Strc Effcts, AcidBasesG

8/6/2019 Strc Effcts, AcidBasesG

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T electron delocalization

(Resonance)

Structural Effects

CH2 CH Cl..

..:

..

..:Cl

CO

NH2

NH2

..

..

..

..

....

CH3 C

O

O H

: :


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T electron delocalization (Resonance)

Structural Effects


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Structural Effects


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Structural Effects

O: :

::

C

O

2

..O O

+

-: : :

....


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HyperconjugationStructural Effects

CH

H

H

CH CH2

CH

H

CH CH2..- -..

C

H

CH CH2

H-

..CH CH CH2

HH

+

H+

H+


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Hyperconjugation

Structural Effects

Dienes

conjugatedcumulative

isolated

CH2

CH CH CH2


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Hyperconjugation

Structural Effects

CH

H H CH

HH

..-

C

H

HH

-

..


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Hyperconjugation

Structural Effects

C

H

H

H

C

HCH

HH

C

-H


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Structural Effects

+3 3

++

-

: 3 O N :-

3. .

+3

3 3


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Inductive EffectStructural Effects

Electron attracting inductiveeffectwith excess positive charges

N+

RR

R

+NH

+NO2

those with electronegative atoms

NH2 OH OCH

F

Cl

Brgroups exhibiting orbital electronegativity

C N N N

those with easily polarizable valence electrons

I-


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Electron attracting inductiveeffect

H3 l

HH

H

H

H3 H2 Cl

H HH

CH3 CH2 OH H

N+

H3C CH3

CH3

--


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Electron repelling inductiveeffectalkyl r s

CH3

negati egr s

COO-

S-

O-

H3C C

O

O

-- H3C S-

CH

H3C

H3C

C N

R C

O

O

C

O

O

O SR

O

O

O


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Steric EffectStructural Effects


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Hydrogen BondingStructural Effects

HF H

O H

N H

H

H

F

O

N

H

H

H

H

H

H

OH

H

H

OH

HO

R

O

H

R

R C

O

O H

H O

C

O

R

C OH

O H

O

F

H

OHOH


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Acids and Bases

BrnstedLowryAcids and Bases

An acid is a proton donor

Abase is a proton acceptor

acid base

H2O H3OHBr Br + +

NH3 H2O+ NH4 + OHacidbase

Note that water can act as an acidora base


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Acids and Bases

BrnstedLowryA

cids and Bases

acid base

H2O H3OHBr Br + +

conjugate

acid

conjugate

base

Every acidbase reaction involvingproton transfer

hastwo conjugateacidbasepairs.

HBr Br+ +H HH

HH


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Acids and Bases

LewisA

cids and BasesALewis acid is an electron pairacceptor.

(think empty orbital)

(think filled orbital)

ALewis base is an electron pairdonor.

The result of a Lewis acidbase reaction is often

called an adduct.


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Acids and Bases

Lewis Acids and Bases

+ OH HH

O

H HH

Lewis acid Lewis base Adduct

Electrondeficient

Electronrich

New covalentbond


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Acids and Bases

LewisA

cids and Bases

Examples of LewisAcids:

Fe3+ AlCl3 H3O+

Examples of Lewis Bases:

H3 H2O Cl


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Acids and Bases


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Acids and Bases

pH = log[H3O+]

HA + H2O A H3O+

Ka=A H O

HA

pKa = log (Ka)


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Acids and Bases

Acid Strength

HA + H2O H O+ + A

-

HA

A-

A-

Has a strong conj. base(@higher energy)

WEAK ACID

STRONG ACID

E

N

E

R

G

Y

ionization

easier

Has a weak conj. base

(@lower energy)


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Acids and Bases


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Acids and Bases

Acid Strength

Factors that influence stability of the

conjugate base include:

Resonance

Electronegativity

Atomic Size

Hybridization

Inductive Effects


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Acids and Bases

Resonance Effects

18

10

5

45

30

25

20

9

28

25

15

pKa Valuesincreasing qualityof resonance

R OH

OH

R C

O

OH

R CH

CH

CH O C

O

CH

R C

O

CH

R C

O

CH2 C R

O

R NH2

NH2

R C

O

NH2


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Acids and BasesThe Acetate Ion

acetateion

aceticacid

CH3 C OH

O

-H+

Base

CH3 C O

O

CH3 C O

O

Resonance Stabilized

Equivalent structures (charges on oxygens)


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Acids and Bases

O

-

More resonance structures, but not more stable than acetate

Nonequivalent structures (note charges on carbon and oxygen)

O O O O O

The Phenolate Ion


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Acids and Bases

ElectronegativitypKa Valuesincreasingelectronegativity

C

O

CH3R 20

15

5

C

O

NH2R

C

O

OHR

CH4

NH3

H2O

HF

>45

34

16

3.5

RCH3

RNH2

ROH

45

35

18


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Acids and Bases

F

Cl

I

Br

Electronegativity

pK

a Values

increasing

size

HF

HCl

HBr

HI

3.5

7

9

10

H2O

H2S

H2Se

H2Te

16

7

4

3

C

O

OHR

C

O

SHR

C

S

SHR

1.36

1.81

1.95

2.16


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Acids and Bases

Inductive Effects

Electron-withdrawing Groups

F, Cl, Br, O,N R, CH , B, Si

electronegative elementspull electron density

away from carbon

alkyl groups and elementsless electronegative than carbon

push electron density toward carbon

Electron-donating Groups

Cl C

H H

CH3

C

HH


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Acids and Bases

Inductive Effects

Chlorine helps to stabilize

CO2by withdrawing

electronsCl C

O

O

HH

This effect diminishes with distanceit extends for about 3 bonds

C C

O

CClHHH

H

HH

O


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Acids and Bases

Inductive Effects

3.13

2.87

2.81

2.66

4.75

2.81

1.29

0.65

pKa Values

increasing

electronegativity

increasing

substitution

I CH2COOH

Br CH2COOH

Cl CH2COOH

F CH2COOH

CH COOH

CH2 COOHCl

CH COOHCl

Cl

C COOHCl

Cl

Cl


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Electrophiles e- pair acceptor

electron poor

may possess formal positive charge

incomplete octets

greatly reduced e- density

those that can yield e- poor species

R

C+

RR

R .

NO2+CH2:

R C+

O

CO2

H2O

H+ BF3

FeBr3

AlCl3X2 in r tic s lvent

X.

R C

O

O OH

SO3

SO3H+

X+

CH3 AlCl4

H H


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Nucleophiles

e- pair donor

excess negative charges fractional negative charges

may have a lone pair e-

RCR

R

: -

OH-

RO-

X-

RC

O

O-

CN-

NH2-

N -

O-


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Strc Effcts, AcidBasesG