Is a negative charge more stable or less stable if it is held closely to an atoms nucleus? WHY? Rank the ability of sp3, sp2, and sp orbitals to stabilize electrons, and explain.

sp3Copyright 2012 John Wiley & Sons, Inc.

sp23 -22

spKlein, Organic Chemistry 1e

3.4 Qualifying Acidity Orbital Compare the acidity of the compounds below by comparing the stabilities of their conjugate bases. How does the type of orbital play a role?

Practice with SKILLBUILDER 3.8.Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity ARIO When assessing the acidity of protons, we GENERALLY use ARIO as our order of priority:1. 2. 3. 4. The type of atom that carries the charge Resonance Induction The type of orbital where the charge resides

Compare ethanol and propylene. Which has a more stable conjugate base? WHY?

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity ARIO ARIO is a good GENERAL guideline, but it can fail. Compare acetylene and ammonia. Using ARIO, which should be a stronger acid?

To determine the ACTUAL acidities, the pKa values must be experimentally measured and compared.

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity ARIO ARIO is a good GENERAL guideline, but it can fail. Using the pKa values, you can never go wrong. Which acid is truly stronger? Which direction will the following equilibrium favor?

Practice with SKILLBUILDER 3.9.Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.4 Qualifying Acidity ARIO For each of the molecules below, rank the labeled Hydrogen atoms in order of increasing pKa value.

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.5 Predicting Equilibrium Position If pKa values are KNOWN, they are a sure-fire way to predict the position of an equilibrium. If pKa values are NOT KNOWN, the relative stability of conjugates should be explored.

Practice with SKILLBUILDER 3.10.

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.5 Predicting Equilibrium Positions Choosing a Reagent pKa values help us choose appropriate reagents for acid/base reaction: Select an ACID from Table 3.1 that could PROTONATE each of the following molecules.pKa = 25 pKa = 19.2

pKa = 4.75

pKa = 2.9

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.5 Predicting Equilibrium Position Choosing a Reagent Another important skill is to be able to choose an appropriate REAGENT for a acid/base reaction: Choose a CONJUGATE BASE from Table 3.1 that could effectively DEPROTONATE each of the following four molecules.

Practice with SKILLBUILDER 3.11.Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.6 Choosing a Solvent Another key skill is to choose an appropriate SOLVENT for acid/base reactions: The solvent should be able to surround (solvate) the reactants and facilitate their collisions without reacting itself. Because water can act as an acid or a base, it has a LEVELING EFFECT on strong acids and bases: Acids stronger than H3O+ cannot be used in water. WHY? Bases stronger than OH- cannot be used in water. WHY?

Copyright 2012 John Wiley & Sons, Inc.

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Klein, Organic Chemistry 1e

3.6 Choosing a Solvent Because water can act as an acid or a base, it has a LEVELING EFFECT on strong acids and bases: Acids stronger than H3O+ cannot be used in water. For example, wate