Ch_322a_3.010

Ch_322a_3.010 - Acid Strength: Structure and Reactivity...

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Acid Strength: Structure and Reactivity Relationships There are several important relationships between acid strength ( K a ) and structure of compounds. (1) Bond Strength H -F H -Cl H -Br H -I pK a 3.2 -7 -9 -10 increasing acid strength increasing H-X bond strength H-A + H 2 O H 3 O + + A - K a The acidity of H-A decreases as the bond strength increases. An example is the order of acidity of the hydrogen halides (H-X) . Order of Base Strength F - > Cl - > Br - > I -
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(2) Electronegativity The acidity of A-H increases, as the electronegativity of A increases. An example is the very dramatic increase in acidity of the second row A-H compounds. C N O F electronegativity H 3 C- H H 2 N- H HO- H F- H pK a >50 38 15.7 3.2 increasing acid strength
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Influences of Electronegativity on Acidity Increasing electronegativity of the central atom enhances acidity in two ways: (1) The polarity of the covalent A-H bond increases with increasing electronegativity of atom A, resulting in a more electropositive H . As positive charge density on H increases, its reactivity towards a
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Ch_322a_3.010 - Acid Strength: Structure and Reactivity...

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