CHEM200-Unit2
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Transcript of CHEM200-Unit2
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E. D. GLORIAUST – FACULTY OF PHARMACYCHEM200 – ORGANIC CHEMISTRY
Unit II: Acids and Bases
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1. Factors that affect the acidity/basicity of a compound2. Arrhenius Acids and Bases3. Bronsted-Lowry Acids and Bases4. Lewis Acids and Bases5. Ka, pKa, Kb, pKb, pH
Contents
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Acids and Bases
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Johannes Bronsted and Thomas Lowry- Independently proposed that:- An acid is a proton donor- A base is a proton acceptor- The acid-base reaction is a proton transfer
reaction.
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1. Formal Charge2. Electronegativity3. Structural effects
1. Resonance2. Hyperconjugation3. Inductive effect4. Steric effect5. Hydrogen bonding
What are the factors that affect the acidity or basicity of a compound?
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• The charge on an atom in a molecule or polyatomic ionFormal Charge
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1. Write a correct Lewis structure for the molecule or ion.2. Assign to each atom all its unshared (nonbonding)
electrons and one-half its shared electrons.3. Compare the number arrived at in Step 2 with the
number of valence electrons in the neutral, unbonded atom.
To derive a Formal Charge:
Number of valence Number of One-halfFormal Charge = electrons in free -- nonbonding -- bonding
atom electrons electrons
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Example
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Summary of Common Formal Charges
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•H3O+
•CH2O-
Example:
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Electronegativity
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H – F < H – Cl < H – Br < H – I
Variability in size within the Family
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Resonance
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Theory of Resonance – developed by Linus Pauling in the 1930s, states that many molecules and ions are best described by writing two or more Lewis structure and considering the real molecule or ion to be a composite of these structures.
What is Resonance?
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Resonance Contributing Structures – representations of a molecule or ion that differ only in the distribution of valence electrons.Resonance hybrids – A molecule or ion that is best described as a composite of a number of contributing structures.Double-headed arrows – a symbol used to connect contributing structures.
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1. Individual resonance forms are imaginary, not real.2. Resonance forms differ only in the placement of their
nonbonding electrons.3. Different resonance forms of a substance don’t have to
be equivalent.4. The resonance hybrid is more stable than any individual
resonance forms.5. Electron pairs can only move to adjacent positions.
Adjacent positions means neighboring atoms and/or bonds.
6. The Lewis structures that result from moving electrons must be valid and must contain the same net charges as all the other resonance structures.
Rules for Resonance Forms
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1. Do not break a single bond.2. Do not exceed an octet for second row
elements.
The Two Commandments of Resonance
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1. A lone pair next to a pi bond.2. A lone pair next to a positive charge.3. A pi bond next to a positive charge.4. A pi bond between two atoms, where one of those
atoms is electronegative.5. Pi bond going all the way around a ring.
Five patterns in drawing resonance structures
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1. Minimize charges2. Electronegative atoms (N, O, Cl, etc) can bear a
positive charge, but only if they possess an octet of electrons.
3. Avoid drawing a resonance structure in which two carbons bear opposites charges.
3 rules in determining which resonance structures are significant
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Inductive Effect
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• the polarization of electron density transmitted through covalent bonds by a nearby atom of higher electronegativity.
Inductive Effect
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Cl CH2 C OH
O
Cl CH C OH
OCl
MORE ACIDIC
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MORE ACIDIC
H2CH3C CH C OH
OCl
HC CH2 C OH
OCl
H3C
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Resonance and Inductive Effects of Various Functions
IE : EW IE: ED RE: EW RE : ED-- NR3
+ --NH3+ --O- --COO- --COOR --COOH --OH --OR
--COOH --COOR --CH2 --CHR2 --CH2R --CR3
--COR --CHO --NH2 -- NR2
--NO2 --CN --SO2R --SO2NHR --NHR2 --SH --CHO --COR --NO2 --CN--F --Cl --Br --I --CF3
--Ar
--OH --OR--NH2
--SH --SO2R
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Hyperconjugation
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• Any adjacent bonding electron pair will participate in carbocation hyperconjugation. However, only C–H and C–C bonds provide a significant level of increased stability.
Hyperconjugation
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Inc. number of alkyl groups attached to carbocation
Inc. # of bonds adjacent to the
carbocation
Inc. # hyperconjugative interactions
Inc. electron density to the
electron-deficient carbon
Inc. StabilityInc. Acidity
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Hyperconjugation: Carbocation stabilization
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Hyperconjugation: Carbocation stabilization
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Hyperconjugation
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Steric Effect
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CH3 C CH2
CH3
CH3
CH
CH3C OH
O
CH3 C CH2
CH3
CH3
CH
C(CH 3)3C OH
O
MORE ACIDIC
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Hydrogen Bonding
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• is a special type of dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom. It results from the attractive force between a hydrogen atom covalently bonded to a very electronegative atom such as a N, O, or F atom and another very electronegative atom.
Hydrogen Bonding
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C OH
O H
O
FH OH OH
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MORE ACIDIC
HOOC
OHHO
HOOCOH
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m-nitrophenol p-nitrophenol o-nitrophenol
HO
NO2
HO
O2N
HONO2
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• 2.10• 2.17• 2.19• 2.28• 2.29• 2.36• 2.39
Answer the following:
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• Brown, W. H., Poon, T. (2014). Introduction to Organic Chemistry (5th ed.). USA: John Wiley & Sons, Inc.
References