Organic Chemistry Structural Effects

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

3. σ electron delocalization (C-H hyperconjugation)

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?

:O:

H

:O: C H

.. :O

.. :O: N+

.. NH2

Allowed movement of electrons Bond -> lone pair B. Lone pair delocalization

Lone pair -> bond

Bond -> bond

* Only p bonds are broken or formed Types of electron delocalization 1.  electron delocalization 2. 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. 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. Identifying structures with resonance 1. 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 +

+

R N R

+

NH3

NO2

R those with electronegative atoms

F Cl Br groups exhibiting orbital electronegativity NH2

C N

OH

OCH3

N N

those with easily polarizable valence electrons I







CH3

CH2

Cl

-

CH3

 CH2

 O

H



CH3 +





H3C Cl

H3C N CH3 --

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