4 Acid Base 4ng2v

Acid-Base Reactions

Read section 2-1 not covered in notes

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

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

CH3COOH acid

+ ¯OH base

CH3COO ¯

+

conjugate base

H2O conjugate acid

Lewis acid  substance that can accept a pair of electrons

Lewis base  substance that can donate a pair of electrons FeBr3 + Br2 Lewis acid Lewis base

FeBr4 ¯ Lewis base

+

Br + Lewis acid

Acid-base movie Sec.4 - Acid-base

1

pH = -log[H+]

Brønsted-Lowry acids and bases

Review

pOH = -log[OH]

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

General formula for an acid HA; for a base B HA

H+ + A¯

B + H2O

B+H + ¯OH

kb = [¯OH][B+H] [B]

ka = [H+][A¯] [HA] taking the log of both sides Henderson-Hasselbalch equations pH = pka + log[A¯] [HA]

pOH = pkb + log[B+H] [B]

strong acid [H+]  , ka , pka 

strong base [¯OH]  , kb , pkb 

weak 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 Sec.4 - Acid-base

2

Lewis Acids and Bases Makes you think about the electrons and how bonds are formed arrow show bond breaking arrow show bond formation

gained two electrons and becomes negatively charged

new bond

Sec.4 - Acid-base

3

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

Sec.4 - Acid-base

4

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

NH3 is sp3 hybridized. The HOMO (Highest Occupied Molecular Orbital) is shown to the left and is where the nonbonding lone pair resides.

sp2

BF3 is hybridized The LUMO (Lowest Unoccupied Molecular Orbital) is shown to the right. Most of the volume of the LUMO corresponds to the empty p orbital

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

Sec.4 - Acid-base

5

The first step to organic reactions is understanding acids and bases Why do acids have different acidities? What makes a strong acid a strong acid? An acid is strong if it’s conjugate base is stable. If the base is not stable it is reactive and will react with a proton and shift the equilibrium to the left in the following equilibrium.

HA

A¯ + H+

Acidity increases with increasing anion stability

Hence the strength of an acid is inversely related to the strength of its conjugate base. What makes the base stable? You must look at the structure of the molecule. What is the hybridization of the atom? Is the charge on an electronegative atom? Is the electronegative atom large so that the charge is spread over a larger area? Can the charge be delocalized throughout the molecule? All of these must be considered

Sec.4 - Acid-base

6

1. What atom is the charge on? Acidity increases with increasing anion stability

A¯ + H+

HA

The more electronegative an atom, the greater its ability to carry a negative charge. , electronegativity is the measure of an elements affinity for an electron or its ability to accept an electron. 1. (CH3)3C-H

(CH3)3C¯ + H+

2. (CH3)2N-H

(CH3)2N ¯ + H+

3. CH3O-H

4. H-F

CH3O ¯ + H+

F ¯ + H+

increasing electronegativity of the underlined atom; increasing acid strength increasing stability of anion – shifts equilibrium to products A) B) C) D)

1>2>3>4 4>3>2>1 4>2>3>1 1>4>3>2

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

Sec.4 - Acid-base

B 2.0

C 2.5

N 3.0

O 3.5

F 4.0

Al 1.5

Si 1.8

P 2.1

S 2.5

Cl 3.0

Ga 1.8

Ge 2.0

As 2.2

Se 2.6

Br 2.8

Te 2.1

I 2.5

7

A¯ + H+

HA

1. What atom is the charge on?

A larger anion can disperse the negative charge over a larger volume and thus increases the stability of the conjugate base which is more important than electronegativity when comparing elements down a column in the periodic table. 1. conjugate base

H-F

2. H-Cl

3. H-Br

4. H-I

F¯

Cl ¯

Br ¯

I¯

increasing size of halogen increasing acid strength increasing stability of anion decreasing electronegativity of halogen

E) HI > HBr > HCl > HF

A) B) C) D) E)

1>2>3>4 4>2>1>3 4>2>3>1 1>4>3>2 4>3>2>1

B 2.0

C 2.5

N 3.0

O 3.5

F 4.0

Al 1.5

Si 1.8

P 2.1

S 2.5

Cl 3.0

Ga 1.8

Ge 2.0

As 2.2

Se 2.6

Br 2.8

Te 2.1

I 2.5

Thus use electronegativity when comparing atoms in the same row and size when comparing atoms in the same column Sec.4 - Acid-base

8

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

For binary acids in the same group, H–A bond strength decreases with increasing size of A, so acidity increases.

