‫ قطبش پذیری‬-‫ قطبیت‬-‫انرژی پیوند‬-2-1
:‫انرژی پیوند‬-1-2-1
Bond
Energy
(kcal)
Length
(pm)
Ethane
C(sp3)-H
100
110
Ethylene
C(sp2)-H
106
108
Acetylene
C(sp)-H
132
106
Molecule
Advanced Organic Chemistry (Chapter 1)

sh.Javanshir
Comparison of C-C bonds:
a) Bond lengths are strongly depend on
hybridization but little influenced by other factors.
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
similar bonds have fairly constant lengths from one molecule to the next,
though exceptions are known. The variation is generally less than 1%.
Bond Lengths between sp3 Carbons in Some Compounds
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
b) Bond energies are not independent of the
remainder molecule.
Homolytic Bond Dissociation Energies
Bond
Bond Energy (kcal/mol)
CH3CH2-H
98
H2C=CH-CH2-H
85
PhCH2-H
85
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
Stable Allyl and Benzyl Radicals:
C-‫پایداری رادیکال های آلیل وبنزیل از طریق رزونانس باعث می شود پیوند‬
‫ ضعیف تر شود‬H
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
‫‪Dissociation Energies‬‬
‫‪Homolytic Bond‬‬
‫ضعیف شدن پیوند ها با افزایش استخالف ها ناشی از افزایش پایداری رادیکال های‬
‫استخالفی نسبت به رادیکال های نوع اول است‬
‫‪sh.Javanshir‬‬
‫)‪Advanced Organic Chemistry (Chapter 1‬‬
‫انرژی تفکیک هترولیتیک در فاز گازی‬
a) Very High in Gas Phase
Advanced Organic Chemistry (Chapter 1)
b) Low in Solution
sh.Javanshir
‫الکترونگاتیوی و قطبیت‬-2-2-1
Bondpolarity: Unequal distribution of electron density in covalent bond (m=e.d)
Electronegativity: The tendency of an atom to attract electrons can be defined
for an atom or group.
Allred and Rochow defined electronegativity in terms of the electrostatic
attraction by the effective nuclear charge Zeff.
A more recent formulation of electronegativity
where IPs and IPp are the ionization potentials of the s and p electrons and a
and b are the number of s and p electrons, respectively.
8
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
Bond strength is increased by electronegativity differences.
Some Group Electronegativites Relative to H = 2.176
Effect of Polarity on Acidity:
O
H
H C
H
C
O
H
OH
H C
H
C
O
Very little dipolar stabilization of anion
Advanced Organic Chemistry (Chapter 1)
Cl
Cl C
Cl
O
C
O
Cl
OH
Cl C
Cl
C
O
Dipolar stabilization of anion
sh.Javanshir
In Solution:
Acetic Acid > Propionic Acid
 In Gas Phase:
Acetic Acid < Propionic Acid
 Reason: Solvation Parameter in Solution
and Charge Accommodation in Gas Phase

Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
‫تعیین کنفورماسیون‬
‫چگونه میتوان تعیین کرد کدام فرم برتر است؟‬
‫دی کلرواستیلبن‬
‫‪sh.Javanshir‬‬
‫)‪Advanced Organic Chemistry (Chapter 1‬‬
INDUCTIVE EFFECT (IE)
‫اثر القایی‬

The polarization of one bond caused by the
polarization of an adjacent bond is called the
inductive effect.
polarization effects transmitted through σ-bonds
Positive (+I) (electron donating)/ negative (-I) (electron
withdrawing).

Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
The moment of p-cresol (1.57 D) is quite far
from the predicted value of 1.11 D. In some
cases, molecules may have substantial
individual bond moments but no total
moments at all because the individual
moments are canceled out by the overall
symmetry of the molecule.
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
‫اثر میدانی‬


The other effect operates not through bonds, but
directly through space or solvent molecules, and
is called the field effect.
The field effect depends on the geometry of the
molecule but the inductive effect depends only
on the nature of the bonds.
‫ به دلیل اینکه اتم های کلر در‬،‫اثر القایی اتم های کلر یکسان ولی اثر میدانی انها متفاوت است‬
. ‫سمت چپ به گروه کربوکسیل نزدیک تر است‬
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
‫اثر میدانی‬
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir

The field-effect order of alkyl groups
attached to unsaturated systems is
tertiary>secondary>primary> CH3, but this
order is not always maintained when the
groups are attached to saturated systems.
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
‫ سختی ونرمی‬-‫قطبش پذیری‬-3-2-1
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
Polarizibility: A Fundamental Atomic Property Which is Related to
Electronegativity.
Softness: Ease of Distortion
Hardness: Difficulty of Distortion
Factors That cause increasing the Hardness:
a) Increasing The Electronegativity
b) Decreasing The size of the Atom
c) Increasing the oxidation Number in Metal Cations
Lewis Acids: Hard Acids Prefer Hard Bases and Soft Acids Prefer Soft
Bases
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir




Soft Bases: The donor atoms are of low
electronegativity and high polarizability and are easy
to oxidize. They hold their valence electrons loosely.
Hard Bases: The donor atoms are of high
electronegativity and low polarizability and are hard
to oxidize. They hold their valence electrons tightly.
Soft Acids: The acceptor atoms are large, have low
positive charge, and contain unshared pairs of electrons
(p or d) in their valence shells. They have high
polarizability and low electronegativity.
Hard Acids: The acceptor atoms are small, have high
positive charge, and do not contain unshared pairs in
their valence shells. They have low polarizability and
high electronegativity.
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
‫مقیاس سختی‬
h = (EI – A)/2
EI= ‫انرژی یونیزاسیون‬
A = ‫الکترون خواهی‬
‫نرمی‬
s = 1/h
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir


these considerations are important because they
allow us to understand the Lewis acid-base
interactions and reactivity (= hard acids
(species) react preferentially with hard
bases(species) and soft with soft).
In fact the reactivity of the hard centers is
dominated by electrostatic attractions, while the
reactivity of the soft centers is dominated by
mutual electron cloud polarization.
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
‫اصل سختی بیشینه‬
PRINCIPLE OF MAXIMUM
HARDNESS

Pearson(1987) suggested that a chemical system tends
to arrange itself so as to achieve maximum hardness
and the most stable state of the system is that in which η
is maximized.
P
R
TS
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir
‫اصل سختی بیشینه‬
PRINCIPLE OF MAXIMUM HARDNESS
How it works: One can consider the degree of electron transfer
between the reactants. This can be expressed numerically as:
x  y
N =
2h x  h y 
x= absolute electronegativity of species x(y)
h x = hardness of species x (y)
Example:
Consider radical halogenation of an alkane (in our case methane). Methyl
halide (product) is formed as a result of combination of a halogen and
methyl radicals according to the following scheme:

‫کاهش سختی وقطبیت‬
Advanced Organic Chemistry (Chapter 1)
‫افزایش واکنش پذیری‬
‫افزایش درجه انتقال الکترون=> افزایش خصلت یونی‬
(MeI>Br>Cl>F)
CH3  X 
 CH3X
sh.Javanshir
This is also in agreement with the known reactivity of
methyl halides (MeI>Br>Cl>F). Because when
similar (= comparable) bonds are considered, the
reaction partners of highest hardness display higher
net charge transfer. This in turn means higher
exothermicity of the product-bond formation (= more
thermodynamically stable product).
Outcome = Bond between two harder reactants is more
stable than bond between two softer reactants.
Advanced Organic Chemistry (Chapter 1)
sh.Javanshir