ch03 skeletonized
TRANSCRIPT
• 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
• Use curved arrows to draw reasonable products for the following acid/base reaction, and label the conjugates.
3.1 Acids and Bases
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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
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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
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• Identify all acids in the following mechanism.
• Practice with SKILLBUILDER 3.1.
3.2 Curved Arrows in Reactions
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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
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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
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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
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ACID pKa ACID pKa
–9 15.7
–7 16
–2.9 18
–1.74 19.2
4.75 50
• 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
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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
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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
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or
• 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
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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
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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
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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
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• Does induction also play a role in explaining why acetic acid is stronger than ethanol?
3.4 Qualifying Acidity – Induction
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• Explain the pKa differences below.
• Practice with SKILLBUILDER 3.7.
3.4 Qualifying Acidity – Induction
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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
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sp3 sp2 sp
• 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
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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
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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
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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
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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
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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
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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
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pKa = 2.9pKa = 4.75
pKa = 19.2pKa = 25
• 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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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