4_acid_base

8/3/2019 4_acid_base

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Sec.4 - Acid-base 1

Acid-Base Reactions

Bronsted-Lowry acid substance that can donate a hydrogen ion (H+)Bronsted-Lowry base substance that can accept a hydrogen ion (H+)

CH3COOH + OH CH3COO + H2O

acid base conjugate base conjugate acid

Lewis acid substance that can accept a pair of electronsLewis base substance that can donate a pair of electrons

FeBr3 + Br2 FeBr4 + Br+

Lewis acid Lewis base Lewis base Lewis acid

Volhardt: 2-1, 2-2, 8-3, 9-1, 13-2, 19-4

Read section 2-1not covered in notes

Acid-base movie
http://acid-base/acid-base.htmlhttp://acid-base/acid-base.htmlhttp://acid-base/acid-base.htmlhttp://acid-base/acid-base.html


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Sec.4 - Acid-base 2

pH = -log[H+] pOH = -log[OH]

kw = [H+][ OH] = 1 x 10-14

pH + pOH = 14

neutral solution [H+] = [ OH] = 1 x 10-7 pH = 7acidic solution [H+] > 1 x 10-7 pH range 0 to 7basic solution [H+] < 1 x 10-7 pH range 7 to 14

Review

General formula for an acid HA; for a base B

HA H+ + A B + H2O B+H + OH

ka = [H+][A ] kb = [ OH][B+H]

[HA] [B]

taking the log of both sides

Henderson-Hasselbalch equations

pH = pka + log[A ] pOH = pkb + log[B+H]

[HA] [B]

strong acid [H+] , ka, pka strong base [ OH] , kb, pkbweak acid [H+] , ka, pka weak base [ OH] , kb, pkb

relationship between conjugate acid base pairs

ka x kb = kw =1 x 10-14

pka + pkb = 14

Brnsted-Lowryacids and bases


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Sec.4 - Acid-base 3

Lewis Acids and Bases

Makes you think about the electrons and how bonds are formed

arrow show bond formationarrow show bond breaking

new bond

gained two electronsand becomesnegatively charged


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Sec.4 - Acid-base 4

Opposite Charges Attract and React

BF3 has substantial positive charge on the boron

NH3 has substantial negative charge localized at the lone pair

-ve goes to+ve


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Sec.4 - Acid-base 5

BF3 is sp2 hybridized

The LUMO (LowestUnoccupied MolecularOrbital) is shown to theright. Most of thevolume of the LUMOcorresponds to theempty p orbital

The base donates its electrons that occupy the HOMO to theLUMO of the acid to produce a new sigma bond

NH3 is sp3 hybridized.

The HOMO (HighestOccupied MolecularOrbital) is shown tothe left and is wherethe nonbonding lonepair resides.

The van der Waals surface electron density is indicated by the mesh


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Sec.4 - Acid-base 6

Why do acids have different acidities? What makes a strong acid astrong acid? An acid is strong if its conjugate base is stable. If the

base is not stable it is reactive and will react with a proton and shiftthe equilibrium to the left in the following equilibrium.

HA A + H+

Hence the strength of an acid is inversely related to the strength of itsconjugate base. What makes the base stable? You must look at thestructure of the molecule. What is the hybridization of the atom? Is thecharge on an electronegative atom? Is the electronegative atom large so

that the charge is spread over a larger area? Can the charge bedelocalized throughout the molecule? All of these must be considered

Acidity increases with increasing anion stability

The first step to organic reactions is understanding acids and bases


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Sec.4 - Acid-base 7

1. What atom is the charge on?

Acidity increases with increasing anion stabilityThe more electronegative an atom, the greater its ability to carry a negative

charge. Remember, electronegativity is the measure of an elements affinity foran electron or its ability to accept an electron.

(CH3)3C-H (CH3)2N-H CH3O-H H-F

B

2.0C

2.5N

3.0O

3.5F

4.0

Al

1.5Si

1.8P

2.1S

2.5Cl

3.0

Ga

1.8Ge

2.0As

2.2Se

2.6Br

2.8

Te

2.1I

2.5

increasing electronegativity of the underlined atom;increasing acid strength

increasing stability of anion shifts equilibrium to products

(CH3)3C + H+ (CH3)2N + H

+ CH3O + H+ F + H+

HA A + H+

B) HF > CH3OH > CH3)2NH > (CH3)2CH

A) 1 > 2 > 3 > 4B) 4 > 3 > 2 > 1C) 4 > 2 > 3 > 1D) 1 > 4 > 3 > 2

1. 2. 3. 4.


