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Organic Chemistry
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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