CHE2060 Topic 1: Atoms, orbitals & bonding
Atoms, Orbitals & Bonding Topics:
1. Very quick history of chemistry…
2. What is organic chemistry?
3. Atomic models: nuclear to quantum
4. All about orbitals
5. How orbitals fill: electron configuration
6. Basic bonding: valence electrons & molecular orbitals
7. Lewis dot structures of molecules
8. Electronegativity & bond polarity
9. Resonance: a critical concept
10. Orbital hybridization: key to carbon’s “flexibility”
sp3
sp2
sp
11. Free electron pairs & radicals
12. VSEPR: classifying molecular geometry
Daley & Daley, Chapter 1
& orbital hybridization
Atoms, Orbitals & Bonds
CHE2060 Topic 1: Atoms, orbitals & bonding
Electronegativity and bond polarity
Bond polarity & electronegativity
Covalent bonds can be either polar or nonpolar depending on how equally
(or unequally) their e- pair bonds are shared between the 2 bonded atoms.
Electronegativity is used to determine polarity of a bond between two atoms.
“Electronegativity is the ability of an atom in a molecule to attract the
molecule’s electrons toward itself.”
• Higher EN =
stronger
attraction.
• More protons
create more
pull on e-.
• More shells
move vefurther
from
nucleus.
Bruice (2007)
Calculating bond polarity
Non-polar bonds share electrons equally (or evenly) between the 2 atoms.
Polar bonds share electrons unequally.
Degree of polarity is calculated:
Type of bond
ΔEN
Ionic
> 2.0
Polar covalent
polarity (ΔEN) = | ENA – ENB|
0.5 – 2.0
Nonpolar covalent
< 0.5
A dipolar charge is a partial positive or
negative charge on atoms joined by
a polar covalent bond.
Small delta (δ) indicates dipolar charge.
Here δ are shown only for polar bonds.
Use EN values to determine the polarity of each bond.
+ . ..
. C – F:
. ..
H
| .. +
H-C-O
.. - H
|
H
H
\
..
+ C = O
.. /
H
H
|
H-C-H
|
H
Crossed arrows are used to show the direction in which electrons are pulled.
E- move toward the arrow head, leaving behind a positive charge (the cross).
Exercise: non-polar, polar or ionic?
For each pair of atoms:
a) Calculate bond polarities
b) Identify each bond as ionic, polar covalent or non-polar covalent
1)
HBr
|2.1 – 2.8| = 0.7
polar covalent
2) H2O
|2.1 – 3.5| = 1.4
polar covalent
3) LiI
|1.0 – 2.5| = 1.5
polar covalent
4) BrCl
|2.8 – 3.0| = 0.2 non-polar covalent
5) NH3
|3.0 – 2.1| = 0.9
6) KF
|0.8 – 4.0| = 3.2 ionic
polar covalent
Visualizing polarity
The polarity of bonds (or within molecules) can be visualized as
electron density in electrostatic potential maps.
EPMs also show
• “Hot” colors indicate higher electron density.
relative atomic size.
H -I
2.1 – 2.5
H - Br
2.1 – 2.8
H - Cl
2.1 – 3.0
δ+
δ-
H -F
2.1 – 4.0
Bruice (2007)
Neighbors can affect your polarity!
Inductive effects occur when the polarity of bond (A) is increased if its
neighboring bond (B) has a much higher polarity.
H H
Ethane’s C-C
| |
bond is non- H - C – C – H
polar.
| |
H H
vs
H H
| | ..
H - C – C – Cl:
..
| |
H H
But 1-chloroethane has an
atom (Cl) with a higher EN.
The polar C-Cl bond
polarizes the neighboring
C-C bond a bit.
This new polarity is induced.
Field effects are similar, but occur when influencing neighbors are far
from one another in terms of bonds, but are close in 3D structure.
• Field effect polarization can also be caused by a molecule’s environment.
