1.7 Structural Formulas of Organic Molecules

1
1.7Structural Formulas of Organic Molecules
2
Constitution

• The order in which the atoms of a molecule are
  connected is called its constitution or
  connectivity.
• The constitution of a molecule must be determined
  in order to write a Lewis structure.

3
Condensed structural formulas

• Lewis structures in which many (or all) covalent
  bonds and electron pairs are omitted.

can be condensed to
4
Bond-line formulas

• Omit atom symbols. Represent structure by
  showing bonds between carbons and atoms other
  than hydrogen.
• Atoms other than carbon and hydrogen are called
  heteroatoms.

5
Bond-line formulas
is shown as

• Omit atom symbols. Represent structure by
  showing bonds between carbons and atoms other
  than hydrogen.
• Atoms other than carbon and hydrogen are called
  heteroatoms.

6
1.8Constitutional Isomers
7
Constitutional isomers

• Isomers are different compounds that have the
  same molecular formula.
• Constitutional isomers are isomers that differ
  in the order in which the atoms are connected.
• An older term for constitutional isomers is
  structural isomers.

8
A Historical Note
NH4OCN
Ammonium cyanate
Urea

• In 1823 Friedrich Wöhler discovered that when
  ammonium cyanate was dissolved in hot water, it
  was converted to urea.
• Ammonium cyanate and urea are constitutional
  isomers of CH4N2O.
• Ammonium cyanate is inorganic. Urea is
  organic. Wöhler is credited with an important
  early contribution that helped overturn the
  theory of vitalism.

9
Examples of constitutional isomers
..
H

O

H
N
C



O
H
..
Nitromethane
Methyl nitrite

• Both have the molecular formula CH3NO2 but the
  atoms are connected in a different order.

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1.9Resonance
11
Resonance

• two or more Lewis structures may be written for
  certain compounds (or ions)
• Recall from Table 1.5

12
Table 1.5 How to Write Lewis Structures

• If an atom lacks an octet, use electron pairs on
  an adjacent atom to form a double or triple bond.
• ExampleNitrogen has only 6 electrons in the
  structure shown.

13
Table 1.5 How to Write Lewis Structures

• If an atom lacks an octet, use electron pairs on
  an adjacent atom to form a double or triple bond.
• ExampleAll the atoms have octets in this Lewis
  structure.

14
Table 1.5 How to Write Lewis Structures

• Calculate formal charges.
• ExampleNone of the atoms possess a formal
  charge in this Lewis structure.

15
Table 1.5 How to Write Lewis Structures

• Calculate formal charges.
• ExampleThis structure has formal charges is
  less stable Lewis structure.

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Resonance Structures of Methyl Nitrite

• same atomic positions
• differ in electron positions

more stable Lewis structure
less stable Lewis structure
17
Resonance Structures of Methyl Nitrite

• same atomic positions
• differ in electron positions

more stable Lewis structure
less stable Lewis structure
18
Why Write Resonance Structures?

• Electrons in molecules are often
  delocalizedbetween two or more atoms.
• Electrons in a single Lewis structure are
  assigned to specific atoms-a single Lewis
  structure is insufficient to show electron
  delocalization.
• Composite of resonance forms more accurately
  depicts electron distribution.

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Example

• Ozone (O3)
• Lewis structure of ozone shows one double bond
  and one single bond

Expect one short bond and one long
bond Reality bonds are of equal length (128 pm)
20
Example

• Ozone (O3)
• Lewis structure of ozone shows one double bond
  and one single bond

Resonance
21
Example

• Ozone (O3)
• Electrostatic potentialmap shows both
  endcarbons are equivalentwith respect to
  negativecharge. Middle atomis positive.

22
1.10The Shapes of Some Simple Molecules
23
Valence Shell Electron Pair Repulsions

• The most stable arrangement of groups attached
  to a central atom is the one that has the
  maximum separation of electron pairs(bonded or
  nonbonded).

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Table 1.6 Methane

• tetrahedral geometry
• HCH angle 109.5

25
Table 1.6 Methane

• tetrahedral geometry
• each HCH angle 109.5

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Table 1.6 Water

• bent geometry
• HOH angle 105

H
H

O
..
but notice the tetrahedral arrangement of
electron pairs
27
Table 1.6 Ammonia

• trigonal pyramidal geometry
• HNH angle 107

H
H

N
H
but notice the tetrahedral arrangement of
electron pairs
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Table 1.6 Boron Trifluoride

• FBF angle 120
• trigonal planar geometry allows for maximum
  separationof three electron pairs

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Multiple Bonds

• Four-electron double bonds and six-electron
  triple bonds are considered to be similar to a
  two-electron single bond in terms of their
  spatialrequirements.