For binary acids in the same row, H–A polarity increases with increasing electronegativity of A, so acidity increases Sec.4 - Acid-base

9

Which of the hydrogens shown in the following molecule would be most acidic? H 1) O A) 1 B) 2

B 2.0

C 2.5

N 3.0

O 3.5

F 4.0

Al 1.5

Si 1.8

P 2.1

S 2.5

Cl 3.0

Ga 1.8

Ge 2.0

As 2.2

Se 2.6

Br 2.8

Te 2.1

I 2.5

N 2) H

Order the following compounds in decreasing acidity. (most acid to least acidic) 1) CH3OH

A) B) C) D) E)

2) CH3SH

3) CH3CH3

1>2>3 3>2>1 2>3>1 2>1>3 1 > 3 >2 Sec.4 - Acid-base

10

2. The proximity of an electronegative atom to an acidic atom can help stabilize the conjugate base. (Induction) HA A¯ + H+ Inductive effects: Electronegative atoms pull electron density through the sigma bond from adjacent carbons making the carbon more positive. If the carbon is close to the anion this carbon can then pull electron density from the region that has the negative charge to make it more stable. Cl

O

O

O

O

Cl OH

OH

pka

pka

4.83

4.52

H3COH

ClCH2CH2OH

15.5

OH

OH Cl

4.05 increasing acidity increasing anion stability

14.3 increasing acidity increasing anion stability

Sec.4 - Acid-base

2.86

F3CCH2OH 12.4

11

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

HA

A) B) C) D)

A¯ + H+

1>2>3>4 4>2>1>3 4>2>3>1 2>4>3>1

Sec.4 - Acid-base

12

d-

O

Cl

-½

d- Cl d+

d+ d- Cl

d- Cl

O

Cl

O

-1

Cl

-½

ddThe chlorines make the adjacent carbons electron poor. The carbon can then pull electron density from the region that has the negative charge, and this effect will stabilize the negative charge How about these hydrogens Do the electronegative atoms help stabilize the conjugate base

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

d- O

O

These protons are not very acidic

H F

H

Take out

Sec.4 - Acid-base

d-

F

+ H+

+ H+ 13

What effect do alkyl groups Which compound is more acidic? H3 C

O C

H3 C

H3C C

C

CH3

O

H3C

OH

CH3

H3C

O

O

O

more acidic

+ H+

C

C

C

H3C

OH

+ H+ O

more stable conjugate base hyperconjugation through C-H sigma bond. This is not resonance. We will talk more about this later.

H sp3

H

O C

H

only one C-H s bond shown

Sec.4 - Acid-base

sp2

C sp2 O

sp2

Resonance 14

Order the following acids from most acidic to least acidic? A) B) C) D) E)

Alkyl and aryl groups are electron donating through hyperconjugation or inductive effects which makes the anion A) less stable.

H

hyperconjugation through C-H sigma bond.

O

H C

H

B) more stable

1>2>3>4 4>2>3>1 2>3>1>4 3>2>1>4 1>4>2>3

H

C

C H

H

H H

Sec.4 - Acid-base

15

Which of the hydrogens shown in the following molecule would be most acidic? 1) HO F A) 1 B) 2

B 2.0

C 2.5

N 3.0

O 3.5

F 4.0

Al 1.5

Si 1.8

P 2.1

S 2.5

Cl 3.0

Ga 1.8

Ge 2.0

As 2.2

Se 2.6

Br 2.8

Te 2.1

I 2.5

2) HO

Which of the hydrogens shown in the following molecule would be most acidic? 1)

H N

A) 1 B) 2

2)

NH2

Sec.4 - Acid-base

16

3. Resonance Stabilization List the following compounds in order of decreasing acidity (Most acidic to least acidic) 1)

2)

3)

A) B) C) D)

1>2>3 1>3>2 3>2>1 2>3>1

Two resonance structures with the negative charge on two electronegative atoms (oxygen) is more stabilizing than 3 resonance structures with the negative charge on carbons. Sec.4 - Acid-base

17

pKa = 15.5

CH3OH

CH3O ¯ + H+ O

O

O

O

+ H+

C6H5OH pKa = 9.92

O

CH3CO2H

H3C

C

O O

H3C

C

O

+ H+

pKa = 4.74 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

Sec.4 - Acid-base

18

Which of the hydrogens shown in the following molecule would be most acidic? H H2N

A) 1 B) 2

1)

H 2)

A) 1 B) 2

NH 2)

H 1)

H

N

CH3 N

H

H

CH3

H

H

H

CH3 H

H H

H Sec.4 - Acid-base

19

O

H

H 2)

A) 1 B) 2 H

H

1)

H 2)

H

O

O

H H

H

2)

H

1)

1)

Sec.4 - Acid-base

20

4. Hybridization Order the following compounds from strongest to weakest acid 1) (CH3)3CH

pka

A) B) C) D) E)

2) CH2=CH2

3) HCCH

4) NH3

1) (CH3)3C

2) CH2=CH

3) HCC

4) NH2

+ H+

+ H+

+ H+

+ H+

> 70

1>2>3>4 4>3>2>1 2>3>1>4 3>4>2>1 1>4>2>3

44

26

sp3

sp2

sp3

sp

C¯

C¯

N¯

C¯

38

The sp3 negatively charged nitrogen atom is a weaker base than a negatively charged sp3 carbon because it is more electronegative. It is also a weaker base than a sp2 carbon. However it is a stronger base than a sp hybridized carbanion because the sp hybridization of the carbanion has a greater stabilizing effectSec.4 than- increasing Acid-base electronegativity in this case. 21

Which of the hydrogens shown in the following molecule would be most acidic? A) 1 B) 2

1)

H 2)

H

H H

Sec.4 - Acid-base

22

Order the following compounds from strongest to weakest acid

1)

Cl

OH

2)

OH

3) CH3SH

4) CH3OH

A) B) C) D)

Sec.4 - Acid-base

1>2>3>4 4>3>2>1 4>2>3>1 1>4>3>2

23

Bases Strength of Amines The concept of base strength can become confusing when we look at a series of amines in which the base strength varies because of the relative stability of the free amine and it’s conjugate acid. R3N

+

R3NH+

H2O

Free amine (base) acid

+

conjugate acid

HO¯ 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 will shift the equilibrium to the right. that amines are weak bases; there are many other stronger bases for example RO¯ is a much stronger base.

