A BrnstedLowry acid is a H proton donor.

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BrØnsted-Lowry Acids and Bases

• A BrØnsted-Lowry acid is a H (proton) donor.
• A BrØnsted-Lowry base is a proton acceptor.
• A BrØnsted-Lowry acid base reaction results in
  the transfer of a proton from an acid to a base.

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Curved Arrow Symbolism

• The movement of electrons in reactions can be
  illustrated using curved arrow notation. Because
  two electron pairs are involved in this reaction,
  two curved arrows are needed.
• Loss of a proton from an acid forms its conjugate
  base.
• Gain of a proton by a base forms its conjugate
  acid.
• A double reaction arrow is used between starting
  materials and products to indicate that the
  reaction can proceed in the forward and reverse
  directions. These are equilibrium arrows.

Examples
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Acid Strength and pKa

• Acid strength is the tendency of an acid to
  donate a proton.
• The more readily a compound donates a proton, the
  stronger an acid it is.
• Acidity is measured by an equilibrium constant
• When a BrØnsted-Lowry acid HA is dissolved in
  water, an acid-base reaction occurs, and an
  equilibrium constant can be written for the
  reaction.

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Acid Strength and pKa
Because the concentration of the solvent H2O is
essentially constant, the equation can be
rearranged and a new equilibrium constant, called
the acidity constant, Ka, can be defined.
It is generally more convenient when describing
acid strength to use pKa values than Ka values.
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Selected pKa values
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Predicting/Explaining Acidity
Factors that Determine Acid Strength

• Anything that stabilizes a conjugate base A
  makes the starting acid HA more acidic.

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Predicting/Explaining Acidity

• Four factors affect the acidity of HA. These
  are
• Element effects
• Inductive effects
• Resonance effects
• Hybridization effects
• No matter which factor is discussed, the same
  procedure is always followed. To compare the
  acidity of any two acids
• Always draw the conjugate bases.
• Determine which conjugate base is more stable.
• The more stable the conjugate base, the more
  acidic the acid.

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Element Effects
Across a row of the periodic table, the acidity
of HA increases as the electronegativity of A
increases.
Down a group of the periodic table, the acidity
of HA increases as the size of A increases.
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Explanation of Element Effects
Negative charge is more stable when it is
localized on a more electronegative element
Positive or negative charge is stabilized when it
is spread over a larger volume.
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Element Effects Summary
Element EffectsTrends in the Periodic Table.

• Down a column of the periodic table, the acidity
  of HA increases as the size of A increases.
  Size, and not electronegativity, determines
  acidity down a column.
• The acidity of HA increases both left-to-right
  across a row and down a column of the periodic
  table.
• Although four factors determine the overall
  acidity of a particular hydrogen atom, element
  effectsthe identity of Ais the single most
  important factor in determining the acidity of
  the HA bond.

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Inductive Effects

• Example

Explanation
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Inductive Effects Summary

• When electron density is pulled away from the
  negative charge through ? bonds by very
  electronegative atoms, it is referred to as an
  electron withdrawing inductive effect.
• More electronegative atoms stabilize regions of
  high electron density by an electron withdrawing
  inductive effect.
• The more electronegative the atom and the closer
  it is to the site of the negative charge, the
  greater the effect.
• The acidity of HA increases with the presence of
  electron withdrawing groups in A.

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Example Problems
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Resonance Effects

• Example

• 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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Resonance Effects

• Electrostatic potential plots of CH3CH2O and
  CH3COO below indicate that the negative charge
  is concentrated on a single O in CH3CH2O, but
  delocalized over both of the O atoms in CH3COO.

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Hybridization Effects
Let us consider the relative acidities of three
different compounds containing CH bonds.

• The higher the percent of s-character of the
  hybrid orbital containing the lone pair, the
  closer the lone pair is held to the nucleus, and
  the more stable the conjugate base.

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Hybridization Effects
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Predicting Acidity Summary