Transcript Structure and Properties of Organic Molecules

Organic Chemistry, 6th Edition
L. G. Wade, Jr.
Chapter 2
Structure and Properties
of Organic Molecules
Jo Blackburn
Richland College, Dallas, TX
Dallas County Community College District
2006, Prentice Hall
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
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2
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.
=>
Chapter 2
3
Bonding Region
• Electrons are close to both nuclei.
=>
Chapter 2
4
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
5
Bonding
Molecular Orbital
Two hydrogens, 1s constructive overlap
=>
Chapter 2
6
Anti-Bonding
Molecular Orbital
Two hydrogens, destructive overlap.
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Chapter 2
7
H2: s-s overlap
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Chapter 2
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Cl2: p-p overlap
Constructive overlap along the same
axis forms a sigma bond.
=>
Chapter 2
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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
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Pi Bonding
• Pi bonds form after sigma bonds.
• Sideways overlap of parallel p orbitals.
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Chapter 2
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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
12
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
13
sp Hybrid Orbitals
• 2 VSEPR pairs
• Linear electron
pair geometry
• 180° bond angle
Chapter 2
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14
sp2 Hybrid Orbitals
• 3 VSEPR pairs
• Trigonal planar e- pair geometry
• 120° bond angle
=>
Chapter 2
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sp3 Hybrid Orbitals
• 4 VSEPR pairs
• Tetrahedral e- pair geometry
• 109.5° bond angle
Chapter 2
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16
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
O
CH3
C
_
CH2
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Chapter 2
17
Rotation around Bonds
• Single bonds freely rotate.
• Double bonds cannot rotate unless the
bond is broken.
Chapter 2
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18
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
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Structural Isomers
CH3
O CH3
and
CH3
CH2
OH
CH3
and
CH3
=>
Chapter 2
20
Stereoisomers
Br
Br
C
H3C
Br
and
C
CH3
C
CH3
H3C
C
Br
Trans - across
Cis - same side
Cis-trans isomers are also called geometric isomers.
There must be two different groups on the sp2 carbon.
H
H3C
H
C C
H
No cis-trans isomers possible
Chapter 2
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21
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)
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Chapter 2
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Molecular Dipole Moments
• Depend on bond polarity and bond angles.
• Vector sum of the bond dipole moments.
Chapter 2
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23
Effect of Lone Pairs
Lone pairs of electrons contribute to the
dipole moment.
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Chapter 2
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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
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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.
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Chapter 2
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Dipole-Dipole
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Chapter 2
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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
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=>
Dispersions
=>
Chapter 2
29
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
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H Bonds
=>
Chapter 2
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Boiling Points and
Intermolecular Forces
CH3
CH2
CH3
OH
ethanol, b.p. = 78°C
dimethyl ether, b.p. = -25°C
CH3CH2
H3C N CH3
O CH3
N CH3
CH3CH2CH2
N H
H
trimethylamine, b.p. 3.5°C ethylmethylamine, b.p. 37°Cpropylamine, b.p. 49°C
H
CH3
CH3
CH2
OH
CH3
CH2
NH2
ethyl amine, b.p. = 17 ° C
ethanol, b.p. = 78° C
Chapter 2
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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
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Ionic Solute with
Polar Solvent
Hydration releases energy.
Entropy increases.
Chapter 2
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34
Ionic Solute with
Nonpolar Solvent
=>
Chapter 2
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Nonpolar Solute with
Nonpolar Solvent
=>
Chapter 2
36
Nonpolar Solute
with Polar Solvent
=>
Chapter 2
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Classes of Compounds
• Classification based on functional group.
• Three broad classes
Hydrocarbons
Compounds containing oxygen
Compounds containing nitrogen.
=>
Chapter 2
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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
39
Compounds Containing
Oxygen
• Alcohol: R-OH
• Ether: R-O-R'
• Aldehyde: RCHO
• Ketone: RCOR'
O
CH3CH2
C H
O
CH3
C CH3
=>
Chapter 2
40
Carboxylic Acids
and Their Derivatives
•
•
•
•
O
Carboxylic Acid: RCOOH
Acid Chloride: RCOCl
Ester: RCOOR'
Amide: RCONH2
C OH
O
C
O
O
C NH
2
C
Cl
OCH3
=>
Chapter 2
41
Compounds Containing
Nitrogen
• Amines: RNH2, RNHR', or R3N
• Amides: RCONH2, RCONHR, RCONR2
• Nitrile: RCN
CH3
O
N
C N
CH3
=>
Chapter 2
42
End of Chapter 2
Chapter 2
43