Transcript Hein and Arena - University of Wisconsin–Eau Claire

Chapter Outline
4.1 Formal Charge
4.6 Conformations
Structural Formulas
4.7 Cycloalkanes
4.2 Polar Covalent Bonds,
Shape, and Polarity
4.8 Alkenes, Alkynes, and
Aromatic Hydrocarbons
4.3 Noncovalent Interactions
4.4 Alkanes
4.5 Constitutional Isomers
1
Draw Lewis structure of molecules and calc. Formal Charge of an atom
in the molecule:
CH4 NCl3 HCN NH4+ NO3Formal charge (FC) – the charge of an atom in a molecule taken that all its
bonded electrons are shared evenly between other atoms that it is bonded to.
FC = (# of val. e-) – [ (# of atom’s e- around the atom itself ]
Or
FC = (# of val. e-) – [ (# of lone pair e-) + ½ ( # of bonded e-) ]
What does FC help me about a molecule?
2
How do we draw a structural formula?
For small molecules, knowing the number of
covalent bonds that an atom is expected to form
can be a good place to start.
• A carbon atom can form 4 covalent bonds.
• A nitrogen atom can form 3 covalent bonds.
• An oxygen atom can form 2 covalent bonds.
• A halogen atom can form 1 covalent bond.
• A hydrogen atom can form 1 covalent bond.
3
Let’s Try It!
The line-bond
structure of acetate
ion is shown here.
Assign formal
charges to the
atoms in this ion.
4
4.2: Structural Formula of organic compounds
Polar covalent bonds
The structure of an organic molecule can be represented by an
electron dot structure (Lewis Structure) or a line-bond structure
(structural formula).
Lewis structure of the molecule
Structural formula
Molecular formula is C3H8O.
5
4.4: Alkanes
(alkanes are hydrocarbons, contain only carbon and hydrogen
atoms.)
• Alkanes are molecules that consist only of carbon and
hydrogen atoms and contain only single bonds.
• Examples include:
methane (CH4)
ethane (CH3CH3)
propane (CH3CH2CH3)
butane, pentane, hexane, heptane, octane, nonane, decane
• These are examples of normal alkanes, which means that
their carbon chains are unbranched.
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sp3 carbon; Bonding for C with four single bonds:
What is the molecular shape or geometry of the sp3 hybrid orbital?
7
Figure 4.10 Alkanes
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General Properties of Alkanes
• Alkanes contain only nonpolar covalent bonds, are
nonpolar molecules, and are attracted to one another by
London forces.
• The more carbon atoms in a normal alkane, the higher
its boiling point.
• The longer the alkane, the greater it’s surface area and
the stronger the London forces that hold it to other
molecules.
• They are not very reactive, except for combustion
reaction.
9
4.3: Types of Noncovalent interactions:
Noncovalent interactions are interactions that
do not involve the sharing of valence
electrons (or direct bonding).
There are those noncovalent interactions due to the
attraction of permanent charges.
•
•
•
•
•
Hydrogen bonds
Salt bridges
Dipole-dipole interactions
Ion-dipole interactions
London forces
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Figure 4.8
(a) hydrogen
bonds
(b) salt bridges
(c) dipole-dipole
forces
(d) ion-dipole
interactions
(e) coordinate
covalent bonds
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Figure 4.9
12
IUPAC Rules for Naming Alkanes
1. Identify and name the parent or main chain.
2. Identify and name any alkyl groups attached to the main chain.
3. Determine the point of attachment of alkyl groups to the main chain.
4. Construct the name of the alkane by placing the alkyl groups in alphabetical order
and specify their position numbers, followed by the names of the main chain. The
labels di, tri, tetra, etc., are added if two or more identical substituents are present.
5. The lowest number is assigned to the multiply-bonded carbon atoms if there are two
competing branches.
CH3
CH3CHCH2CH2CH2CH3
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14
Example:
Give the correct IUPAC name for this molecule.
15
Let’s Talk About It!
