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Organic Chemistry , 5 th Edition L. G. Wade, Jr. Unite 2, Chapter 2 Structure and Properties of Organic Molecules. _. +. +. -. =>. Wave Properties of Electrons. Standing wave vibrates in fixed location. Wave function,  , mathematical description of size, shape, orientation

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wave properties of electrons

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Wave Properties of Electrons
  • Standing wave vibrates in fixed location.
  • Wave function, , mathematical description of size, shape, orientation
  • Amplitude may be positive or negative
  • Node: amplitude is zero

Chapter 2

wave interactions
Wave Interactions
  • Linear combination of atomic orbitals
    • between different atoms is bond formation
    • on the same atom is hybridization.
  • Conservation of orbitals
  • Waves that are in phase add together.Amplitude increases.
  • Waves that are out of phase cancel out. =>

Chapter 2

sigma bonding
Sigma Bonding
  • Electron density lies between the nuclei.
  • A bond may be formed by s-s, p-p, s-p, or hybridized orbital overlaps.
  • The bonding MO is lower in energy than the original atomic orbitals.
  • The antibonding MO is higher in energy than the atomic orbitals. =>

Chapter 2

h 2 s s overlap
H2: s-s overlap

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Chapter 2

cl 2 p p overlap
Cl2: p-p overlap

Constructive overlap along the same

axis forms a sigma bond.

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Chapter 2

hcl s p overlap
HCl: s-p overlap

Question:

Draw the predicted shape for the bonding molecular orbital and the antibonding molecular orbital of the HCl molecule.

Answer: See bottom of page 42 in your text.

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Chapter 2

pi bonding

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Pi Bonding
  • Pi bonds form after sigma bonds.
  • Sideways overlap of parallel p orbitals.

Chapter 2

multiple bonds

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Multiple Bonds
  • A double bond (2 pairs of shared electrons) consists of a sigma bond and a pi bond.
  • A triple bond (3 pairs of shared electrons) consists of a sigma bond and two pi bonds.

Chapter 2

molecular shapes
Molecular Shapes
  • Bond angles cannot be explained with simple s and p orbitals. Use VSEPR theory.
  • Hybridized orbitals are lower in energy because electron pairs are farther apart.
  • Hybridization is LCAO within one atom, just prior to bonding. =>

Chapter 2

sp hybrid orbitals

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sp Hybrid Orbitals
  • 2 VSEPR pairs
  • Linear electron pair geometry
  • 180° bond angle

Chapter 2

sp 2 hybrid orbitals

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sp2 Hybrid Orbitals
  • 3 VSEPR pairs
  • Trigonal planar e- pair geometry
  • 120° bond angle

Chapter 2

sp 3 hybrid orbitals

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sp3 Hybrid Orbitals
  • 4 VSEPR pairs
  • Tetrahedral e- pair geometry
  • 109.5° bond angle

Chapter 2

sample problems

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Sample Problems
  • Predict the hybridization, geometry, and bond angle for each atom in the following molecules:
  • Caution! You must start with a good Lewis structure!
  • NH2NH2
  • CH3-CC-CHO

Chapter 2

rotation around bonds

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Rotation around Bonds
  • Single bonds freely rotate.
  • Double bonds cannot rotate unless the bond is broken.

Chapter 2

isomerism
Isomerism
  • Molecules which have the same molecular formula, but differ in the arrangement of their atoms, are called isomers.
  • Constitutional (or structural) isomers differ in their bonding sequence.
  • Stereoisomers differ only in the arrangement of the atoms in space. =>

Chapter 2

stereoisomers

Trans - across

Cis - same side

No cis-trans isomers possible

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Stereoisomers

Cis-trans isomers are also called geometric isomers.

There must be two different groups on the sp2 carbon.

Chapter 2

bond dipole moments
are due to differences in electronegativity.

depend on the amount of charge and distance of separation.

In debyes,

 = 4.8 x  (electron charge) x d(angstroms)

=>

Bond Dipole Moments

Chapter 2

molecular dipole moments

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Molecular Dipole Moments
  • Depend on bond polarity and bond angles.
  • Vector sum of the bond dipole moments.
  • Lone pairs of electrons contribute to the dipole moment.

Chapter 2

intermolecular forces
Intermolecular Forces
  • Strength of attractions between molecules influence m.p., b.p., and solubility; esp. for solids and liquids.
  • Classification depends on structure.
    • Dipole-dipole interactions
    • London dispersions
    • Hydrogen bonding =>

Chapter 2

dipole dipole forces
Dipole-Dipole Forces
  • Between polar molecules
  • Positive end of one molecule aligns with negative end of another molecule.
  • Lower energy than repulsions, so net force is attractive.
  • Larger dipoles cause higher boiling points and higher heats of vaporization. =>

Chapter 2

dipole dipole
Dipole-Dipole

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Chapter 2

london dispersions

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London Dispersions
  • Between nonpolar molecules
  • Temporary dipole-dipole interactions
  • Larger atoms are more polarizable.
  • Branching lowers b.p. because of decreased surface contact between molecules.

Chapter 2

dispersions
Dispersions

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Chapter 2

hydrogen bonding
Hydrogen Bonding
  • Strong dipole-dipole attraction
  • Organic molecule must have N-H or O-H.
  • The hydrogen from one molecule is strongly attracted to a lone pair of electrons on the other molecule.
  • O-H more polar than N-H, so stronger hydrogen bonding =>

Chapter 2

h bonds
H Bonds

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Chapter 2

solubility
Solubility
  • Like dissolves like
  • Polar solutes dissolve in polar solvents.
  • Nonpolar solutes dissolve in nonpolar solvents.
  • Molecules with similar intermolecular forces will mix freely. =>

Chapter 2

ionic solute with polar solvent
Ionic Solute with Polar Solvent

Hydration releases energy.

Entropy increases.

=>

Chapter 2

classes of compounds
Classes of Compounds
  • Classification based on functional group
  • Three broad classes
    • Hydrocarbons
    • Compounds containing oxygen
    • Compounds containing nitrogen =>

Chapter 2

hydrocarbons
Hydrocarbons
  • Alkane: single bonds, sp3 carbons
  • Cycloalkane: carbons form a ring
  • Alkene: double bond, sp2 carbons
  • Cycloalkene: double bond in ring
  • Alkyne: triple bond, sp carbons
  • Aromatic: contains a benzene ring =>

Chapter 2

compounds containing oxygen

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Compounds Containing Oxygen
  • Alcohol: R-OH
  • Ether: R-O-R'
  • Aldehyde: RCHO
  • Ketone: RCOR'

Chapter 2

carboxylic acids and their derivatives

=>

Carboxylic Acids and Their Derivatives
  • Carboxylic Acid: RCOOH
  • Acid Chloride: RCOCl
  • Ester: RCOOR'
  • Amide: RCONH2

Chapter 2

compounds containing nitrogen

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Compounds Containing Nitrogen
  • Amines: RNH2, RNHR', or R3N
  • Amides: RCONH2, RCONHR, RCONR2
  • Nitrile: RCN

Chapter 2