acidity and basicity

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Acidity and

Basicity

Ms. Anjelyn del Rosario

UNIVERSITY OF THE PHILIPPINES MANILA Padre Faura, Ermita, Manila

A.Y. 2009 – 2010, Second Semester

CHEMISTRY ORGANIC

Chemistry 31 • Second Sem 2009-2010

+ H2O acid

+ base

solute which produces H3O+

ion when dissolved in H2O

solute which produces OH-

ion when dissolved in H2O

example:

HCl H3O+ + Cl-

example:

NH3 H2O NH4+ + OH-

H2SO4 (aq) + H2O(l) HSO4- (aq) + H3O

+(aq)

HSO4- (aq) + H2O(l) SO4

2- (aq) + H3O

+(aq)

1. Arrhenius Concept classifies solutes according to their behavior in H2O

KOH(aq) OH- (aq) + K+

(aq)

CO32-

(aq) + H2O(l) OH- (aq) + HCO3

- (aq)

Acid- Base Theories

HA (aq) + H2O(l) A- (aq) + H3O

+(aq) MOH (aq) M+

(aq) + OH-(aq)

a proton donor

binary acids <HnX>, oxyacids

<(HO)nXOm>, aquocomplexes

of metal ions,…

a proton acceptor

chemical species that has an

atom with an available electron

for pairing

2. Brønsted-Lowry Concept based on the behavior of a substance during proton (H+) transfer; H+

(aq) + H2O(l) H3O+

(aq)

Acid- Base Theories

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Acid- Base Theories

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• Proton donors and acceptors.

• H+ or H3O+ = a proton

2. Brønsted-Lowry Concept

Conjugate acid-base pair a pair of substances that are related through the loss or gain of a proton; in any acid-base reaction we can identify two sets of conjugate acid-base pairs.

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Acid- Base Theories

2. Brønsted-Lowry Concept

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Acid- Base Theories

Explanation of Acid Strength

1. Positivity of H: The more positive H is, the more acidic it is, the higher the acid strength. RCOOH > H-O-H > R-O-H

2. Stability of the conjugate base: the more stable the resulting conjugate base, the more willing the acid is to donate its proton, the higher the acid strength RCOOH > phenol > HOH > ROH

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Structural Effects on Acidity and Basicity

In the Bronsted-Lowry Concept:

ACID strength – proton-donating ability BASE strength – proton-accepting ability Factors Affecting Acidity and Basicity

1. Direction of polarity of H-X bond 2. Strength of the H-X bond 3. Stability of the conjugate base 4. Type of solvent

ACID and BASE strength

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Factors Affecting Acidity and Basicity

1. Direction of polarity of H-X bond

The H in an H-X bond should be partially positive. The more polar the bond, the stronger the acid.

2. Strength of the H-X bond

Stronger bonds are less easily dissociated than

weaker bonds.

3. Stability of the conjugate base

In general, the more stable the conjugate base, the

stronger the acid.

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ACID strength

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• Anything that stabilizes a conjugate base A:¯ makes the

starting acid H—A more acidic.

• Four other factors affect the acidity of H—A. These are:

Element effects (trends in periodic table)

Inductive effects (electronegativity)

Resonance effects (multiple resonance structures)

Hybridization effects (sp, sp2, sp3)


ACID strength

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• No matter which of these factors is discussed, to compare

the acidity of any two acids:

o Always look at the conjugate bases.

o Determine which conjugate base is more stable.

o The more stable the conjugate base, the more acidic the acid.

• The strengths of a conjugate acid and its conjugate base are inversely related.

• A strong conjugate base has a weak conjugate acid.

• A weak conjugate base has a strong conjugate acid.


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Element Effects—Trends in the Periodic Table.

Across a row of the periodic table, the acidity of H—A

increases as the electronegativity of A increases.

Factors that determine ACID strength

Down a column of the periodic table, the acidity of H—A increases

as the size of A increases.

