Download ORGANIC REACTION TYPES:

ELECTRONEGATIVITY:
One way to estimate the degree of ionic or covalent character in a chemical bond is to
compare electronegativities of atoms involved. Electronegativity is a measure of the force of
an atom’s attraction for electrons that it shares in a chemical bond with other atoms.
 In the 1930’s, Linus Pauling assigned electronegativity values to all elements relative to F
(the most electronegative element), which he gave a value of 4.0 .
Linus Pauling's Table of Electronegativities
H
2.1
Li
1.0
Na
1.0
K
0.9
Rb
0.9
Cs
0.8
Fr
0.8
Be
1.5
Mg
1.2
Ca Sc Ti V Cr Mn Fe
1.0 1.3 1.4 1.5 1.6 1.6 1.7
Sr Y Zr Nb Mo Tc Ru
1.0 1.2 1.3 1.5 1.6 1.7 1.8
Ba La Hf Ta W Re Os
1.0 1.1 1.3 1.4 1.5 1.7 1.9
Ra Ac
1.0 1.1
Co Ni Cu Zn
1.7 1.8 1.8 1.6
Rh Pd Ag Cd
1.8 1.8 1.6 1.6
Ir Pt Au Hg
1.9 1.8 1.9 1.7
B
2.0
Al
1.5
Ga
1.7
In
1.6
Tl
1.6
C
2.5
Si
1.8
Ge
1.9
Sn
1.8
Pb
1.7
N
3.0
P
2.1
As
2.1
Sb
1.9
Bi
1.8
O
3.5
S
2.5
Se
2.4
Te
2.1
Po
1.9
F
4.0
Cl
3.0
Br
2.8
I
2.5
At
2.1

Bear in mind that these values can vary slightly depending upon the chemical
environment and so the values are average values.
Note that EN increases across any period and decreases down any group (in most cases).

Pure covalent bonds involve equal sharing of the bonding electron pairs. Pure covalent
bonds occur when both atoms involved have equal EN (i.e., EN = 0) ...
For example, H2, N2, O2, F2, Cl2, Br2, I2, Cx, S8, PH3, and CS2 are all pure covalent.



Nonpolar covalent bonds are those in which EN  0.4. Examples include all the pure
covalent compounds listed above as well as compounds, e.g., CH4 (EN = 0.4) and BH3
(calculate EN)
Polar covalent bonds are those in which the bonding pair of electrons is unequally
shared (0.5  EN  1.7). Examples include HBr, HCl, etc. In polar covalent
compounds, the more electronegative atom has a partial negative charge ( -) and the less
electronegative atom has a partial positive charge ( +).
Ionic bonds are those in which EN  1.8 and are generally considered to have complete
charge separation, i.e., considered to be made up of cations and anions. Examples include
ORGANIC CHEMISTRY INTRO
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P ..shell
O
N
M
L
K
Nucleus
(n
(n
......ns
.. (n
Compare
.shell
...shell
(n.
.
.
.this
:O
F
:
O
S
S
P
N
Cl
Cl
Si
C
.
P
B
Al
N
Si
C
Al
B
=
6)
5)
4)
3)
2)
1)
...... ..table
.. 2)f
6s .. 1
5s
4s
3s
2s
1s
1,
6p
5p
4p
3p
with
a 2p
..
H O
..
periodic
 33,C:N1s
6d  5
5d
4d
3d
Cl
S
Si
P
....The
table.
5,
6f..C2s
5f
7,
7
7,
Li 4f
filling
6g N9
5g
9, 6h
Na
Clis the
order
C
3s
 11S..
same
as
K P4s
the layout
Cl
of the
Rb
5ss-,
p-, d- and
Cs 6s on
4f
f-blocks
the
Fr 7s
5f
periodic
table, i.e.,
this is the
filling
order.
Note the
patterns.
The orbital
filling
order is:
ns, (n-2)f,
(n-1)d, np.
The porbitals
begin
filling after
the 2s
orbital.
The dorbitals
begin
filling after
the 4s
orbital.
The forbitals
begin
filling after
the 6s
orbital.
(n- np
1)d
He
NaCl, Li
N, and CaO. Exceptions are HF and alkali metal iodides. EN in HF is 1.9 but
2p Ne3
this compound behaves as a polar covalent compound. EN in LiI is only 1.5 but LiI
3p Arionically. BF3 is also anomalous. It is covalent although EN = 2.0.
behaves
3d 4p Kr
 A scale of bond type versus EN follows...
4d 5p Xe
5d
6p
RnE
6d
7p
Uuo
0
1.8
0.4
n o n p o la r
0.5
covalent
p o la r c o va le n t
3.2
io n ic
1.7
p u re c o va le n t
In polar covalent bonds, the electron distribution is said to be polarized, i.e., not equally
distributed but closer to the more electronegative atom. For example, in HCl....
EN = (3.0 – 2.1) = 0.9 i.e., polar covalent
A separation of '+' and '-' charge is called a dipole. Dipoles are sometimes illustrated with an
arrow pointing toward the more electronegative atom.
+
The tail of the arrow is crossed to look like a + sign.
The head of the arrow points in the direction of electron shift.
H  Cl
The shifting of electron density through sigma bonds due to EN differences between atoms is
called an 'inductive effect'. Electropositive elements (metals) such as Zn and Hg, inductively
donate electrons through sigma (single) bonds with carbon. Electronegative elements
(nonmetals) such as oxygen and chlorine inductively withdraw electrons from carbon through
their sigma bonds with carbon.
Problem: Calculate EN values and show bond dipoles and dipole arrows.
H
H
C
H
:O:
C
H
..
O
..
H
H
H
C
C
H
H
..
H
N
H
acetic acid
ethyl amine
ORGANIC CHEMISTRY INTRO
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