Structural Effects And Acid-base Property (org Chem)

Roldan M. de Guia Department of Biochemistry Faculty of Pharmacy University of Santo Tomas

Structural Effects π electron delocalization (Resonance) CH2

.. : CH Cl ..

.. : Cl ..

.. NH2

.. O .. C .. NH2

:O: CH3

C

.. O .. H

Structural Effects

π electron delocalization (Resonance)

Structural Effects

π electron delocalization (Resonance)

Structural Effects

π electron delocalization (Resonance)

:O:

.. :O

H

.. :O: N+

:O: C H

.. NH2

Structural Effects

Hyperconjugation

H H C CH CH2 H +

H

..-

H C CH CH2 H

H

..+ H C CH CH2 H

H

..H C CH CH2 H

+

Structural Effects

Hyperconjugation

Dienes

CH2

CH CH CH2

conjugated

isolated

cumulative

Structural Effects

Hyperconjugation

H

H

H C H

H C H

H H C H

+

-

+

..

.. -

Structural Effects

Hyperconjugation H

O

H C C H

O

H

+

H O H C C H H

O

Structural Effects +

CH3

+

CH3

CH3

+

CH2

CH2 H3C

H -

:CH3

-

O2N

C: H H

. CH

3

C. H

C

+

CH3

Structural Effects

Inductive Effect

Electron attracting inductive with excess positive charges effect +

+

R N R

NH3

+

NO2

R those with electronegative atoms NH2

OH

F

OCH3

Cl Br groups exhibiting orbital electronegativity C N

N N

those with easily polarizable valence electrons I

-

Structural Effects

Inductive Effect

Electron attracting inductive effect δ+ δ− H3C Cl δ+

δ++

δ−

CH3

CH2

Cl

CH3

CH3 +

H3C N CH3 --

δ+ CH2

δ− O

H

δ+

Structural Effects

Inductive Effect

Electron repelling inductive alkyl groups effect CH3

negative groups -

COO

O H3C C

--

S

-

H3C

-

H3C H3C

O

O

O C

O

S

CH C N

O

R C

-

O

O O

R

S O O

Structural Effects

Steric Effect

Structural Effects

Hydrogen Bonding F

H δ+

O

H δ+

N

R

O

H δ+

F

H

O

H

N

H

δ+

δ+ δ+

H

O

O H

F

O H O

H O H H

H O R

R C

H

C R O H O

H C O O H

OH

OH

Acids and Bases

Brønsted–Lowry Acids and Bases  An acid is a proton donor  A base is a proton acceptor HBr

+ H2O

acid

base

NH3 + H2O base acid

Br + H3O

NH4 + OH

Note that water can act as an acid or a base

Acids and Bases

Brønsted–Lowry Acids and Bases HBr HBr acid

+

+ H2O H

O

Br + H3O H

base

Br

+

conjugate base

O H

H

H

conjugate acid

Every acid–base reaction involving proton transfer has two conjugate acid–base pairs.

Acids and Bases

Lewis Acids and Bases  A Lewis acid is an electron pair acceptor. (think empty orbital)

 A Lewis base is an electron pair donor. (think filled orbital) The result of a Lewis acid–base reaction is often called an adduct.

Acids and Bases

Lewis Acids and Bases H

+

H

O

O

H

Lewis acid

Lewis base

Electron deficient

Electron rich

H

H

H

Adduct New covalent bond

Acids and Bases

Lewis Acids and Bases Examples of Lewis Acids: Fe3+ AlCl3

H3O+

Examples of Lewis Bases: NH3 H2O

Cl

−

Acids and Bases

Acids and Bases  pH = – log[H3O+] A + H3O

HA + H2O

Ka =

A

H3O

HA

pKa = – log (Ka)

Acids and Bases

Acid Strength HA + H2O

H3O+ +

A-

- WEAK ACID A Has a strong conj. base E N E R G Y

(∴ higher energy)

HA

ionization easier

- STRONG ACID A Has a weak conj. base (∴ lower energy)

Acids and Bases

Acids and Bases

Acid Strength  Factors that influence stability of the conjugate base include: • • • • •

Resonance Electronegativity Atomic Size Hybridization Inductive Effects

Acids and Bases increasing quality of resonance

R OH

Resonance Effects pKa Values 18

R CH3 CH3

30

CH3O C CH3 O

25

OH 10 O R C OH

5

45

R NH2

NH2 25

O

R C CH3 O

28

20

O

R C CH2 C R 9

O R C NH2

15

Acids and Bases

 The Acetate Ion

O

O CH3

C OH

-H+

CH3

C O

Base

acetic acid

O CH3

C O

acetate ion Resonance Stabilized Equivalent structures (charges on oxygens)

Acids and Bases

 The Phenolate Ion O

O

O

O

O

O

More resonance structures, but not more stable than acetate Nonequivalent structures (note charges on carbon and oxygen)

Acids and Bases

Electronegativity

increasing electronegativity

pKa Values O

CH4

>45

RCH3

45

NH3

34

RNH2

35

H2O

16

ROH

18

HF

3.5

R C CH3 20 O R C NH2 15 O R C OH

5

Acids and Bases

Electronegativity

increasing size

pKa Values O

HF

3.5

F–

1.36 Å

H2O

16

HCl

–7

Cl–

1.81 Å

H2S

7

H2Se

4

H2Te

3

HBr HI

–9 –10

Br I–

–

1.95 Å 2.16 Å

R C OH O R C SH S R C SH

Acids and Bases

Inductive Effects Electron-withdrawing Groups

δ− Cl

δ+ C

F, Cl, Br, O, N

Electron-donating Groups

δ+ CH3

δ− C

R, CH3, B, Si

electronegative elements alkyl groups and elements less electronegative than carbon pull electron density push electron density toward carbon away from carbon

Acids and Bases

Inductive Effects Chlorine helps to stabilize – CO2– by withdrawing electrons

δ− Cl

δ+O C O

This effect diminishes with distance—it extends for about 3 bonds

δ − δ + δ−δ+ δ− δ+ Cl C C C

O

O

Acids and Bases increasing electronegativity

I Br Cl F

Inductive Effects

CH2COOH

pKa Values 3.13

CH2COOH 2.87 CH2COOH 2.81 CH2COOH

CH3 Cl

COOH

CH2

CH

4.75

COOH 2.81

Cl Cl

increasing substitution

COOH

1.29

Cl 2.66

Cl

C Cl

COOH

0.65

Electrophiles

 e- pair acceptor

R

 electron poor  may possess formal positive charge

R

 incomplete octets  greatly reduced e- density  those that can yield e- poor species

X2 in protic solvent

R

O C

O

OH

SO3 +

SO3H

δ+ CH3

R C+

+

X

AlCl3 δ−

AlCl4

R.

X.

: CH2

NO2+

CO2

R C+ O

H2O

H+

BF3 FeBr3

Nucleophiles  e- pair donor  excess negative charges

OH-

 fractional negative charges

RO-

 may have a lone pair e-

R R C: R

NH2-

O R

C

-

O

CN-

X-

N3O-