Review of molecular orbitals

1
Lecture 3

• Review of molecular orbitals
• Acid dissociation

2
Acetylene

• A linear molecule
• High electron density in the C-C region
• Why?
• Its all in the orbitals.

3
Hybrid Carbon AOs

• What orbitals do we need for a linear arrangement
  around a carbon atom?
• The C-H bond
• 1S can have overlap with the 2S orbital and only
  one of the P orbitals.
• We will hybridize these two orbitals -- SP
  orbitals
• The other two P orbitals are orthogonal

4
Hybrid SP Orbitals
5
Atomic Orbitals in Acetylene
6
Add em Up

• Carbon-Hydrogen ? bonds

Bonding
7
Add em Up

• Carbon-Carbon ? bonds

Bonding
Antibonding
8
Sigma Bonds in Acetylene
9
Add em Up

• C-C ? bonds

Bonding
10
Calculated Orbitals

• Quantum chemistry software can calculate the
  orbitals to a high degree of precision.

11
Pi System of Acetylene
12
Orbitals of Acetylene
10 Atomic orbitals (4 SP, 4 P, 2 S) give 10
molecular orbitals
How do we fill these orbitals with electrons?
5 bonds 0 lone pairs 5 electron pairs
13
Chemical Consequences
LUMO
HOMO
14
Acids and Bases

• Bronstead
• Anything that can give up a proton is an acid
• Anything that can accept a proton is a base
• Lewis
• Anything that can accept an electron pair can be
  an acid
• Anything with an electron pair can be a base

15
Naked H Does Not Exist

• Heterolytic cleavage of HCl in a vacuum is nearly
  impossible
• Homolytic cleavage is the only observed product
• In Water, we get complete dissociation of HCl.
  Why?

16
Water Stabiles Chloride Ions
17
Water Coordinates Proton

• Water has two electron pairs than can accept the
  proton
• In this case, water is the Lewis base and proton
  is the Lewid acid.
• The resulting hydronium ion is a Brönstead acid

18
Hydronium ion stabilized by Water
19
Acids Need Bases

• If a molecule is an acid, it can donate a proton
  to a base.
• Strong acids are lower in energy of they can
  donate the proton
• Weak acids are higher in energy without the proton

• If a molecule can accept a proton from an acid,
  it is a base.
• Strong bases are lower in energy when protonated.
• Weak bases are higher in energy when protonated

20
Strong and Weak Acids
21
Rules of Thumb

• Acids can be ordered in strength for similar
  molecules according to the atom type
• Halogen gt Oxygen gt Nitrogen gt Carbon

22
Rules of Thumb

• The acid with the higher pKa value is the
  stronger base in its basic form.
• pKa is the affinity value for the acid and the
  proton
• High - acid holds proton tightly
• Low - acid easily gives up proton
• A given proton will go to the base whose acid
  form has the higher pKa value

23
Assignment

• Allene is an unusual molecule with a pair of
  connected double bonds.
• What is the angle between the planes of hydrogens
  and either end?
• (Is the molecule flat, like ethylene, or what?)
• Use a quick orbital diagram of the ? system to
  back up your answer.

24
For Next Class

• Read Chapter 1.11 and 1.12 and tutorial on acids
  and bases.
• Read chapter 2.1 to 2.8
• www.upei.ca/chem441