Chapter 4 Acid - Base Chemistry

4-

Brønsted-Lowry and Lewis Acids/Bases Acid Dissociation Constants, pKa, the Relative strength of Acids and Bases. [electron pushing, arrows, electronic view rxs.]

Equilibrium in Acid-Base Reactions Molecular structure and acidity [periodicity: electronegativity & size of atom hybridization, inductive effects, resonance]

See the Chapter Summary

H

H

CH3

F H3C

H

O

HO

H3C F H3C

4-

OH C

C

C

H

H

O

+

+

C

H

Chapter 4

δ+ Br Mg

H

H

δ− CH3 LiCu

OH

C

H

O

CH2

I C

H3C D

acid-base strength

CH3

general, Bronsted Lowry, Lewis Acid-Base

h t n

CH3

H

s i s e

2

H

(-)

I

(-):CN:

O

O

NC D H3C

H

H3C

X

Cl

H

H 3C O

H2 / M

O

sy

structure effects on acidity

b5

O OH

ab

H

O

O

b2

b4 OH

oad

OH

j

b4

ma

p

k i

c3

b1

OR

Br

OH O

is r

OH

Br

4-

hes

OR

b7

b6

O

syn t

road trip started

Cl

O

c4

b1 b

u

OR

v

OR

h z

Nuc(Anionic)

b3 g

r

t

Br

c a

Br

OH

l

OH

m

o

f

Br

n

1 other product OHOH

e

2

R

a7 Br O Br

R

NH2

Li

CuLi

OH

O

a2 y

OH

w

u

OR

u

a8

b

c

x

H O H

OH OH

2 other product

HO

H

H

CH3

OH C

H3C F H3C

F H3C

H

4-

O

HO C

C H

H O

+

C

H

Chapter 4

δ+ Br Mg

H

OH

C

H

O

CH2

O

O

H

δ− CH3

I C

H3C D

acid-base strength

general, Bronsted Lowry, Lewis Acid-Base

th n y

CH3

H

s i s e

2

H

(-)

I

(-):CN:

H

LiCu

NC D H3C

+

H3C

X

Cl

H

H 3C O

H2 / M

O

CH3

s

structure effects on acidity

Arrhenius Acids and Bases

4-

acid: substance that produces H3O+ ions aqueous solution

H+(aq) + H2O(I)

H3O+(aq) hydronium ion

base: substance that produces OH- ions in aqueous solution

KOH

H 2O

K+ + -OH(aq)

Brønsted-Lowry Definitions Acid: a proton donor Base: a proton acceptor

4-

Conjugate Acids & Bases Brønsted-Lowry does not require water only H+ transfer

4-

Pi Electrons As Basic Sites

4-

pi electrons - base - donate a pair of e-s

Result: formation of a carbocation, C has 6-e-s, ∴+1charge

Lewis Acids and Bases Lewis acid: molecule/ion that accepts a pair of electrons Lewis base: molecule/ion that donates a pair of electrons

forms a new covalent bond

4-

Lewis Acids and Bases examples

carbocations very strong Lewis acid

4-

H

H

CH3

OH C

H3C F H3C

F H3C

H

4-

O

HO C

C H

H O

+

C

H

Chapter 4

δ+ Br Mg

H

OH

C

H

O

CH2

O

O

H

δ− CH3

I C

H3C D

acid-base strength

general, Bronsted Lowry, Lewis Acid-Base

th n y

CH3

H

s i s e

2

H

(-)

I

(-):CN:

H

LiCu

NC D H3C

+

H3C

X

Cl

H

H 3C O

H2 / M

O

CH3

s

structure effects on acidity

Acids & Base Strengths

4-

How is acid [base] strength expressed/compared?

By equilibrium constant e.g.: dissociation (ionization) of acetic acid O

+ H3C C O H

H O H

pKa = -log Ka

O H3C C O

+

H

O H

H

4-

3.18 HF

Acid-Base Equilibria

4-

What favors the direction of acid - base reactions?

Strong acid - completely to product(s) Weak - incomplete, gives an Equilibrium direction favors reaction of stronger acid-base pair

pKa 4.76

pKa 9.24

Acid-Base Equilibria

4-

acetic acid + sodium bicarbonate (omit Na+ ) CO2 + H 2O pKa= 4.76

6.36

H

H

CH3

OH C

H3C F H3C

F H3C

H

4-

O

HO C

C H

H O

+

C

H

Chapter 4

δ+ Br Mg

H

OH

C

H

O

CH2

O

O

H

δ− CH3

I C

H3C D

acid-base strength

general, Bronsted Lowry, Lewis Acid-Base

th n y

CH3

H

s i s e

2

H

(-)

I

(-):CN:

H

LiCu

NC D H3C

+

H3C

X

Cl

H

H 3C O

H2 / M

O

CH3

s

structure effects on acidity

Acid-Base Theories

4-

Brønsted-Lowry: acid-proton donor, base-proton acceptor

Lewis: acid-accept pair electrons, base-donate pair electrons

How does Structure effect Acidity?

4-

relative acidities the more A:(-) stable, greater the acidity of H-A

Ways to stabilize A:(-) the negative charge ON a more electronegative atom ON larger atom RESONANCE delocalized STABILIZED by inductive effect IN an orbital with more s character

electronegativity

4-

within a period - the greater the electronegativity of A:(-) the more A:(-) is stablized ∴ the stronger the acid incerasing acidity Conjugate base

H3C N H H H pKa 51 H3C C H H

pKa 38

CH3 O

CH3 N

H

CH3

H C H

increasing basicity

pKa 16 H3C O H

increasing acidity

Acid

4-

Size of A:(-)

Within a column -

the larger the atom bearing the (-), the greater its stability H3C S H pKa 7.0

+ CH3 O

CH3 S

+

H3C O H pKa 16

4-

inductive effect, electron-withdrawing -covalent bonds transmit electronegativity (polarizing) effects -push or pull shared e(-)s of adjacent atoms

decreases w/ distance

4-

inductive effect: O OH pKa 4.82

Cl

butanoic and chlorobutanoic acids Cl

O OH pKa 4.52

O

O OH

pKa 3.98

OH Cl pKa 2.83

4-

Hybridization greater the % s character with (-) the more stable the anion Conjugate pKa (s-character) Base

Weak Acid

H C C H H

H

C C H H H H H C C H H H

HO-H

water

H C C

25

(50%)

44

(33%)

H

C C H H H H C C H H H

HO–

51 15.7

(25%)

Conjugate Acids & Bases

4-

curved arrows show the flow of electrons in an acid-base reaction

recall resonance - also uses curved arrows

4-

Resonance delocalized of charge on A:

(-)

Compare alcohol and carboxylic acid acidity H H H C + H C O O HH H H

CH3

O C O

+

H

H O H

H H C H C O HH

CH3

CH3

O C O

+ H O

+ H O

H

pKa = 15.9

H

H H

pKa = 4.76

O C O

resonance stabilization (of A:-)

Use of resonance theory molecules may have 2 or more sites that can accept a H+ e.g. carboxylic acids, esters, and amides protonation favored where the charge is more delocalized which oxygen is protonated?

4-

Use of resonance theory resonance octets greater contribution even with plus charge on O

4-

Use of resonance theory

4-

H+ on the hydroxyl

∴ H+ on the carbonyl

can “write” contributing structures create & separate charge

Acid - Base Chemistry.pdf

acid-base. strength structure effects. on acidity. general,. Bronsted Lowry,. Lewis. Acid-Base. Page 4 of 28. Acid - Base Chemistry.pdf. Acid - Base Chemistry.pdf.

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