ORGANIC CHEMISTRY 3540

COVALENT BONDING, HYBRIDIZATION

COVALENT BONDING

METHANE

ETHANE

ETHYLENE

ACETYLENE

REVIEW

THERE ARE SEVEN C₄H₁₀O ISOMERS.

DETERMINE THE STRUCTURE OF EACH ISOMER BELOW BASED ON:

• C-13 NMR DATA (NUMBER OF PEAKS SHOWS NUMBER OF DIFFERENT KINDS OF CARBONS).
• IR DATA (PEAK AT 3300-3700 CM ^-1 OF A {CARBON, HYDROGEN, AND OXYGEN COMPOUND} PROVES THE PRESENCE OF AN -OH GROUP).

TWO LINES IN C-13 NMR AND ABSENCE OF AN IR PEAK AT 3300-3700 CM ^-1.

THREE LINES IN C-13 NMR AND PRESENCE OF AN IR PEAK AT 3300-3700 CM ^-1.

FOUR LINES IN C-13 NMR AND ABSENCE OF AN IR PEAK AT 3300-3700 CM ^-1.

COVALENT BONDING

VALENCE BOND THEORY

MOLECULAR ORBITAL THEORY

VALENCE BOND THEORY

• COVALENT BONDS ARE FORMED BY THE OVERLAP OF ATOMIC ORBITALS.
• EXAMPLES
• EACH BONDED ATOM MAINTAINS ITS OWN ATOMIC ORBITALS, NONBONDING ELECTRONS
• BUT THE ELECTRONS IN THE OVERLAPPING ORBITALS ARE SHARED BY BOTH ATOMS.
• THE GREATER THE AMOUNT OF ORBITAL OVERLAP, THE STRONGER THE BOND.

MOLECULAR ORBITAL THEORY

• MATHEMATICALLY COMBINE ATOMIC WAVE EQUATIONS TO GET MOLECULAR WAVE EQUATIONS
• OBTAIN THE SAME NUMBER OF MOLECULAR ORBITALS AS THE NUMBER OF ATOMIC ORBITALS COMBINED (2 AO'S YIELDS 2 MO'S)
• ONE MO IS LOWER IN ENERGY THAN AO'S
  • MO IS A BONDING ORBITAL
• ONE MO IS HIGHER IN ENERGY THAN AO'S
  • MO IS AN ANTIBONDING ORBITAL

METHANE

C + 4 H ---> CH₄

• VALENCE BOND THEORY PREDICTS TWO DIFFERENT SIGMA BONDS WITH DIFFERENT ANGLES, LENGTHS, ENERGIES
• EXPERIMENTALLY:
• FOUR EQUIVALENT BONDS (SAME ENERGY AND LENGTH BUT DIFFERENT ORIENTATION), BOND ANGLES = 109.5 DEGREES (POINTS TO CORNERS OF A TETRAHEDRON).
• HOW?
  • USE MOLECULAR ORBITAL APPROACH.
  • MATHEMATICALLY COMBINE ONE 2s WAVE EQUATION WITH THE THREE 2p WAVE EQUATIONS.
  • GET FOUR NEW EQUATIONS WHICH ARE FOUR NEW ORBITALS!
  • CALL NEW ORBITALS HYBRID ORBITALS
  • SPECIFICALLY sp³ HYBRID ORBITALS
  • CONSEQUENCES
    • BETTER OVERLAP WITH LARGER LOBE
    • STRONGER BONDS
    • ALL SAME LENGTH AND ENERGY

NOTE: CARBON CAN BOND TO CARBON USING HYBRID ORBITALS

ETHANE, C₂H₆

OTHER HYBRID ORBITALS

ETHYLENE, C₂H₄

• sp² HYBRID ORBITALS IN ETHYLENE
  • UNUSED p ORBITAL USED FOR pi ORBITAL!

ACETYLENE, C₂H₂

• sp HYBRID ORBITALS IN ACETYLENE
  • TWO UNUSED p ORBITALS USED FOR TWO pi ORBITALS!

SUMMARY

sp³, 4 SIGMA BONDS, 109.5 DEGREES

sp², 3 SIGMA BONDS, 1 PI BOND, 120 DEGREES

sp, 2 SIGMA BONDS, 2 PI BONDS, 180 DEGREES

SUMMARY IF NONBONDING ELECTRONS ARE INVOLVED OR IF LESS THAN AN OCTET:

SUM OF SIGMA BONDS AND NONBONDING PAIRS = 4: sp³, 109.5 DEGREES

SUM OF SIGMA BONDS AND NONBONDING PAIRS = 3: sp², 120 DEGREES

SUM OF SIGMA BONDS AND NONBONDING PAIRS = 2: sp, 180 DEGREES

HOW DO WE SHOW 3-DIMENSIONAL SHAPES USING 2-DIMENSIONAL PAPER?

WE NEED A CODE!