Chapter 2 Structure and Properties of Organic Molecules
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Transcript of Chapter 2 Structure and Properties of Organic Molecules
Chapter 2Structure and Properties
of Organic Molecules
Organic Chemistry, 6th EditionL. G. Wade, Jr.
Jo BlackburnRichland College, Dallas, TX
Dallas County Community College District2006,Prentice Hall
Chapter 2 2
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.
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Chapter 2 3
Wave Interactions• Linear combination of atomic orbitals
on different atoms produce molecular orbitals
on the same atom give hybrid orbitals.
• Conservation of orbitals.• Waves that are in phase add together.
Amplitude increases.• Waves that are out of phase cancel out.
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Chapter 2 4
Bonding Region
• Electrons are close to both nuclei.
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Chapter 2 5
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 6
Bonding Molecular Orbital
Two hydrogens, 1s constructive overlap
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Chapter 2 7
Anti-Bonding Molecular Orbital
Two hydrogens, destructive overlap.
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Chapter 2 8
H2: s-s overlap
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Chapter 2 9
Cl2: p-p overlap
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Constructive overlap along the sameaxis forms a sigma bond.
Chapter 2 10
HCl: s-p overlap
Question: What is the predicted shape for the bonding MO and the antibonding MO of the HCl molecule?
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Chapter 2 11
Pi Bonding• Pi bonds form after sigma bonds.
• Sideways overlap of parallel p orbitals.
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Chapter 2 12
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.
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Chapter 2 13
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 14
sp Hybrid Orbitals
• 2 VSEPR pairs
• Linear electron pair geometry
• 180° bond angle
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Chapter 2 15
sp2 Hybrid Orbitals• 3 VSEPR pairs
• Trigonal planar e- pair geometry
• 120° bond angle
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Chapter 2 16
sp3 Hybrid Orbitals
• 4 VSEPR pairs
• Tetrahedral e- pair geometry
• 109.5° bond angle
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Chapter 2 17
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-CC-CHO
CH3 C
O
CH2
_
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Chapter 2 18=>
• Single bonds freely rotate.
• Double bonds cannot rotate unless the bond is broken.
Rotation around Bonds
Chapter 2 19
Isomerism
• Same molecular formula, but different arrangement of atoms: isomers.
• Constitutional (or structural) isomers differ in their bonding sequence.
• Stereoisomers differ only in the arrangement of the atoms in space. =>
Chapter 2 20
Structural Isomers
CH3 O CH3 and CH3 CH2 OH
CH3
CH3
and
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Chapter 2 21
Stereoisomers
C CBr
CH3
Br
H3CC C
CH3
Br
Br
H3Cand
Cis - same side Trans - across
Cis-trans isomers are also called geometric isomers.There must be two different groups on the sp2 carbon.
C CH3C
H H
H No cis-trans isomers possible
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Chapter 2 22
Bond Dipole Moments• are due to differences in electronegativity.• depend on the amount of charge and
distance of separation.• In debyes,
x (electron charge) x d(angstroms)
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Chapter 2 23
Molecular Dipole Moments
• Depend on bond polarity and bond angles.
• Vector sum of the bond dipole moments.
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Chapter 2 24
Effect of Lone Pairs
Lone pairs of electrons contribute to the dipole moment.
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Chapter 2 25
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 interactionsLondon dispersionsHydrogen bonding
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Chapter 2 26
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 27
Dipole-Dipole
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Chapter 2 28
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.
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Chapter 2 29
Dispersions
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Chapter 2 30
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 31
H Bonds
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Chapter 2 32
Boiling Points and Intermolecular Forces
CH3 CH2 OH
ethanol, b.p. = 78°C
CH3 O CH3
dimethyl ether, b.p. = -25°C
trimethylamine, b.p. 3.5°C
N CH3H3C
CH3
propylamine, b.p. 49°C
CH3CH2CH2 N
H
H
ethylmethylamine, b.p. 37°C
N CH3CH3CH2
H
CH3 CH2 OH CH3 CH2 NH2
ethanol, b.p. = 78° C ethyl amine, b.p. = 17 ° C
Chapter 2 33
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 34
Ionic Solute with Polar Solvent
Hydration releases energy.Entropy increases. =>
Chapter 2 35
Ionic Solute withNonpolar Solvent
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Chapter 2 36
Nonpolar Solute withNonpolar Solvent
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Chapter 2 37
Nonpolar Solute with Polar Solvent
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Chapter 2 38
Classes of Compounds
• Classification based on functional group.
• Three broad classesHydrocarbonsCompounds containing oxygenCompounds containing nitrogen.
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Chapter 2 39
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 40
Compounds Containing Oxygen
• Alcohol: R-OH
• Ether: R-O-R'
• Aldehyde: RCHO
• Ketone: RCOR'
CH3CH2 C
O
H
CH3 C
O
CH3
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Chapter 2 41
Carboxylic Acids and Their Derivatives
• Carboxylic Acid: RCOOH
• Acid Chloride: RCOCl
• Ester: RCOOR'
• Amide: RCONH2
C
O
OH
C
O
Cl
C
O
OCH3C
O
NH2
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Chapter 2 42
Compounds Containing Nitrogen
• Amines: RNH2, RNHR', or R3N
• Amides: RCONH2, RCONHR, RCONR2
• Nitrile: RCN
N
O
CH3
CH3 C N
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Chapter 2 43
End of Chapter 2