Structural Effects

STRUCTURAL EFFECTS

Gladys I. Bathan

STRUCTURAL EFFECTS

Effect of the structure on STABILITY and

REACTIVITY of the organic compound

STRUCTURAL EFFECTS

I. Resonance

II. Inductive effect

III. Steric effect

IV. Hydrogen bonding

I. RESONANCE

A molecule’s structure can have several forms that differ only in the placement of electrons

I. RESONANCE

Electrons are shared between more than two atoms (electrons are delocalized)

I. RESONANCE

Structural forms are called contributing structures or resonance structures or canonical structures

I. RESONANCE

TRUE STRUCTURE - Hybrid of all contributing or resonance structures

http://www.masterorganicchemistry.com/2011/11/30/introduction-to-resonance-2-curved-arrows/

I. RESONANCE

How to show the movement of electrons?

http://www.masterorganicchemistry.com/2011/11/30/introduction-to-resonance-2-curved-arrows/

I. RESONANCE

Allowed movement of electrons

bond lone pair

lone pair bond

bond bond* Only p bonds are broken or formed

I. RESONANCE

bond lone pair

http://www.masterorganicchemistry.com/2011/11/30/introduction-to-resonance-2-curved-arrows/

I. RESONANCE

lone pair bond

http://www.masterorganicchemistry.com/2011/11/30/introduction-to-resonance-2-curved-arrows/

I. RESONANCE

bond bond

http://www.masterorganicchemistry.com/2011/11/30/introduction-to-resonance-2-curved-arrows/

I. RESONANCE

Types of electron delocalization

A. - electron delocalization

B. lone pair delocalization

C. σ electron delocalization

(C-H hyperconjugation)

A. - electron delocalization

p Electrons - electrons found in api bond

- electron delocalization is observed in compounds containing conjugated double or triple bonds.

A. - electron delocalization

isolated conjugated cumulated alkene alkene alkene

A. - electron delocalization

Klein, Organic Chemistry 2e

B. lone pair delocalization

Lone pairs from O

Klein, Organic Chemistry 2e

B. lone pair delocalization

Lone pairs from N

Klein, Organic Chemistry 2e

B. lone pair delocalization

Allylic lone pairs (Atom with lone pair next to a pi bond)

Klein, Organic Chemistry 2e

B. lone pair delocalization

CONH2

NH2

..

..

..

......

CH3 CO

O H

: :

OH

: :

::

C

O

H NH2

..N

O O+

-: : :

....

B. lone pair delocalization

C. Hyperconjugation

Electron delocalization involving sigma bonds (C-C, C-H) and pi bonds.

More canonical structures lead to better stability of the molecule

C. Hyperconjugation

Klein, Organic Chemistry 2e

C

H

H H C

H

HH+

..-

C

H

HH+

-..

C. Hyperconjugation

How many canonical structures?

C

H

H

H

CO

HCH

H HC

O-H

+

O O

C. Hyperconjugation

How many canonical structures?

I. RESONANCE

Molecules with resonance structures meet the ff criteria:

Criterion 1: Have at least one pi bond.

Criterion 2: Have at least one of the following

a) a second, conjugated pi bond, or

b) an allylic or α-atom with at least one lone

pair of electrons, or

c) an allylic atom with a vacant p-orbital.

http://www.organicchem.org/oc2web/lecture/outlines/resonance.pdf

Identifying structures with resonance

1. Does the molecule have at least

one pi bond?

YES : a. number the two atoms of

the pi bond “1” and “2”

b. go to step 2.

NO : molecule does not have

resonance. http://www.organicchem.org/oc2web/lecture/outlines/resonance.pdf

Identifying structures with resonance

2. Circle the atom(s) directly bonded

to atom “1”.

3. Circle the atom(s) directly bonded

to atom “2”.

4. Label the circled atoms “3”, “4”,

“5”, and “6”.

http://www.organicchem.org/oc2web/lecture/outlines/resonance.pdf

Identifying structures with resonance

5. Is atom 3, 4, 5 or 6 part of another pi bond?

YES : molecule has resonance

NO : Go to step 6.

http://www.organicchem.org/oc2web/lecture/outlines/resonance.pdf

Identifying structures with resonance

6. Does atom 3, 4, 5 or 6 have at

least one lone pair of electrons

or a single unpaired electron?

YES : the molecule has resonance

NO : Go to step 7.

http://www.organicchem.org/oc2web/lecture/outlines/resonance.pdf

Identifying structures with resonance

7. Does atom “3, 4, 5 or 6 have a

vacant p-orbital (usually a

carbocation)?

YES : molecule has resonance

NO : molecule has no resonance

http://www.organicchem.org/oc2web/lecture/outlines/resonance.pdf

Identifying structures with resonance

7. Does atom “3, 4, 5 or 6 have a

vacant p-orbital (usually a

carbocation)?

YES : molecule has resonance

NO : molecule has no resonance

http://www.organicchem.org/oc2web/lecture/outlines/resonance.pdf

II. INDUCTIVE EFFECT

Electronic effects of an atom or functional group contributed through single bonds

Factors: electronegativity, position within the structure

II. INDUCTIVE EFFECT

Inductive effect weakens with increasing distance from the substituent

Inductive effect weakens with decreasing electronegativity of the substituent

II. INDUCTIVE EFFECT

A substituent may WITHDRAW or RELEASE electrons

ELECTRON-WITHDRAWING SUBSTITUENT

-helps to offset negative chargeELECTRON-RELEASING

SUBSTITUENT

-helps to offset the positive charge

Electron attracting/withdrawing inductive effect with excess positive charges

N+

RR

R

+NH3

+NO2

those with electronegative atoms

NH2 OH OCH3

F

ClBr

groups exhibiting orbital electronegativity

C N N N

those with easily polarizable valence electrons

I-

Electron attracting/withdrawing inductive effect

H3C Cl

CH3 CH2 Cl

CH3 CH2 OH

N+

H3C CH3

CH3

--

Electron releasing/donating inductive effect alkyl groups

CH3

negative groups

COO-

S-

O-

H3C CO

O

-- H3C S-

CHH3C

H3CC N

R CO

OC

O

OO SR

O

O

O

III. STERIC EFFECT

Effect of atom or functional group on CROWDING in the molecule

III. STERIC EFFECT

http://www.utdallas.edu/~scortes/ochem/OChem1_Lecture/Class_Materials/08_conform_anal.pdf

Most stable conformation has the least Van der Waals repulsion

III. STERIC EFFECT