Transcript BONDS - chemistrysemester55

LECTURE 1:
MOLECULAR STRUCTURE
1B. Hybridization, Bonding, Resonance and
Intermolecular Attractions
LECTURE OUTLINE
1. Bonding
2. Hybridization
3. Sigma and Pi Bonding
4. Electron Delocalization and Resonance
5. Dipole Moment and Intermolecular Forces
BONDING
⋆ Organic Chemistry is concerned almost
exclusively with covalent bonds.
⋆ All bonds are formed because there is an
electrostatic attraction between the nuclei
and the two electrons in the bond.
HYBRIDIZATION
http://www.youtube.com/watch?v=3pV690yJBms
First consider the atomic orbitals of Carbon:
It appears that the carbon atom can only form
2 bonds in this state.
HYBRIDIZATION
Now there are 4 degenerate orbitals
Since 1(s) + 3(p) orbitals went into the hybridization, all four orbitals
are sp3 hybridized.
HYBRIDIZATION
Now there are 4 degenerate orbitals
Since 1(s) + 3(p) orbitals went into the hybridization, all four orbitals
are sp3 hybridized.
HYBRIDIZATION
Other hybridizations of the carbon atom and “p character”
http://www.youtube.com/watch?v=g1fGXDRxS6k
HYBRIDIZATION
To determine the hybridization of carbon in a molecule
Count electron “zones” and ensure that the zones add up to
the exponents.
HYBRIDIZATION
HYBRIDIZATION and VSEPR
Molecular Shapes!
Benzene
SIGMA(σ) AND PI(π) BONDS
A sigma bond results from the direct overlap of atomic
orbitals.
A pi bond results from the lateral overlap of atomic orbitals.
Since the overlap is weaker, the π bond is easier to break.
SIGMA(σ) AND PI(π) BONDS
A triple bond results from one sigma, and two pi orbitals
overlapping.
Since the overlap is weaker, the π bonds are easier to break.
ELECTRON DELOCALIZATION
http://www.learnerstv.com/animation/animation.php?ani=187&cat=chemistry
http://www.youtube.com/watch?v=bSLxoX27Gt4
There are only 2 rules for drawing resonance structures:
1. Don’t have more than eight valence electrons (but less is okay).
2. Don’t break sigma bonds.
3. Hint: Start the movement from any lone pair or double bond
ELECTRON DELOCALIZATION
Some things to know about aromatic regions of
compounds:
1. The are planar.
2. They are conjugated unsaturated systems
3. Rings obey Huckel’s Rule (4n + 2 π electrons)
DIPOLE MOMENT
A dipole moment occurs when the majority of negative
charge is at one end of a molecule or bond.
INTERMOLECULAR FORCES
1. Induced Dipole
2. Instantaneous Dipole
3. Hydrogen Bond
4. London Dispersion Forces or Van de Waals Forces
PRACTICE QUESTIONS
1. What is the hybridization for the carbon indicated by the arrow?
a) sp
b) sp2
c) sp3
d) sp3d2
PRACTICE QUESTIONS
2. Which of the following is an invalid resonance structure of
pyridinium?
a)
c)
b)
d)
+
+N
N
+N
+ N
H
H
H
H