Chap 04 Acids and Bases

7/29/2019 Chap 04 Acids and Bases

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Acids and Bases


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Different Definitions of Acids and

Bases Arrhenius definitions for aqueous solutions.

acid: a substance that produces H+ (H3O+) ions aqueous solution

base: a substance that produces OH- ions in aqueous solution

H+

(aq) + H2O( l) H3 O+

(aq)

Hydronium ionBronsted-Lowry definitions for aqueous and non-aqueous solutions.

Conjugate acid base pair: molecules or ions interconverted by transfer of

a proton.

acid: transfers the proton.

base: receives the proton.


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Lewis Acids and Bases

Focuses on the electrons not the H+.

An acid receives electrons from the base making a new bond.

Acid electron receptor.

Base electron donor.

H

O

H

H+

H

O

H

H

base

acid

lone pairs pi bonding electrons sigma bonding electrons

Energy

Basicity

Types of electrons:


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Acid Base Eqilibria

The position of the equilibrium is obtained by comparing the pKa values

of the two acids. Equivalently, compare the pKb values of the twobases.


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Acid Base Eqilibria

Same equilibrium with electron pushing (curved arrows).


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Lone Pair acting as Base.

Note the change in formal charges. As reactant oxygen hadcomplete ownership of lone pair. In product it is shared.

Oxygen more positive by 1.

Similarly, B has gained half of a bonding pair; more negative by

1.


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An example: pi electrons as bases

Bronsted

Lowry Acid

Bronsted

Lowry Base

The carbocations are

conjugate acids of the

alkenes.

For the moment, just note that

there are two possible

carbocations formed.


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Sigma bonding electrons as bases.

Much more unusual!!

Super acid

A very, very

electronegative F!!

A very positive S!! The

OH becomes very

acidic because thatwould put a negative

charge adjacent to the

S.


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Trends for Relative Acid Strengths

Totally ionized in

aqueous solution.

Aqueous Solution

Totally unionized

in aqueous

solution


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What affects acidity?

1. Electronegativity of the atom holding the negative charge.

CH3OH CH3O-

+ H+

CH3NH2 CH3NH-+ H

+

CH3CH3 CH3CH2-+ H

+

Increasing

electronegativity of

atom bearing negative

charge. Increasing

stability of anion.

Increasing

acidity.

Increasing basicity

of anion.

2. Size of the atom bearing the negative charge in the anion.

CH3OH CH3O-

+ H+; pKa = 16

CH3SH CH3S-

+ H+; pKa = 7.0

Increasing size of

atom holding negativecharge. Increasing

stability of anion.

Increasing

acidity.

Increasing basicity

of anion.


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OO

What affects acidity? - 23. Resonance stabilization, usually of the anion.

OH

phenol, PhOH

OO

ethanol, EtOH

CH3CH2OH CH3CH2O- + H+

Increasing

resonance

stabilization.

Increasedanion

stability.Acid

ity

Increasing

basicity of

the anion.

No resonance structures!!

OHOH

etc.

Note that phenol itself enjoys resonance but

charges are generated, costing energy, making

the resonance less important. The more

important resonance in the anion shifts the

equilibrium to the right making phenol more

acidic.


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An example: competitive Bases &

Resonance Two different bases or two sites in the same molecule may

compete to be protonated (be the base).

O

O H

acetic acid

H+O

O H

H

H+

O

O H

H

Acetic acid can be protonated at two sites.

Which conjugate acid is

favored?

The more stable one!Which is that?

Recall resonance

provides additional

stability by moving pi or

non-bonding electrons.

Pi bonding

electrons

converted to

non-bonding.

O

O H

H

O

O H

H

Non-bonding

electrons

converted to

pi bonding.

No valid resonance

structures for this

cation.


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An example: competitive Bases &

Resonance

H+O

O H

H

O

O H

H

O

O H

H

O

O H

acetic acid

All atoms obey octet

rule!

All atoms obey octet

rule!

The carbon is electron

deficient 6 electrons, not 8.

Lesser importance

Comments on the importance of the resonance structures.


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What affects acidity? - 34. Inductive and Electrostatic Stabilization.

F3CCH2O-

+ H+

H3CCH2O- + H+H3CCH2OH

F3CCH2OH

Due to electronegativity

of F small positive

charges build up on C

resulting in stabilization

of the anion.

Increasing anion

stability.Acidity.Increasing anion

basicity.d++

Effect drops off with distance. EtOH pKa = 15.9


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What affects acidity? - 4

5. Hybridization of the atom bearing the charge. H-A H+ + A:-.sp3 sp2 sp

More s character, more stability, more electronegative, H-A

more acidic, A:- less basic.

IncreasingAc

idityofHA

Increasing

B

asicityofA-

Note. The NH2-

is more basic

than the RCC-

ion.

Know this order.


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Example of hybridization Effect.

RCCH + AgNO3 AgCCR (ppt)

acid base

terminal alkyne

non-terminal alkyne

RCCR + LiCH2CH2CH2CH3 No Reaction

RCCH + LiCH2CH2CH2CH3 HCH2CH2CH2CH3 + RCCLi

RCCR + AgNO3 NR


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Example

Para nitrophenol is more acidic than phenol. Offer an explanation

OH

OH

N

O O

O

O

N

O O

+ H

+ HThe lower lies further to the right.

Why? Could be due to destabilization

of the unionized form, A, or

stabilization of the ionized form, B.

A B


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OH

N

O O

Examine the equilibrium for p-nitrophenol. How does

the nitro group increase the acidity?

O

N

O O

+ H

Resonance structures A, B and C are comparable to those in the phenol itself and

thus would not be expected to affect acidity. But note the + to attraction here

OH

N

O O

OH

N

O O

OH

N

O O

OH

N

O O

A B C D

Structure D occurs only due to thenitro group. The stability it

provides will slightly decrease

acidity.

Examine both sides

of equilibrium. Whatdoes the nitro group

do?

First the unionized

acid.

Note carefully that in these resonance

structures charge is created: + on the

O and in the ring or on an oxygen.

This decreases the importance of the

resonance.


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OH

N

O O

O

N

O O

+ H

Resonance structures A, B and C are comparable to those in the phenolate anion

itself and thus would not be expected to affect acidity. But note the + toattraction here

Structure D occurs only due to the

nitro group. It increases acidity.The greater amount of significant

resonance in the anion accounts for

the nitro increasing the acidity.

Now look at the anion. What does the nitro group do? Remember we

are interested to compare with the phenol phenolate equilibrium.

In these resonance structures charge

is not created. Thus these structuresare important and increase acidity.

They account for the acidity of all

phenols.

O

N

O O

O

N

O O

O

N

O O

O

N

O O

A B C D

Sample Problem


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3. (3 pts) Which is the stronger base and why?

HN

vs

HN O

Sample Problem

H2N H2NO H2N O

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Chap 04 Acids and Bases