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Sec.4 - Acid-base 8

1. What atom is the charge on?

A larger anion can disperse the negative charge over a larger volume and thusincreases the stability of the conjugate base which is more important thanelectronegativity when comparing elements down a column in the periodic

table.

H-F H-Cl H-Br H-I

B

2.0C

2.5N

3.0O

3.5F

4.0

Al

1.5Si

1.8P

2.1S

2.5Cl

3.0

Ga

1.8

Ge

2.0

As

2.2

Se

2.6

Br

2.8Te

2.1I

2.5

Thus use electronegativity when comparing atoms in the samerow and size when comparing atoms in the same column

F Cl Br I conjugatebase

HA A + H+

increasing size of halogen increasing acid strengthincreasing stability of anion

decreasing electronegativity of halogen

1. 2. 3. 4.

E) HI > HBr > HCl > HF

A) 1 > 2 > 3 > 4B) 4 > 2 > 1 > 3C) 4 > 2 > 3 > 1D) 1 > 4 > 3 > 2E) 4 > 3 > 2 > 1


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Sec.4 - Acid-base 9

The electrostatic potential maps show all the hydrohalic acids are polar.The variation in polarity is less significant than the bond strength whichdecreases from 567 kJ/mol for HF to 299 kJ/mol for HI.

Can also look at bond strength

For binary acids in the same group, HA bond strength decreases withincreasing size of A, so acidity increases.

For binary acids in the same row, HA polarity increases with increasingelectronegativity of A, so acidity increases


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Sec.4 - Acid-base 10

Order the following compounds in decreasing acidity. (most acid to least acidic)

B

2.0C

2.5N

3.0O

3.5F

4.0

Al

1.5Si

1.8P

2.1S

2.5Cl

3.0

Ga

1.8Ge

2.0As

2.2Se

2.6Br

2.8

Te

2.1I

2.5

1) CH3OH 2) CH3SH 3) CH3CH3

A) 1 > 2 > 3B) 3 > 2 > 1C) 2 > 3 > 1D) 2 > 1 > 3E) 1 > 3 >2

Which of the hydrogens shown in the following moleculewould be most acidic?

O

H

N

H

1)

2)

A) 1B) 2


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2. The proximity of an electronegative atom to an acidic atom canhelp stabilize the conjugate base. (Induction)

Inductive effects: Electronegative atoms pull electron density through the sigmabond from adjacent carbons making the carbon more positive. If the carbon is closeto the anion this carbon can then pull electron density from the region that has thenegative charge to make it more stable.

pka

4.83 4.52 4.05 2.86

increasing acidity

increasing anion stability

H3COH ClCH2CH2OH F3CCH2OH

pka 15.5 14.3 12.4increasing acidity

increasing anion stability

HA A + H+

OH

O

OH

O

ClOH

OCl

OH

O

Cl


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2. The proximity of an electronegative atom to an acidic atom canhelp stabilize the conjugate base. (Induction)

HA A + H+

A) 1 > 2 > 3 > 4B) 4 > 2 > 1 > 3C) 4 > 2 > 3 > 1D) 2 > 4 > 3 > 1


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O

O

Cl

Cl

Cl

-

-

d+

d-d-

d-O

Cl

Cl

Cld+

d-

d-d-

- 1

The chlorines make the adjacent carbons electron poor. Thecarbon can then pull electron density from the region that has thenegative charge, and this effect will stabilize the negative charge

O

H

F

H

How about these hydrogensDo the electronegative atoms

help stabilize the conjugate base

F

O-

d-

This is destabilizing. The lone pairs onthe electronegative atom and the lonepair on the carbon repel each other.

+ H+

+ H+

These protons arenot very acidic

Take out


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What effect do alkyl groups

CCH3C

H3C CH3

O

OH

Which compound is more acidic?

C

O

H3C OH

CC

H3C

H3C CH3

O

O

C

O

H3C O

+ H+

+ H+

H

C

O

C

HH O

Resonance

hyperconjugationthrough C-H sigma

bond. This is notresonance. We willtalk more about

this later.

more acidic

more stable conjugate base

sp2

sp2

sp2sp3

only one C-H sbond shown


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Order the following acids from most acidic to least acidic?

