Chapter 2 Acid and Bases

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Chapter 2 Acid and Bases
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BrØnsted Acids and Bases
A BrØnsted acid is a H donor. A BrØnsted
base is a H acceptor.
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Examples of BrØnsted Acids
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Examples of BrØnsted Bases
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Conjugate Acid-Base Pairs
(Conjugate Base )
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What is a conjugate base?
A conjugate base is a derivative of a Bronsted
acid
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p. 152
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Conjugate Base Workshop
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Determine the conjugate base of the
following substances.
HF
HOH
HCl
HCN
HCO3
H2S
p. 152
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Conjugate Acid
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What is a conjugate acid?
A conjugate acid is a derivative of a Bronsted
base.
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Conjugate Acid Workshop
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Determine the conjugate acid of the
following substances.
HPO42
HOH
NO3
HCOO
CH3NH2
HCO3
p. 152
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Predicting Products in Proton Transfer Rxns
(See Smartboard)
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BrØnsted-Lowry Acids and Bases
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Acids and Bases
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Reactions of BrØnsted Acid and Bases
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Acid Strength
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Acid Strength and pKa
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Acid Strength and pKa
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Acid Strength and pKa
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Acid Strength and pKa
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pKas you will need to know RCOOH 5 PhOH,
HCN 10 H2O 15.7 ROH 16 NH3 - 38 RH
gt 50
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Factors Affecting Acid Strength
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• Four factors can affect the pKa of an acid
• Resonance ( e.g., compare pKas of ethanol and
  ethanoic acid)
• Inductive (e.g. compare ethanol and its trifluoro
  derivative)
• Hybridization (e.g. compare ethane and ethyne)
• Element (e.g. compare H2O, HF and HBr)

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Resonance Structures for the Acetate Ion
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E-maps for the Ethoxide and Acetate Ions
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Why is the trifluoro derivative a stronger acid?
Always compare the relative stability of the
conjugate bases.
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Compare the relative stability of the conjugate
bases
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Example of E-maps showing an electron
withdrawing inductive effect
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Hybridization Again compare the relative
stability of the conjugate bases
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Why is HF a stronger acid than H2O but a weaker
acid than HBr? Again, compare the relative
stability of the conjugate bases
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Compare the relative stability of the conjugate
bases
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Element EffectsTrends in the Periodic Table.
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Acids and Bases
Factors that Determine Acid StrengthResonance
Effects

• Resonance is a third factor that influences
  acidity.
• In the example below, when we compare the
  acidities of ethanol and acetic acid, we note
  that the latter is more acidic than the former.

• When the conjugate bases of the two species are
  compared, it is evident that the conjugate base
  of acetic acid enjoys resonance stabilization,
  whereas that of ethanol does not.

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Acids and Bases
Factors that Determine Acid StrengthHybridization
Effects
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Acids and Bases
Commonly Used Acids in Organic Chemistry
In addition to the familiar acids HCl, H2SO4 and
HNO3, a number of other acids are often used on
organic reactions. Two examples are acetic acid
and p-toluene-sulfonic acid (TsOH).
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Acids and Bases
Commonly Used Acids in Organic Chemistry
Common strong bases used in organic reactions are
more varied in structure.
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Acids and Bases
Commonly Used Acids in Organic Chemistry
It should be noted that

• Strong bases have weak conjugate acids with high
  pKa values, usually gt 12.
• Strong bases have a net negative charge, but not
  all negatively charged species are strong bases.
  For example, none of the halides F, Cl, Br,
  or I, is a strong base.
• Carbanions, negatively charged carbon atoms, are
  especially strong bases. A common example is
  butyllithium.
• Two other weaker organic bases are triethylamine
  and pyridine.

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Acids and Bases
Lewis Acids and Bases

• The Lewis definition of acids and bases is more
  general than the BrØnsted-Lowry definition.
• A Lewis acid is an electron pair acceptor.
• A Lewis base is an electron pair donor.
• Lewis bases are structurally the same as
  BrØnsted-Lowry bases. Both have an available
  electron paira lone pair or an electron pair in
  a ? bond.
• A BrØnsted -Lowry base always donates this
  electron pair to a proton, but a Lewis base
  donates this electron pair to anything that is
  electron deficient.

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Acids and Bases
Lewis Acids and Bases

• A Lewis acid must be able to accept an electron
  pair, but there are many ways for this to occur.
• All BrØnsted-Lowry acids are also Lewis acids,
  but the reverse is not necessarily true.
• Any species that is electron deficient and
  capable of accepting an electron pair is also a
  Lewis acid.
• Common examples of Lewis acids (which are not
  BrØnsted-Lowry acids) include BF3 and AlCl3.
  These compounds contain elements in group 3A of
  the periodic table that can accept an electron
  pair because they do not have filled valence
  shells of electrons.

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Acids and Bases
Lewis Acids and Bases

• Any reaction in which one species donates an
  electron pair to another species is a Lewis
  acid-base reaction.
• In a Lewis acid-base reaction, a Lewis base
  donates an electron pair to a Lewis acid.
• Lewis acid-base reactions illustrate a general
  pattern in organic chemistry. Electron-rich
  species react with electron-poor species.
• In the simplest Lewis acid-base reaction one bond
  is formed and no bonds are broken. This is
  illustrated in the reaction of BF3 with H2O. H2O
  donates an electron pair to BF3 to form a new
  bond.

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Lewis Acids and Bases
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What are Lewis Acids and Bases ?
Acids are e- pair acceptors Bases are e- pair
donors
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How are Lewis Acid-Bases similar to
Electrophiles/Nucleophiles?
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What is an electrophile ( E) ? .any Lewis
acid or electron deficient cmpd that can accepta
pair of electrons. What is a nucleophile
(Nu)? .any Lewis base or electron rich cmpd
that can donate a pair of electrons to an E
excluding Bronsted acids
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Lewis Acids and Bases
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Acids and Bases
Lewis Acids and Bases

• In some Lewis acid-base reactions, one bond is
  formed and one bond is broken. To draw the
  products of these reactions, keep in mind the
  following steps
• Always identify the Lewis acid and base first.
• Draw a curved arrow from the electron pair of the
  base to the electron-deficient atom of the acid.
• Count electron pairs and break a bond when needed
  to keep the correct number of valence electrons.