This document discusses isomers and organic compound structure. It begins by explaining that organic molecules can be described at different structural levels, from molecular formula to configuration. Compounds with the same molecular formula but different connectivity are called constitutional isomers. It also discusses condensed and bond-line structural formulas as shortcuts. The document goes on to cover resonance structures and how delocalized electrons lead to resonance hybrids rather than discrete structures. It concludes by discussing molecular geometries based on valence shell electron pair repulsion theory and how to determine a molecule's dipole moment from its structure.

1. 8/22/19
1
1
Section 1.6
STRUCTURAL FORMULAS OF
ORGANIC COMPOUNDS:
ISOMERS
©2020 McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution
permitted without the prior written consent of McGraw-Hill Education.
2©2020 McGraw-Hill Education.
Levels of Organic Structure
We think about the structures of organic molecules at
various levels
• The composition of a molecule is captured by its molecular
formula
C4H10O
• The constitution (connectivity) of a molecule is captured
by a structural formula showing how atoms are connected
• The configuration of a molecule is captured by a structural
drawing showing the positions of atoms in space
Compounds can have identical structures on a broad level but differ on
a more specific level; such compounds are called isomers.

2. 8/22/19
2
3©2020 McGraw-Hill Education.
Constitutional Isomers
Constitutional isomers have the same
composition (molecular formula) but differ in how
their atoms are connected
Ethanol and diethyl ether both have the molecular
formula C2H6O, but they have different connectivity
Constitutional isomers have different physical and
chemical properties!
4©2020 McGraw-Hill Education.
Condensed and Bond-line Formulas
Organic chemists make use of structural shortcuts to make
structure drawings easier to parse
Condensed formulas are textual and list atoms in order of
their connectivity
Bond-line formulas omit labels for carbon atoms and omit
hydrogens where they are implied (assume the octet rule is
satisfied)

3. 8/22/19
3
5©2020 McGraw-Hill Education.
Expanding a Bond-line Formula
To expand a bond-line formula into a full Lewis structure,
add in all implied carbons, hydrogens, and lone pairs
1. Draw carbon atoms at each vertex and at the end of each line
6©2020 McGraw-Hill Education.
Expanding a Bond-line Formula-1
To expand a bond-line formula into a full Lewis structure,
add in all implied carbons, hydrogens, and lone pairs
1. Draw carbon atoms at each vertex and at the end of each line
2. Add implicit hydrogens to the carbons such that the octet rule is
satisfied (consistent with formal charges as well)

4. 8/22/19
4
7©2020 McGraw-Hill Education.
Expanding a Bond-line Formula-2
To expand a bond-line formula into a full Lewis structure,
add in all implied carbons, hydrogens, and lone pairs
1. Draw carbon atoms at each vertex and at the end of each line
2. Add implicit hydrogens to the carbons such that the octet rule is
satisfied (consistent with formal charges as well)
3. If missing, add implicit lone pairs on heteroatoms and/or
carbons
8©2020 McGraw-Hill Education.
Section 1.7
RESONANCE AND CURVED
ARROWS

5. 8/22/19
5
9©2020 McGraw-Hill Education.
Resonance
The assumption of electron localization built into Lewis
structures is not always true!
Molecules containing delocalized electrons have multiple
Lewis-structural representations called resonance structures
For example, protonated formaldehyde has two
important resonance forms
O
H
H H
δ+
δ+
The true resonance hybrid contains a
partial double bond between C and O and
partial positive charge on C and O.
The compound does not convert between A and B;
it is in reality a weighted sum of structures A and B!
10©2020 McGraw-Hill Education.
Effects of Resonance
A Lewis structure of ozone suggests that the two O–O
bonds are not equivalent
However, experiment confirms that they are equivalent—they have the
same length and the outer oxygens have the same partial negative charge!
Resonance structures reveal that the bond orders of both O–O bonds are
between 1 and 2 and that both outer oxygens share negative charge
The resonance hybrid
Resonance represents the delocalization and
stabilization of charge. Molecules with resonance
are stabilized!

6. 8/22/19
6
11©2020 McGraw-Hill Education.
Rules of Resonance
When can resonance be considered?
• Rule 1. The connectivity and positions of the atoms must
remain the same in all resonance structures
A and B are not resonance structures
because they differ in connectivity.
They are constitutional isomers.
• Rule 2. Each contributing structure must have the same
number of electrons and the same net charge
A, C, and D are resonance structures.
All have a net charge of zero,
although the formal charges of atoms
differ between them.
12©2020 McGraw-Hill Education.
Rules of Resonance-1
When can resonance be considered?
• Rule 3. Each contributing structure must have the same
number of unpaired electrons
F contains two unpaired electrons
while A, C, and D contain none. F is
not an important resonance form of A.
• Rule 4. Atoms of second-row elements must not violate
the octet rule
G and H are valid resonance forms,
but I is not because within it, nitrogen
violates the octet rule.