A) 1 > 2 > 3 > 4

B) 4 > 2 > 3 > 1C) 2 > 3 > 1 > 4D) 3 > 2 > 1 > 4E) 1 > 4 > 2 > 3

Alkyl and aryl groups areelectron donating through

hyperconjugation or inductiveeffects which makes theanion

hyperconjugationthrough C-H sigmabond. C

O

C

HH

C

HH

H

H

H

H

A) less stable.B) more stable


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B

2.0C

2.5N

3.0O

3.5F

4.0

Al

1.5Si

1.8P

2.1S

2.5Cl

3.0

Ga

1.8

Ge

2.0

As

2.2

Se

2.6

Br

2.8

Te

2.1I

2.5


HO F

HO

1)

2)

A) 1B) 2

1)

2)N

H

NH2

Which of the hydrogens shown in the following molecule would be most acidic?

A) 1

B) 2


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3. Resonance Stabilization

List the following compounds in order of decreasing acidity (Most acidic to leastacidic)

1)

Two resonance structures with the negative charge on two electronegativeatoms (oxygen) is more stabilizing than 3 resonance structures with thenegative charge on carbons.

A) 1 > 2 > 3

B) 1 > 3 > 2C) 3 > 2 > 1D) 2 > 3 > 1

2) 3)


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O

O O O

CH3C

O

O CH3C

O

O

CH3O + H+

+ H+

+ H+

Phenol has more resonance structures than acetic acid but acetic acid is more acidic, why?

Which atom is able to handle a negative charge better, carbon or oxygen?

Oxygen can. Thus resonance onto two oxygens is more stabilizing than three carbons

CH3OH

C6H5OH

CH3CO2H

pKa = 15.5

pKa = 4.74

pKa = 9.92


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1)

2)

NH

H2NA) 1B) 2

1)

2)

H

H H

A) 1B) 2

CH3

HH

H

H

CH3

H

H

HNCH3N

HH

H


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1)

2)O

H H

H H

A) 1B) 2

1)

2)O

H

H H

1)

2)O

H

H

H


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pka > 70 44 26 38

4. Hybridization

Order the following compounds from strongest to weakest acid

1) (CH3)3CH 2) CH2=CH2 3) HCCH 4) NH3

sp3

C

sp2

C

sp3

N

sp

C

A) 1 > 2 > 3 > 4B) 4 > 3 > 2 > 1C) 2 > 3 > 1 > 4D) 3 > 4 > 2 > 1E) 1 > 4 > 2 > 3

The sp3 negatively charged nitrogen atom is a weaker base thana negatively charged sp3 carbon because it is moreelectronegative. It is also a weaker base than a sp2 carbon.However it is a stronger base than a sp hybridized carbanionbecause the sp hybridization of the carbanion has a greater

stabilizing effect than increasing electronegativity in this case.

1) (CH3)3C 2) CH2=CH 3) HCC 4) NH2+ H+ + H+ + H+ + H+


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Which of the hydrogens shown in the following molecule would be most acidic?

1)2)

H

H

H

H

A) 1B) 2


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OH OH1) 2) 3) CH3SH 4) CH3OHCl

A) 1 > 2 > 3 > 4B) 4 > 3 > 2 > 1C) 4 > 2 > 3 > 1D) 1 > 4 > 3 > 2

Order the following compounds from strongest to weakest acid


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The concept of base strength can become confusing when we look at a series ofamines in which the base strength varies because of the relative stability of the

free amine and its conjugate acid.

R3N + H2O R3NH+ + HO

Free amine (base) acid conjugate acid conjugate base

From the above equation we can see that anything that stabilizes the free amine(base) relative to the conjugate acid makes the base less basic (shifts the

equilibrium to the left).

Anything that stabilizes the conjugate acid makes the base more basic and willshift the equilibrium to the right.

Remember that amines are weak bases; there are many other stronger bases forexample RO is a much stronger base.

Bases Strength of Amines


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A) 1 > 2 > 3 > 4B) 4 > 3 > 1 > 2C) 4 > 2 > 3 > 1D) 1 > 4 > 3 > 2

Order the following compounds from strongest to weakest base

pKb = 3.3pKb = 9.37pKb = 9.85 pKb = 13


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Order the bases from strongest to weakest.