Image placing a large magnet near one end of a long non-polar molecule.
While the yellow atoms are far
apart if the peel is stretched out,
they are close when its 3D
structure is restored. So they
affect one another’s polarity
D&D p.56
Formal charge shows extreme polarities
If a molecule carries a charge, we calculate formal charge to show where that
charge is located; which atom carries the charge.
• Start with a Lewis structure.
Formal charge = (# ve-) – (dots + sticks)
Calculated for each atom.
Draw Lewis structures & calculate formal charges for these molecules:
1) CH4
Generally, the most
2) H3O+1
H
..
EN atom carries
H-O–H
3) CH3O-1
| ..
H
|
H-C–H
|
H
|
H
H = 1 – (0+1) = 0
O = 6 – (2 + 3) = +1
So O carries the charge.
H = 1 – (0+1) = 0
C = 4 – (0 + 4) = 0
Makes sense since there is no charge.
H - C – O:
..
|
H
the charge.
H = 1 – (0+1) = 0
C = 4 – (0 + 4) = 0
O = 6 – (6 + 1) = +1
So O carries the charge.
D&D p.56-9
Exercise: calculating formal charge
Draw a Lewis structure & calculate formal charges for each atom.
1) CH3F
H
H = 1 – (0+1) = 0
H = 1 – (0+1) = 0
H
2) CH3OH2+1
| .. C = 4 – (0 + 4) = 0
C = 4 – (0 + 4) = 0
| ..
+1
3) CH3
H - C – ..F: F = 7 – (6 + 1) = 0
H - C – O – H O = 6 – (2 + 3) =
4) NH3BF3
|
| |
+1
-1
5) :CH3
H
H H
6) :CH2
7) HNO2
H
| H = 1 – (0+1) = 0
H – C C = 4 – (0 + 3) = +1
|
H
H
|
H – C:
H = 1 – (0+1) = 0
C = 4 – (2 + 2) = 0
..
H :F:
| | ..
H – N – B – ..F:
| |
H :F:
..
H = 1 – (0+1) = 0
N = 5 – (0 + 4) = +1
B = 3 – (0 + 4) = -1
F = 7 – (6 + 1) = 0
Net = zero
H
| H = 1 – (0+1) = 0
H – C:
C = 4 – (2 + 3) = -1
|
H
.. H = 1 – (0+1) = 0
H–N=O
.. N = 5 – (0 + 4) = +1
|
Single O = 6 – (6 + 1) = -1
:O:
..
Double O = 6 – (4 + 2) = o
Net = zero
D&D p.59
Dipole moment: polarity & geometry
Dipole moments are permanent partial charges created by
unsymmetrical separation of charges.
Which of these molecules has a permanent dipole moment?
δ+
H
δ+
H
C
Cl
δ-
δ+
H
δCl
C
C
Cl
δ-
Cl
δ-
C
H
δ+
Here the dipoles cancel each
other producing no overall
molecular polarity, or
dipole moment.
H&P p.7
Polarity varies with atomic context
The electronegativity values of H & O are constant, and always differ by 1.4.
However, the degree of molecular polarity also depends on overall
molecular charge, and to some extent, geometry & shape.
These molecules all have
dipole moments.
least reactive
Which of these molecules is the most reactive?
The least reactive?
most reactive
Bruice (2007)
Polarities of typical organic bonds
Note that bond dipole takes molecular geometry into account along
with differences in electronegativity values.
Bond
Bond dipole (D)
Bond
Bond dipole (D)
C-F
1.53
H-F
1.82
C - Cl
1.59
H - Cl
1.08
C - Br
1.48
H- Br
0.82
C-I
1.29
H-I
0.44
C-N
0.22
H-N
1.32
C-O
0.85
H-O
1.54
C-H
0.35
What trends do you see here?
Critical point to remember:
Chemical reactivity of a bond increases with its increasing polarity!
D&D p.54-5