7. 8/22/19
7
13©2020 McGraw-Hill Education.
Rules of Resonance-2
Which resonance form is most important?
• Rule 5. Resonance structures containing more covalent
bonds are typically more important
Structure J with a double bond is the
major contributor and is more important
than structure K with a single bond (see
also Rule 6).
• Rule 6. Resonance structures with minimal separation
of opposite charges are most important
Structure L lacks formal charges and is
more important than structure M, which
contains formal charges.
14©2020 McGraw-Hill Education.
Rules of Resonance-3
Which resonance form is most important?
• Rule 7. Resonance structures with negative charge on
the most electronegative atom and positive charge on
the most electropositive atom are most important
In the major contributor N, the more electronegative
oxygen atom bears the negative charge.

8. 8/22/19
8
15©2020 McGraw-Hill Education.
The Importance of Resonance
Recognizing resonance and drawing valid resonance
structures are important skills for at least two reasons:
1. Resonance indicates that molecules are stabilized
due to the delocalization of charge.
Carbonate anion is fairly stable despite its
overall charge of −2 due to delocalization
of the charge over three oxygen atoms.
2. Resonance forms containing formal charges can
reveal hidden points of reactivity in organic
molecules.
Minor contributor K reveals that
carbon in C=O can act as a
Lewis acid!
16©2020 McGraw-Hill Education.
Section 1.8
SULFUR AND PHOSPHORUS-
CONTAINING ORGANIC
COMPOUNDS AND THE OCTET RULE

9. 8/22/19
9
17©2020 McGraw-Hill Education.
Expanded Octets of Third-row Atoms
Atoms of elements in the third row can bear more than
eight electrons in covalent compounds due to
hypervalency
Compounds of phosphorus and sulfur are the most
important examples of this phenomenon
P S
P S S
Important examples include PCl5,
phosphates, sulfates, sulfonates,
and SF4.
18©2020 McGraw-Hill Education.
Examples of Expanded Octets
Trimethylphosphine oxide has two important resonance
forms; form G is more important (see Rule 1)
Phosphates in molecules such as adenosine triphosphate
(ATP) contain expanded octets at phosphorus

10. 8/22/19
10
19©2020 McGraw-Hill Education.
Section 1.9
MOLECULAR GEOMETRIES
20©2020 McGraw-Hill Education.
Valence Shell Electron Pair Repulsion
VSEPR theory: In covalent compounds, valence electron
pairs strive to be as far away from one another as possible
to minimize electron-electron repulsion
This effect is one basis of the geometries of molecules. For
example, methane’s four bonding pairs point to the vertices
of a tetrahedron
We can distinguish between the electron-group
arrangement of bonding pairs and lone pairs and the
molecular geometry defined by the bonds and atoms only

11. 8/22/19
11
21©2020 McGraw-Hill Education.
A Survey of VSEPR Geometries
22©2020 McGraw-Hill Education.
Electron Groups
Electron groups are collections of electrons situated close
in space. Two types:
• Lone (unshared, non-bonding) electron pairs
• Bonding electrons – multiple bonds are
treated as a single group
X
treated as one
electron group
The number of electron groups around an atom dictates the
geometry at that atom
Unshared electron pairs are considered “larger” and “more
repulsive” than bonded pairs

12. 8/22/19
12
23©2020 McGraw-Hill Education.
Section 1.10
MOLECULAR DIPOLE MOMENTS
24©2020 McGraw-Hill Education.
Applying Geometry and Polarization
With knowledge of the geometry of a molecule and the
polarization of its bonds, we can deduce the molecular
dipole moment
Individual bond dipoles are added in a vector sense to
produce the overall molecular dipole vector
Compounds containing polarized bonds may still be
nonpolar overall!

13. 8/22/19
13
25©2020 McGraw-Hill Education.
Nonpolar and Polar Molecules
Carbon tetrachloride (CCl4) is a nonpolar molecule. The four
C–Cl bond dipoles add to a net molecular dipole of zero
Methylene chloride (CH2Cl2) is a polar molecule. The C–H
and C–Cl bond dipoles add to give a net molecular dipole
26©2020 McGraw-Hill Education.
Identifying Molecular Dipoles
Symmetric molecules and those with weakly polarized or
unpolarized bonds are nonpolar; all others are polar
The polarization of a molecule determines its chemical
behavior (reactivity) and physical properties