ORGCHEM LECSTRUCTURAL EFFECTS- Effect of the structure on STABILITY and REACTIVITY of the organic compound

I. Resonance- A molecule’s structure can have several forms that differ only in the placement of electrons- Electrons are shared between more than two atoms (electrons are delocalized)- Structural forms are called contributing structures or resonance forms

TRUE STRUCTURE - Hybrid of all contributing or resonance structures

How to show the movement of electrons?

Allowed movement of electrons

Bond -> lone pair

Lone pair -> bond

Bond -> bond

* Only p bonds are broken or formed

Types of electron delocalization1. electron delocalization2. lone pair delocalization

3. σ electron delocalization (C-H hyperconjugation)

OH

: :

::

C

O

H NO O

+

-: : :

....

NH2

..

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 Double bonds can be:

I. ResonanceMolecules 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.

Identifying structures with resonance1. Does the molecule have at least one pi bond? YES: a. number the 2 atoms of the pi bond “1” & “2” b. go to step 2. NO: molecule does not have 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”. 5. Is atom 3, 4, 5 or 6 part of another pi bond? YES: molecule has resonance NO: Go to step 6. 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. 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

II. INDUCTIVE EFFECT Electronic effects of an atom or functional

group contributed through single bonds Factors: electronegativity, position within the

structure Inductive effect weakens with increasing

distance from the substituent Inductive effect weakens with decreasing

electronegativity of the substituent A substituent may WITHDRAW or

RELEASE electrons ELECTRON-WITHDRAWING

SUBSTITUENT -helps to offset negative charge

ELECTRON-RELEASING SUBSTITUENT -helps to offset the positive charge

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-

CH3 CH2 Cl

CH3 CH2 OH

H3C Cl

N+

H3C CH3

CH3

--

III. STERIC EFFECT- Effect of atom or functional group on CROWDING in the molecule