Sec.4 - Acid-base

24

Order the following compounds from strongest to weakest base

pKb = 9.85

pKb = 13

pKb = 9.37

pKb = 3.3 A) B) C) D)

Sec.4 - Acid-base

1>2>3>4 4>3>1>2 4>2>3>1 1>4>3>2

25

Order the bases from strongest to weakest. Cl

O NH2

NH2 1

2

N

NH

H 3

4

Sec.4 - Acid-base

NH2

A) B) C) D) E)

4>3>2>5>1 1>3>2>5>4 3>2>5>1>4 2>3>1>4>5 5>3>2>4>3

5

26

Amides O

O

C

C

O d-

pkb = 13.37

NH2

H3C

H+

H3C

NH2

Very weak bases because of resonance onto electronegative oxygen atom.

d- O d+ C

H3C

NH2

H3C

d+

C

+

NH3

Destabilizing

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

H

H3 C

O

O

C

C

NH2

H3 C

resonance stabilized

NH

Sec.4 - Acid-base

pka = 0.6 27

A) B) C) D)

Order the bases from strongest to weakest

d+ 1)

d-

N

2)

H

O

d-

N

H2O d+

H N H

+ HO¯

d- O

dCl

H

N

N

H2O d+

H

N

H

d+ + HO¯

pkb = 5.67

N

4)

H2O

+ HO¯

Destabilizing

pkb = 2.88

d+

H

d+

H2O

N

3)

Cl

1>2>3>4 4>3>2>1 4>2>3>1 1>4>3>2

H

+ HO¯ Destabilizing

pkb = 8.75

Sec.4 - Acid-base

pkb = 11.45

28

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

A) Left B) Right

CH3CH2O

H

H CH3CH2O

+ HO

O O A) Left B) Right

A) Left B) Right

H O

O

+

O

C

H

H3CH2C N

+

HC

C

N

+

H

H3CH2C

O

O H3C

H

+

H3CH2C HC

O H

O

H3CH2C

A) Left B) Right

+

C H

H + CH3CH2

+

CH3CH3

H Sec.4 - Acid-base

29

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

O

O

O

OH +

+

Cl3C

CCl3

H3C

CH3

Cl3C

CCl3

H3C

CH3

A) Left B) Right

O

O

S H

+

O

S +

O

S

SH

A) Left B) Right

O H A) Left B) Right

O

O O

+

H3C

S

O

O

O O

+

H3C

S

OH

O

Sec.4 - Acid-base

30

If one equivalence of NaOH is added to a solution containing the compound which proton would predominately be removed? 1)

OH 3)

Cl

OH

Cl

A) 1 B) 2 C) 3

OH 2)

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

H N

N

2)

1)

H

Sec.4 - Acid-base

31

Order the bases from strongest to weakest. NH2

H N

N

1

2

A) B) C) D)

1>2>3 1>3>2 3>2>1 2>3>1

3

Sec.4 - Acid-base

32

NH2

H N

Strongest base

A)

B)

N

Strongest base

NH2

A)

N

B)

H N

Strongest base

H3C

1)

Strongest base

Strongest acid

2)

A) CH3CH2OH

C

A) B) C) D)

C

3)

1>2>3 1>3>2 3>2>1 2>1>3

B) CH3CH2O

+

A) CH3CH2OH

B) CH3CH2OH2 Sec.4 - Acid-base

33

Strongest to weakest base. 1) CH3ONa

2) CH3SNa

3)CH3OH

Strongest to weakest acid. 1) CH3SH

2) HI

2) C6H5OH

1>2>3 1>3>2 3>2>1 2>1>3

3) CH3NH2

A) B) C) D)

1>2>3 1>3>2 3>2>1 2>1>3

3) CH3CO2H

A) B) C) D)

1>2>3 1>3>2 3>2>1 2>1>3

Strongest to weakest acid.

1) H2SO4

A) B) C) D)

Sec.4 - Acid-base

34

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

G  -RT lnKeq

HA

A¯ + H+

G° = -ve value the reaction favors the formation of products and if smaller than – 13 kJ mol-1 the is said to go to completion (>99% converted) G° = +ve value the reaction favors the reactants and the formation of products is unfavorable.

G° = H° - TS° Exothermic reaction H ° is -ve Endothermic reaction H ° is +ve S° is +ve - Change from a more ordered system to less a ordered system S° is -ve - Change from a less ordered system to a more ordered system Sec.4 - Acid-base

We will come back to this when we look at reactions 35