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Chapter 3 Acids and Bases. The Curved-Arrow Notation. Arrhenius Acids and Bases. Acid : a substance that, when dissolved in water, increases the concentration of H + (protons) HCl H + + Cl - HCl + H 2 O H 3 O + + Cl -. H 2 O. - PowerPoint PPT Presentation
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Chapter 3Acids and Bases. The Curved-Arrow Notation
Arrhenius Acids and Bases
• Acid: a substance that, when dissolved in water, increases the concentration of H+ (protons)• HCl H+ + Cl-
• HCl + H2O H3O+ + Cl-
H2O
• Base: a substance that, when put in water, increases the concentration of OH- ions or a substance that accepts H+ ions• NaOH(aq) Na+(aq) + OH-(aq)
Bronsted-Lowry Acids and Bases
• Acid: proton (H+) donor• Base: proton (H+) acceptor
Lewis Acids and Bases• Lewis Acid
– Electron deficient/poor– Electron acceptor – Electrophile– Tend to have less than an
octet• Lewis Base
– Electron rich– Electron donor– Nucleophile– Must have a lone pair of
electrons• product called an adduct
Fluorine is electron rich
• Lewis acids tend to react so as to fulfill their valence-shell octet
• Note the conservation of charge• Recall: FC = # valence e-’s – ( # LP e-’s + ½ # of bonding e-’s)
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ProblemsComplete the following Lewis acid-base reactions. Draw in any missing electrons, label the nucleophiles and electrophiles, identify the adduct, and calculate any formal charges needed for the adduct
OH
H C H
+ OH-1
Curved-Arrow Notation
• A tool for tracking electrons in a chemical reaction
• Electrons flow from the electron donor (Lewis base) to the electron acceptor (Lewis acid)
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ProblemsUse the curved arrow notation to derive a structure for your product in each of the following reactions
CH3NH2 + H+
Electron Pair Displacement Reactions
• Not all acceptors are electron-deficient• When an atom is NOT electron deficient, an
electron pair must depart from the atom before it receives another electron pair
• This preserves the octet rule
10
Curved-Arrow Notation for Displacement• Displacement reactions require two arrows• Watch for conservation of total charge!
• Donated electron pairs can also originate from a lone pair or a bond
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The Wrong Way
• Curved-arrows show the movement of electron pairs not nuclei
• Electrons are responsible for chemistry!
12
ProblemsProvide a curved arrow notation for each of the following reactions
H2O + HCl H3O+ + Cl-
+CH2CH2 + Br2
Problems
For each of the following reactions, give the product that results
Two Reactions Represented by Curved Arrows
• Most reactions in O-chem involve moving electrons
– Every reaction involving electron pairs fits into one of these two categories:
1) Lewis base + Lewis acid2) Electron-pair displacement reactions
• Reactions may be a combination of the two types above
153.3 Review of the Curved-Arrow Notation
Problems
• For the following reactions, indicate whether you have a Lewis acid-base reaction or an electron pair displacement reaction
Curved-Arrow Notation for Resonance
• Resonance structures differ only by movement of electrons (and usually electron pairs)
• Curved-arrow notation is ideal to help derive resonance contributors
• Note: the interconversion of resonance structure by movement of electron is NOT a reaction
173.3 Review of the Curved-Arrow Notation
Problems• Using the curved arrow
notation, derive resonance structures for the following compounds:
1) Benzene2) Aniline3) Diazomethane
BrØnsted Acid-Bases Reactions
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• A Bronsted acid-base reaction involves an electron-pair displacement on a proton
• Bronsted Acid: A species that donates a H+
– Keeps the electrons that were bonding to H• Bronsted Bases: A Lewis base that donates its electron pair to
a proton (in order to grab it)
Conjugate Acids and Bases
• When a BrØnsted acid loses a proton, its conjugate base is formed
• When a BrØnsted base gains a proton, its conjugate acid is formed
203.4 BrØnsted-Lowry Acids and Bases
Amphoteric Compounds
• Compounds that can act as either an acid or a base are called amphoteric
• Observe the behavior of a compound in a reaction to classify it as an acid or base