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• Size, and not electronegativity, determines acidity down a column.

• The acidity of H—A 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 effects—the identity of A—is the single most important factor in determining the acidity of the H—A bond.


Element Effects—Trends in the Periodic Table.

Positive or negative charge is stabilized when it is spread over

a larger volume.

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Inductive Effects

• An inductive effect is the pull of electron density through

bonds caused by electronegativity differences between atoms.

• In the example below, when we compare the acidities of ethanol and 2,2,2-trifluoroethanol, we note that the latter

is more acidic than the former.


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• The reason for the increased acidity of 2,2,2-

trifluoroethanol is that the three electronegative fluorine atoms stabilize the negatively charged conjugate base.

• This effect is limited to a three bond distance.

Inductive Effects


Resonance Effects

• Electron delocalization

• Resonance structures – Individual Lewis structures in

cases where two or more Lewis structures are equally good descriptions of a single molecule.

• ethanol vs. acetic acid


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• 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.

Resonance Effects


• Resonance delocalization makes CH3COO¯ more stable than CH3CH2O¯, so

CH3COOH is a stronger acid than CH3CH2OH.

• The final factor affecting the acidity of H—A is the hybridization.

• The higher the percent of s-character of the hybrid orbital, the

closer the lone pair is held to the nucleus, and the more stable

the conjugate base.

Let us consider the relative acidities of three different compounds

containing C—H bonds.

Hybridization Effects


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Figure. Electrostatic Potential plots

Hybridization Effects


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Summary of the factors that determine acidity

Factors that determine ACID strength Factors that determine ACID strength


Exercise:

For each of the following compounds, indicate which is the stronger acid:

For each of the following compounds, indicate which is the stronger base:


Exercise:

“Hard” acids and bases have low polarizability. Their orbitals do not change their shapes, so the interaction is mostly driven by electrostatic forces. “Soft” acids and bases have high polarizability. Their orbitals change their shapes and drive the interaction.

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Aromaticity and acidity Compound pKa1 pKa2 Struct. Effects

A

Benzoic Acid 4.20

o-methylbenzoic Acid 3.91

o-chlorobenzoic acid 2.94

o-tertbutylbenzoic acid 3.46 STERIC and POLAR o-nitrobenenzoic acid 2.17

B

O-hydroxybenzoic acid 2.98 Intramolecular H-Bond 2,6-dihydroxybenzoic acid 2.30

C

Pthalic Acid 2.98 5.28

H-bond and Polar Isophthalic Acid 3.46 4.46

Terephthalic Acid 3.51 4.82

acidity

Compound pKa1 pKa2 Structural Effects

D

p-methoxybenzoic acid 4.47

p-cyanobenzoic acid 3.55

p-methylbenzoic acid 4.34 Electron delocalization p-nitrobenzoic acid 3.44

acidity

Compound Ka Ethanol 1 .0 x 10-18

Phenol 1.2 x 10-10

o-nitrophenol 6.8 x 10-8

m-nitrophenol 5.0 x 10-9

p-nitrophenol 7.0 x 10-8

2,6-dinitrophenol 1.0 x 10-4

2,4-dintrophenol 5.6 x 10-4

2-nitroresorcinol 1.59 x 10-6

4-nitroresorcinol 1.04 x 10-6

acidity

Compound pKb Compound pKb

Ammonia 4.75 o-methylaniline 9.62

Methylamine 3.36 m-methylaniline 9.33

Dimethylamine 3.23 p-methylaniline 9.00

Trimethylamine 4.20

o-nitroaniline 14.28

Ethylamine 3.33 m-nitroaniline 11.55

Diethylamine 3.07 p-nitroaniline 13.02

Aniline 9.38

N-methyl aniline 9.60 p-methoxyaniline 8.71

p-hydroxyaniline 8.50

triethylamine 3.42

Basicity

Strength of Organic Bases when Acid is a Proton

acidity and basicity

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