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Table 1.6 Formaldehyde

• HCH and HCOangles are close to 120
• trigonal planar geometry

31
Table 1.6 Carbon Dioxide

• OCO angle 180
• linear geometry

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1.11Molecular Dipole Moments
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Dipole Moment

• A substance possesses a dipole moment if its
  centers of positive and negative charge do not
  coincide.
• ? e x d
• (expressed in Debye units)

not polar
34
Dipole Moment

• A substance possesses a dipole moment if its
  centers of positive and negative charge do not
  coincide.
• ? e x d
• (expressed in Debye units)

polar
35
Molecular Dipole Moments
?
?-
?-

• molecule must have polar bonds
• necessary, but not sufficient
• need to know molecular shape
• because individual bond dipoles can cancel

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Molecular Dipole Moments
Carbon dioxide has no dipole moment ? 0 D
37
Figure 1.7
Dichloromethane
Carbon tetrachloride
? 0 D
? 1.62 D
38
Figure 1.7
Resultant of thesetwo bond dipoles is
Resultant of thesetwo bond dipoles is
? 0 D
Carbon tetrachloride has no dipolemoment
because all of the individualbond dipoles cancel.
39
Figure 1.7
Resultant of thesetwo bond dipoles is
Resultant of thesetwo bond dipoles is
? 1.62 D
The individual bond dipoles do notcancel in
dichloromethane it hasa dipole moment.
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1.12Acids and BasesThe Arrhenius View
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Definitions

• Arrhenius
• An acid ionizes in water to give protons. A base
  ionizes in water to give hydroxide ions.
• Brønsted-Lowry
• An acid is a proton donor. A base is a proton
  acceptor.
• Lewis
• An acid is an electron pair acceptor. A base is
  an electron pair donor.

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Arrhenius Acids and Bases

• An acid is a substance that ionizes to give
  protons when dissolved in water.

A base is a substance that ionizes to give
hydroxide ions when dissolved in water.
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Arrhenius Acids and Bases

• Strong acids dissociate completely in water.
  Weak acids dissociate only partially.

Strong bases dissociate completely in water.
Weak bases dissociate only partially.

M

44
Acid Strength is Measured by pKa
pKa log10Ka
45
1.13Acids and BasesThe Brønsted-Lowry View

• Brønsted-Lowry definitionan acid is a proton
  donora base is a proton acceptor

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A Brønsted Acid-Base Reaction

• A proton is transferred from the acid to the base.

.
.
B

H
A
H
A
B

base
acid
47
A Brønsted Acid-Base Reaction

• A proton is transferred from the acid to the base.

.
.
B

H
A
H
A
B

base
acid
conjugate acid
conjugate base
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Proton Transfer from HBr to Water
hydronium ion
H
H
..

..

.
.
.
.
O
H
Br
H
Br
O


..
..
H
H

• base acid conjugate conjugate acid base

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Equilibrium Constant for Proton Transfer
..

.
.

O

H
Br
H
O
..
H3OBr
Ka
HBr

• Takes the same form as for Arrhenius Ka, but H3O
  replaces H. H3O and H are considered
  equivalent, and there is no difference in Ka
  values for Arrhenius and Brønsted acidity.

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Equilibrium Constant for Proton Transfer
..

.
.

O

H
Br
H
O
..
H3OBr
Ka
HBr
pKa log10 Ka
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Water as a Brønsted Acid
H
H
..

..

.
.
N

H
OH
H
OH
N

..
..
H
H

• base acid conjugate conjugate acid base

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Dissociation Constants (pKa) of Acids

• strong acids are stronger than hydronium ion

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Important Generalization!

• The stronger the acid, the weaker the conjugate
  base.

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Dissociation Constants (pKa) of Acids

• weak acids are weaker than hydronium ion

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Dissociation Constants (pKa) of Acids

• alcohols resemble water in acidity their
  conjugatebases are comparable to hydroxide ion
  in basicity

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Dissociation Constants (pKa) of Acids

• ammonia and amines are very weak acidstheir
  conjugate bases are very strong bases

57
Dissociation Constants (pKa) of Acids
Acid
p
K
C
on
j
.

B
a
se
a
26
43
45
62
CH3CH3

• Most hydrocarbons are extremely weak acids.