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Brønsted-Lowry definitions: Acids donate a proton. Bases accept a proton. Recall: 3.1 Acids and Bases Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 3 -1 Conjugate Conjugate

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• Brønsted-Lowry definitions:

– Acids donate a proton.

– Bases accept a proton.

• Recall:

3.1 Acids and Bases

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -1

Conjugate

base

Conjugate

acid

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

3.1 Acids and Bases

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -3

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• 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!

3.2 Curved Arrows in Reactions

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -4

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• 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.

3.2 Curved Arrows in Reactions

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -5

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• Identify all acids in the following mechanism.

• Practice with SKILLBUILDER 3.1.

3.2 Curved Arrows in Reactions

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -7

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• 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.

3.3 Quantifying Acidity and Basicity –Acidity

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -8

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• Quantitative strength analysis—using 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.

3.3 Quantifying Acidity and Basicity –Acidity

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -9

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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.

• Practice with SKILLBUILDER 3.2.

3.3 Quantifying Acidity

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -11

ACID pKa ACID pKa

–9 15.7

–7 16

–2.9 18

–1.74 19.2

4.75 50

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• 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.

3.3 Quantifying Basicity

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -12

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• 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.

3.3 Using pKas to Analyze Equilibria

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -13

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• 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?

3.4 Qualifying Acidity

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -15

or

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• 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.

3.4 Qualifying Acidity

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -16

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• ARIO—The 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.

3.4 Qualifying Acidity –The Type of Atom

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -17

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• ARIO—Resonance 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.

3.4 Qualifying Acidity – Resonance

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -18

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• ARIO—Induction 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?

3.4 Qualifying Acidity – Induction

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -19

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• Does induction also play a role in explaining why acetic acid is stronger than ethanol?

3.4 Qualifying Acidity – Induction

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -20

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• Explain the pKa differences below.

• Practice with SKILLBUILDER 3.7.

3.4 Qualifying Acidity – Induction

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -21

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• 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 atom’s nucleus? WHY?

• Rank the ability of sp3, sp2, and sp orbitals to stabilize electrons, and explain.

3.4 Qualifying Acidity – Orbital

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -22

sp3 sp2 sp

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• 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.

3.4 Qualifying Acidity – Orbital

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -23

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• When assessing the acidity of protons, we GENERALLY use ARIO as our order of priority:

1. The type of atom that carries the charge

2. Resonance

3. Induction

4. The type of orbital where the charge resides

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

3.4 Qualifying Acidity – ARIO

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -25

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• 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.

3.4 Qualifying Acidity – ARIO

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -26

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• 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.

3.4 Qualifying Acidity – ARIO

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -27

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• For each of the molecules below, rank the labeled Hydrogen atoms in order of increasing pKa value.

3.4 Qualifying Acidity – ARIO

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -28

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• 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.

3.5 Predicting Equilibrium Position

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -31

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• 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.

3.5 Predicting Equilibrium Positions – Choosing a Reagent

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -33

pKa = 2.9pKa = 4.75

pKa = 19.2pKa = 25

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• 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.

3.5 Predicting Equilibrium Position –Choosing a Reagent

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -34

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• 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?

3.6 Choosing a Solvent

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -35

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• 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, water would not be an appropriate solvent for the following reaction. WHY?

3.6 Choosing a Solvent

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -36

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• Because water can act as an acid or a base, it has a LEVELING EFFECT on strong acids and bases:

– Bases stronger than OH- cannot be used in water. For example, water would not be an appropriate solvent for the following reaction. WHY?

– Which of the following solvents would be a better choice?

3.6 Choosing a Solvent

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -37

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• Because they are so similar, ARIO cannot be used to explain the pKa difference between ethanol and tert-butanol.

• Considering that pKa values are measured in solution, how might the solvent act to make ethanol a slightly stronger acid? Think about how the solvent could stabilize its conjugate base.

3.7 Solvation

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -38

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• The solvent must form ion–dipole attractions to stabilize the formal negative charge.

• As the tert-butoxide is sterically hindered, it won’t be as well solvated as the ethoxide.

3.7 Solvation

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -39

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• SOLVATION is important in reactions. Solvent is often needed to stabilize transition states, intermediates, and/or products to allow a reaction to occur.

• Explain why the pKa for acetic acid is 4.75 in water while it is 23.5 in CH3CN.

• Practice with CONCEPTUAL CHECKPOINT 3.33.

3.7 Solvation

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -40

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• Counterions are also known as spectator ions. They are always present, as they balance the overall charge.

• Here is a reaction with the counterion shown:

• Here is a reaction without the counterion shown even though it is present:

• Why are they often left out when writing a reaction mechanism?

3.8 Counterions

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -41

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• Lewis acid/base definition:

– A Lewis acid accepts a pair of electrons.

– A Lewis base donates a pair of electrons.

• Brønsted-Lowry acids are also Lewis acids.

• Brønsted-Lowry bases are also Lewis bases.

• Explain how this reaction fits both definitions.

3.9 Lewis Acids and Bases

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -42

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• Lewis acid/base definition:

– A Lewis acid accepts a pair of electrons.

– A Lewis base donates a pair of electrons.

• Some Lewis acid/base reactions cannot be classified using the Brønsted-Lowry definition.

• Explain how this reaction fits the Lewis definition but not the Brønsted-Lowry definition.

• Practice with SKILLBUILDER 3.12.

3.9 Lewis Acids and Bases

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e3 -43