Transcript Electron Pair Displacement Reactions

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)
H2O
• HCl
• HCl + H2O
H+ + ClH3O+ + Cl-
• 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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Problems
Complete 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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Problems
Use 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
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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!
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Problems
Provide 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 acid
2) Electron-pair displacement reactions
• Reactions may be a combination of the two
types above
3.3 Review of the Curved-Arrow Notation
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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
3.3 Review of the Curved-Arrow Notation
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Problems
•
Using the curved arrow
notation, derive
resonance structures for
the following compounds:
1) Benzene
2) Aniline
3) Diazomethane
BrØnsted Acid-Bases Reactions
• 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)
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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
3.4 BrØnsted-Lowry Acids and Bases
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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
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
•
•
•
•
H2O
FHCO3SO42-
•
•
•
•
H2O
HCO3HPO42H2S
Organic Reactions
• The BrØnsted-Lowry acid-base concept is
central to many reactions in organic chemistry
• For example:
• …looks similar to:
3.4 BrØnsted-Lowry Acids and Bases
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Nucleophiles and Electrophiles
• Nucleophile = Lewis base (“nucleus loving”)
3.4 BrØnsted-Lowry Acids and Bases
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Nucleophiles and Electrophiles
• Electrophile = Lewis acid (“electron loving”)
• The atom that receives the electron pair
3.4 BrØnsted-Lowry Acids and Bases
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Leaving Groups
• The group or atom that receives electrons
from the breaking bond is a leaving group
3.4 BrØnsted-Lowry Acids and Bases
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Leaving Groups
• Can also be applied to Lewis acid-base
dissociation reactions
3.4 BrØnsted-Lowry Acids and Bases
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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.
H 3 O+
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:
3.4 BrØnsted-Lowry Acids and Bases
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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
3.4 BrØnsted-Lowry Acids and Bases
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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
3.4 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
3.4 BrØnsted-Lowry Acids and Bases
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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
3.6 Relationship of Structure to Acidity
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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
3.6 Relationship of Structure to Acidity
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Problems
• Which of the following is the stronger acid?
1) PH3 or SH2
2) H2O or SH2
3) NH3, NH4+, or NH2-
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The Polar Effect
• Which of the following is more acidic?
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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
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• Consider the following series:
• Which molecule is the most acidic and why?
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Problems
• Rank the following molecules, in each series
according to order of increasing acidity and explain
your reasoning
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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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