Structural Effects and Acid-Base Property (ORG CHEM)

8/14/2019 Structural Effects and Acid-Base Property (ORG CHEM)

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Roldan M. de GuiaDepartment of BiochemistryFaculty of Pharmacy

University of

Santo Tomas


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electron delocalization

(Resonance)

Structural Effects

CH2 CH Cl..

..:

..

..:Cl

CO

NH2

NH2

..

..

..

..

..

..CH3 C

O

O H

: :


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electron delocalization (Resonance)Structural Effects


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http://en.wikipedia.org/wiki/Image:Benz4.png


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OH

: :

::

C

O

H

NH2..

N

O O

+

-

: : :....


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HyperconjugationStructural Effects

C

H

H

H

CH CH2

CHH

CH CH2

..- -..

CH

CH CH2

H-..

CH CH CH2

HH

+

H

+

H+


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Hyperconjugation

Structural Effects

Dienes

conjugated

cumulative

isolated

CH2 CH CH CH2


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Hyperconjugation

Structural Effects

C

H

H H C

H

HH+

..-

C

H

HH+

-..


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Hyperconjugation

Structural Effects

C

H

H

H

C

O

H

CH

HH

CO

-H+

OO


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Inductive EffectStructural Effects

Electron attracting inductiveeffectwith excess positive charges

N+

RR

R

+NH3

+NO2

those with electronegative atoms

NH2 OH OCH3

F

Cl

Brgroups exhibiting orbital electronegativity

C N N N

those with easily polarizable valence electrons

I-


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Electron attracting inductiveeffect

H3C Cl

+

+

CH3 CH2 Cl

++

+

CH3 CH2 OH +

N+

H3C CH3

CH3

--


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Electron repelling inductiveeffectalkyl groups

CH3

negative groups

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

+F H

O H

N H

+

+

F

O

N

H

H

H

+

+

+

OH

H

HO

H

HO

R

O

H

R

R C

O

O H

H O

C

O

R

C O

H

O H

O

F

H

OHOH


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Acids and Bases

BrnstedLowry Acids and Bases

An acid is a proton donor

A base 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

BrnstedLowry Acids and Bases

acid base

H2O H3OHBr Br + +

conjugate

acid

conjugate

base

Every acidbase reaction involving proton transfer

has two conjugate acidbase pairs.

HBr Br+ +O

H H

O

H

HH


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Acids and Bases

Lewis Acids and Bases

A Lewis acid is an electron pairacceptor.(think empty orbital)

(think filled orbital)

A Lewis base is an electron pairdonor.

The result of a Lewis acidbase reaction is often

called an adduct.


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Acids and Bases


+

O

H HH

O

HH

H

Lewis acid Lewis base Adduct

Electrondeficient

Electronrich

New covalentbond


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Acids and Bases


Examples of Lewis Acids:

Fe3+ AlCl3 H3O+Examples ofLewis Bases:

NH3

H2

O Cl


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Acids and Bases


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Acids and Bases

pH = log[H3O+]

HA + H2O A H3O+

Ka = AH

3O

HA

pKa = log (Ka)

id d


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Acids and Bases

Acid Strength

HA + H2O H3O+ + A-

HA

A-

A-

Has a strong conj. base( higher energy)

WEAK ACID

STRONG ACID

E

N

E

R

GY

ionization

easier

Has a weak conj. base

( lower energy)

A id d


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Acids and Bases

A id d B


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Acids and Bases

Acid StrengthFactors that influence stability of the

conjugate base include:

Resonance

Electronegativity

Atomic Size

Hybridization

Inductive Effects

A id d B


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Acids and Bases

Resonance Effects

18

10

5

45

30

25

20

9

28

25

15

pKa Valuesincreasing quality

of resonance

R OH

OH

R C

O

OH

R CH3

CH3

CH3O C

O

CH3

R C

O

CH3

R C

O

CH2

C R

O

R NH2

NH2

R C

O

NH2

A id d B


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Acids and BasesThe Acetate Ion

acetate ion

acetic acid

CH3 C OH

O

-H+

Base

CH3 C O

O

CH3 C O

O

Resonance Stabilized

Equivalent structures (charges on oxygens)

A id d B


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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

A id d B


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Acids and Bases

ElectronegativitypKaValuesincreasingelectronegativity

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

A id d B


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Acids and Bases

F

Cl

I

Br

ElectronegativitypKaValues

increasingsize

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

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1.81

1.95

2.16

A id d B


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Acids and Bases

Inductive Effects

Electron-withdrawing Groups

F, Cl, Br, O, N R, CH3, 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

+CH3 C

+

A id d B


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Acids and Bases

Inductive Effects

Chlorine helps to stabilizeCO2

by withdrawing

electronsCl C

O

O

+

This effect diminishes with distanceit extends for about 3

bonds

C C

O

CCl

+++ O

A id d B


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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

CH3 COOH

CH2 COOHCl

CH COOHCl

Cl

C COOHCl

Cl

Cl

Electrophiles


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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+

R

R

R.

NO2+CH2:

R C+

O

CO2

H2O

H+BF

3

FeBr3

AlCl3X2 in protic solvent

X.

R C

O

O OH

SO3

SO3H+

X+

CH3 AlCl4

+


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Nucleophiles

e- pair donor

excess negative charges

fractional negative charges

may have a lone pair e-

R

CR

R

: -

OH-

RO-

X

-

RCO

O-

CN-

NH2-

N3-

O-


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Structural Effects and Acid-Base Property (ORG CHEM)