1.15How Structure Affects Acid Strength

The Main Ways Structure Affects Acid StrengthThe strength of the bond to the atom from which the proton is lost. The electronegativity of the atom from which the proton is lost.Changes in electron delocalization on ionization.

Bond StrengthBond strength is controlling factor when comparing acidity of hydrogen halides.

Bond StrengthRecall that bond strength decreases in a group in going down the periodic table.Generalization: Bond strength is most important factor when considering acidity of protons bonded to atoms in same group of periodic table (as in HF, HCl, HBr, and HI). Another example: H2S (pKa = 7.0) is a stronger acid than H2O (pKa = 15.7).

The Main Ways Structure Affects Acid StrengthThe strength of the bond to the atom from which the proton is lost. The electronegativity of the atom from which the proton is lost.Changes in electron delocalization on ionization.

ElectronegativityElectronegativity is controlling factor when comparing acidity of protons bonded to atoms in the same row of the periodic table.

Electronegativity

ElectronegativityThe equilibrium becomes more favorable as A becomes better able to bear a negative charge.Another way of looking at it is that H becomes more positive as the atom to which it is attached becomes more electronegative.

Bond strength versus ElectronegativityBond strength is more important when comparing acids in which the proton that is lost is bonded to atoms in the same group of the periodic table.Electronegativity is more important when comparing acids in which the proton that is lost is bonded to atoms in the same row of the periodic table.

Acidity of Alcohols

Acidity of AlcoholsElectronegative substituents can increase the acidity of alcohols by drawing electrons away from the OH group.CH3CH2OHCF3CH2OH1611.3weakerstrongerpKa

Inductive EffectThe greater acidity of CF3CH2OH compared to CH3CH2OH is an example of an inductive effect.Inductive effects arise by polarization of the electron distribution in the bonds between atoms.

Electrostatic Potential MapsThe greater positive character of the proton of the OH group of CF3CH2OH compared to CH3CH2OH is apparent in the more blue color in its electrostatic potential map. CH3CH2OHCF3CH2OH

Another example of the inductive effect4.70.50weakerstrongerpKa

The Main Ways Structure Affects Acid StrengthThe strength of the bond to the atom from which the proton is lost. The electronegativity of the atom from which the proton is lost.Changes in electron delocalization on ionization.

Electron DelocalizationIonization becomes more favorable if electron delocalization increases in going from right to left in the equation.Resonance is a convenient way to show electron delocalization.

Nitric AcidpKa = -1.4+

Nitric AcidNitrate ion is stabilized by electron delocalization.

Nitric AcidNegative charge is shared equally by all three oxygens.

Acetic AcidO O HH pKa = 4.7+CH3

Nitric AcidAcetate ion is stabilized by electron delocalization.

Nitric AcidNegative charge is shared equally by both oxygens.OOC CH3

1.16Acid-Base Equilibria

GeneralizationThe equilibrium in an acid-base reaction is favorable if the stronger acid is on the left and the weaker acid is on the right.Stronger acid + Stronger baseWeaker acid + Weaker base

Example of a strong acidOH+++The equilibrium lies to the side of the weaker acid. (To the right)

Example of a weak acid++The equilibrium lies to the side of the weaker acid. (To the left)

Important PointsA strong acid is one that is stronger than H3O+. A weak acid is one that is weaker than H3O+.A strong base is one that is stronger than HO. A weak base is one that is weaker than HO.The strongest acid present in significant quantities when a strong acid is dissolved in water is H3O+. The strongest acid present in significant quantities when a weak acid is dissolved in water is the weak acid itself.

Predicting the Direction of Acid-Base ReactionsHOC6H5+The equilibrium lies to the side of the weaker acid. (To the right) Phenol is converted to phenoxide ion by reaction with NaOH.OC6H5+ HOH

Predicting the Direction of Acid-Base ReactionsThe equilibrium lies to the side of the weaker acid. (To the left) Phenol is not converted to phenoxide ion by reaction with NaHCO3.

1.17Lewis Acids and Lewis Bases

DefinitionsArrheniusAn acid ionizes in water to give protons. A base ionizes in water to give hydroxide ions.Brnsted-LowryAn acid is a proton donor. A base is a proton acceptor.LewisAn acid is an electron pair acceptor. A base is an electron pair donor.

Lewis Acid-Lewis Base ReactionsThe Lewis acid and the Lewis base can be either a neutral molecule or an ion.

Example: Two Neutral MoleculesF3B+Lewis acidLewis baseProduct is a stable substance. It is a liquid with a boiling point of 126C. Of the two reactants, BF3 is a gas and CH3CH2OCH2CH3 with a boiling point of 34C.

Example: Ion + Neutral moleculeReaction is classified as a substitution. But notice how much it resembles a Brnsted acid-base reaction.

Example: Ion + Neutral moleculeBrnsted acid-base reactions are a subcategory of Lewis acid-Lewis base reactions.

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