Write the correct IUPAC name for each molecule.
16
4.5: Isomers:
Molecules that have the same molecular
formula, but different atomic connections (or
bonding) are called constitutional isomers.
Constitutional isomers of C4H10.
Note: Constitutional isomers have the
same formula but different names.
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Example 1: Let’s Try It!
Are the two alkanes constitutional isomers or are they
identical?
Example 2:
18
4.6: Conformations
•
•
•
Rotation about single bonds allows most molecules to assume a
number of different 3-dimensional shapes.
The shapes that a molecule can be in due to bond rotations are
called conformations.
The different conformations of a molecule
–
have the same molecular formula
–
have the same atomic connections
–
have a different 3-dimensional shape
–
are interchanged by the rotation of single bonds
19
Figure 4.13 Conformations of Butane
Rotation about the bond between carbons
2 and 3 in butane gives rise to different
conformations for the molecule.
Why should I care about
conformation?
20
4.7: Cycloalkanes
• In some alkanes
the carbon atoms
are joined into
rings. These are
called
cycloalkanes.
21
• When naming cycloalkanes, the ring is usually designated as the parent
chain or main chain, which is named by combining “cyclo” with the
appropriate prefix name.
• When only one branch is attached to a cycloalkane, the carbon atom of the
ring that is bondedd to the branch is assigned carbon #1, but we don’t write
it. Why?
• When a ring holds more than one branches, the ring is numbered from the
position and in the direction that gives the lowest numbers to the branches.
If one of the branches’s name starts with “e” and a second one with “m”,
assign #1 to the one that starts with “e”, so as to alphabetize the first letter
of the branch names, but still count in the direction that will result in the
lowest numbers.
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Stereoisomers
• The limited rotation of the carbon-carbon single bonds in
cycloalkanes has an interesting side effect in that it allows for
the existence of stereoisomers, molecules that
– have the same molecular formula
– have the same sequence of atomic connections
– have a different 3-dimensional shape
– are interchanged only by breaking bonds
H
H
C Cl
Br
Cl
F
Cl
Br
F
Br
F
Cl
H C
C
Br
C H
F
23
Geometric Isomers
• When stereoisomers exist because of restricted bond rotation,
the stereoisomers are called geometric isomers.
• Geometric isomers come in pairs – one is called cis and
another trans.
CH3
CH3
CH3
C C
H
H
H
C C
H
CH3
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25
4.8: Alkenes, Alkynes, and Aromatic Compounds
Unsaturated Hydrocarbons
• Alkenes contain carbon-carbon double bonds.
• Alkynes contain carbon-carbon triple bonds.
• Aromatic compounds contain benzene rings.
26
Alkenes contain C=C
Alkynes contain C≡C
27
sp2 carbon (Bonding in the C=C)
What geometrical shape is the sp2 carbon?
28
sp carbon (Bonding in the triple bond):
What geometrical shape exists around the C with a triple
bond?
29
30
Properties of Unsaturated Hydrocarbons
• Like alkanes, the unsaturated hydrocarbons have only
nonpolar covalent bonds and are nonpolar molecules.
• London forces hold members of these hydrocarbon
families to one another.
• Increasing size leads to stronger London force
attractions between molecules and higher melting and
boiling points.
• They undergo combustion reaction well like the
alkanes, and plus they are more reactive.
31
Naming alkenes and alkynes
1. When using IUPAC rules to name alkenes or alkynes,
the main chain is the longest chain of carbon atoms that
contains the carbon-carbon double or triple bond.
2. Begin numbering the parent chain at the end nearer to
the double or triple bond.
• Alkenes end with “ene”
• Alkynes end with “yne”
CH3
H2C C CH2CH3
CH3CHCH2CH CH2
CH2CH2CH3
CH2
CH CHCH2CH2CH3
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33
Aromatic compounds:
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH2OH
O
O
C OH
C O CH2CH3
34
Figure 4.20 Polycyclic Aromatic
Hydrocarbons (PAHs)
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