• Water is amphoteric
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Problems
• H2O
• F-• HCO3
-
• SO42-
• H2O
• HCO3-
• HPO42-
• H2S
Identify the conjugate acids for the compounds on the left and the conjugate bases for the compounds on the right. Also, identify all amphoteric compounds
Organic Reactions
• The BrØnsted-Lowry acid-base concept is central to many reactions in organic chemistry
• For example:
• …looks similar to:
243.4 BrØnsted-Lowry Acids and Bases
Nucleophiles and Electrophiles
• Nucleophile = Lewis base (“nucleus loving”)
253.4 BrØnsted-Lowry Acids and Bases
Nucleophiles and Electrophiles
• Electrophile = Lewis acid (“electron loving”)• The atom that receives the electron pair
263.4 BrØnsted-Lowry Acids and Bases
Leaving Groups
• The group or atom that receives electrons from the breaking bond is a leaving group
273.4 BrØnsted-Lowry Acids and Bases
Leaving Groups
• Can also be applied to Lewis acid-base dissociation reactions
283.4 BrØnsted-Lowry Acids and Bases
Problems• Classify each of the following reactions as a Bronsted acid-base reaction or
a Lewis acid-base association/dissociation. Identify each species in the following reactions as a Bronsted acid, Bronsted base, Lewis acid, Lewis base, nucleophile, electrophile, and/or leaving group. Draw in the appropriate curved arrow notation where appropriate.
H3O+
HCl
Strengths of BrØnsted Acids
• A measure of the extent of proton release to a BrØnsted base
• The standard base traditionally used is water
• The equilibrium constant is:
333.4 BrØnsted-Lowry Acids and Bases
The Dissociation Constant
• As [H2O] effectively remains constant:
• Each acid has its own dissociation constant• A large Ka = many H+ transferred
– Strong acid– Weak conjugate base
343.4 BrØnsted-Lowry Acids and Bases
The pKa Scale and pH
• pKa = -log Ka
• pKa values are more manageable than Ka values
• Stronger acids have smaller pKa values
• pH is a measure of [H+], a property of a solution (recall: pH = - log[H3O+])
• pKa is a measure of acid strength, a fixed property
373.4 BrØnsted-Lowry Acids and Bases
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Strengths of BrØnsted Bases
• Directly related to Ka/pKa of the conjugate acid
• If a base is weak, its conjugate acid is strong• If a base is strong, its conjugate acid is weak
403.4 BrØnsted-Lowry Acids and Bases
Problems
1) Write out the dissociation constant expression for formic acid, HCO2H, in water
2) Identify the conjugate acid-base pairs in the equation for problem #1
3) Using the Ka for formic acid, calculate the pKa
4) What is the Ka for acetic acid if its pKa = 4.74? Is acetic acid’s conjugate base weaker or stronger than the conjugate base of HF? HF’s Ka = 7.2 x 10-4
Relationship of Structure to Acidity• Which of the following molecules is the
weakest acid? Which is the strongest?• HF• HCl• HBr• HI
• What about these:
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The Element Effect
• Evaluate the atom attached to the proton• The acidities of Bronsted acids (H-A) increase
down a group• Acidities increase as the atomic # of A
increases• Due to decrease in bond strength
• The acidities of Bronsted acids (H-A) increase across a period from left to right• Due to increasing electronegativity of A
433.6 Relationship of Structure to Acidity
45
The Charge Effect
• Who is more acidic, H2O or H3O+?
• Positively charged compounds attract electrons better than neutral ones
• pKa of H2O = 15.7
• pKa of H3O+ = -1.7
463.6 Relationship of Structure to Acidity
Problems
• Which of the following is the stronger acid?1) PH3 or SH2
2) H2O or SH2
3) NH3, NH4+, or NH2
-
47
The Polar Effect
• Which of the following is more acidic?
48
The Polar Effect
• The presence of electronegative substituents has an acid strengthening effect = polar effect or inductive effect– Such substituents are said to be electron
withdrawing• Acids with stable conjugate bases tend to be more
acidic– Resonance = stabilization
49
• Consider the following series:
• Which molecule is the most acidic and why?
50
Problems
• Rank the following molecules, in each series according to order of increasing acidity and explain your reasoning
51
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Homework Problems
• 3.1 – 3.13, 3.19 – 3.21, 3.24 – 3.37, 3.39, 3.44, 3.49, 3.50
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