O

NH2NH

2

N

HNH

1 2 3 4

NH2

Cl

5

A) 4 > 3 > 2 > 5 > 1B) 1 > 3 > 2 > 5 > 4C) 3 > 2 > 5 > 1 > 4D) 2 > 3 > 1 > 4 > 5

E) 5 > 3 > 2 > 4 > 3


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C

H3C

O

NH2

C

H3C

O

NH2

C

H3C

O

NH2

Amides

+d+

Very weak bases because of resonance ontoelectronegative oxygen atom.

pkb = 13.37

d-

C

O

NH3H3C

d+

d-

Destabilizing

H+

In fact the oxygen is protonated with a strong acid not the nitrogen

C

O

NH2H3C

H

C

O

NHH3C

H

resonance stabilized

pka = 0.6


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Order the bases from strongest to weakest

N H

Cl

N

H

NO

H

N N

Cl

pkb = 2.88 pkb = 5.67 pkb = 8.75 pkb = 11.45

d+

d+

d-d+-

N

H

H NO

H

H

H2O

d+

d+d-

N H

d+d-

Destabilizing Destabilizing

A) 1 > 2 > 3 > 4B) 4 > 3 > 2 > 1C) 4 > 2 > 3 > 1D) 1 > 4 > 3 > 2

1) 2) 3) 4)

H2O H2O H2O

+ HO + HO + HO + HO


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Predict which way the equilibrium is shifted in the following reactions.Also show the flow of electrons in the acid-base reaction

A) LeftB) Right

A) LeftB) Right

A) LeftB) Right

A) LeftB) Right

CH3CH2OH

HO CH3CH2O

H

O H

HC C H N

H3CH2C

H3CH2C

HC C N

H3CH2C

H3CH2C

H

H3C

O

CH

HH

CH3CH2

O

CH3CH3

O

O

H

O

O

OO

+ +

+ +

++

+ +H


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Predict which way the equilibrium is shifted in the following reactions.Also show the flow of electrons in the acid-base reaction

A) LeftB) Right

A) LeftB) Right

A) LeftB) Right

O

O

H

+

+

+

O

CCl3Cl3C

HO

CH3H3C

OH

CH3H3C

O

CCl3Cl3C

+

S

S

O

O

+SH

S

S

O

H3C O

O

O

O

H+ S

O

H3C OH

O

O

O


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If one equivalence of NaOH is added to a solution containing the compound whichproton would predominately be removed?

If one equivalence of LiCH3 is added to a solution containing the compound whichproton would predominately be removed?

1)

2)

OH

OH

OHCl

Cl

3)

1)

2)N N

H

H

A) 1B) 2C) 3


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Order the bases from strongest to weakest.

1 2 3

HN N

NH2 A) 1 > 2 > 3B) 1 > 3 > 2C) 3 > 2 > 1

D) 2 > 3 > 1


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HN

N

NH2

NH2

NHN

H3C C C

A) B)

A) B)

1) 2) 3)

A) CH3CH2OH B) CH3CH2OH2+

A) 1 > 2 > 3B) 1 > 3 > 2C) 3 > 2 > 1D) 2 > 1 > 3

Strongest base

Strongest base

Strongest base

A) CH3CH2OH B) CH3CH2OStrongest base

Strongest acid

St t t k t b


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1) CH3ONa 2) CH3SNa 3)CH3OH

A) 1 > 2 > 3B) 1 > 3 > 2C) 3 > 2 > 1D) 2 > 1 > 3

1) CH3SH 2) HI 3) CH3NH2

1) H2SO4 2) C6H5OH 3) CH3CO2H

A) 1 > 2 > 3B) 1 > 3 > 2C) 3 > 2 > 1D) 2 > 1 > 3

A) 1 > 2 > 3B) 1 > 3 > 2

C) 3 > 2 > 1D) 2 > 1 > 3

Strongest to weakest base.

Strongest to weakest acid.

Strongest to weakest acid.


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Sec 4 Acid base 35

We are always looking at an equilibrium and which way it isshifted in a specific environment

HA A + H+

eqKRTG ln

G = -ve value the reactionfavors the formation of productsand if smaller than 13 kJ mol-1the is said to go to completion(>99% converted)

G = +ve value the reactionfavors the reactants and theformation of products isunfavorable.

G = H - TS

Exothermic reaction H is -veEndothermic reaction H is +ve

We will come backto this when welook at reactions

S is +ve - Change from a more ordered system to less a ordered systemS is -ve - Change from a less ordered system to a more ordered system

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