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Chemistry 31

Isomerism
UNIVERSITY OF THE PHILIPPINES MANILA
Padre Faura, Ermita, Manila
SS, 2009 – 2010
Ms. Anjelyn del Rosario

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Isomerism
Isomers
 Compounds that have the same molecular formula but are
not identical

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Isomerism
The Two Major Classes of Isomers:
• The two major classes of isomers are constitutional isomers and
stereoisomers.
Constitutional/structural isomers have different IUPAC names,
the same or different functional groups, different physical
properties and different chemical properties.
Stereoisomers differ only in the way the atoms are oriented in
space. They have identical IUPAC names (except for a prefix like
cis or trans). They always have the same functional group(s).
• A particular three-dimensional arrangement is called a
configuration. Stereoisomers differ in configuration.

Isomerism

Figure 5.3 A comparison of constitutional isomers
and geometric stereoisomers

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Constitutional Isomers

Structural Isomerism
• Structural isomers are molecules CH2 CH2
with the same chemical formulas CH3 CH2 CH3
but different molecular structures n-pentane, C5H12
- different “connectivity”.
• They arise because of the many
CH2 CH3
ways to create branched CH3 CH
hydrocarbons.
• a.k.a. “Constitutional Isomers” CH3
2-methylbutane, C5H12

The First 10 “Normal” Alkanes
Name Formula M.P. B.P. # Structural Isomers
• Methane CH4 -183 -162 1
• C1 - C4 are Gases
Ethane C2H6 -172 -89 1
• Propane C3H8 -187 -42 1
•
at Room Temperature
Butane C4H10 -138 0 2
• Pentane C5H12 -130 36 3
• Hexane C6H14 -95 68 5
• C5 - C16 are Liquids
Heptane C7H16 -91 98 9
• Octane C8H18 -57 126 18
•
at Room Temperature
Nonane C9H20 -54 151 35
• Decane C10H22 -30 174 75

1. Chain Isomers
 have the same number of C and H atoms but different points
of attachment
CH2 CH2
CH3 CH2 CH3
n-pentane, C5H12

CH2 CH3
CH3 CH

CH3
2-methylbutane, C5H12

2. Position Isomers
 Differ in the positions of substituents or multiple bonds

3. Functional Isomers
 Differ in their functional groups.

2. Stereochemistry
Stereochemistry
 the study of the spatial arrangement of atoms in molecules.

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Stereochemistry

Example of Stereoisomers:
Stereoisomers differ
only in the way the
atoms are oriented in
space. They have
identical IUPAC names
(except for a prefix like
cis or trans). They
always have the same
functional group(s).

Conformational Isomers
I. Rotation about
Single Bonds


Conformation: any three-dimensional arrangement of atoms in
a molecule that results from rotation about a single bond
• Conformer: a specific conformation
1. Rotation about single bonds
2. Amine inversion
• Molecules constantly rotate through all the
possible conformations.

Conformational Isomers: Rotation about Single Bonds

Example: Ethane


• Staggered conformation: a conformation about a carbon-
carbon single bond in which the atoms or groups on one
carbon are as far apart as possible from the atoms or
groups on an adjacent carbon

Example: Ethane
H
H H

H H
H


• Eclipsed conformation: a conformation about a carbon-
carbon single bond in which the atoms or groups of atoms
on one carbon are as close as possible to the atoms or
groups of atoms on an adjacent carbon

Example: Ethane
H
H

H
H HH


• Torsional strain
– also called eclipsed interaction strain
– strain that arises when nonbonded atoms separated by three
bonds are forced from a staggered conformation to an eclipsed
conformation
– the torsional strain between eclipsed and staggered ethane is
approximately 12.6 kJ (3.0 kcal)/mol

+12.6 kJ/mol


• Dihedral angle (Q): the angle created by two intersecting
planes


Example: Butane (C1-C2 bond)


Example: Butane (C2-C3 bond)

(Anti is Greek for “opposite of ”; gauche is French for “left.”)


Eclipsed Butane
– calculated energy difference between (a) the non-energy-
minimized and (b) the energy-minimized eclipsed
conformations is 5.6 kJ (0.86 kcal)/mol


• anti conformation
– a conformation about a single bond in which the groups lie at
a dihedral angle of 180°
CH 3
H H

H H
CH 3


Angle strain
 the strain induced in a molecule when the bond angles are
different from the ideal tetrahedral bond angle of 109.5°.

Torsional strain
 the strain caused by repulsion between the bonding electrons of
one substituent and the bonding electrons of a nearby
substituent.

Steric strain
 strain caused by atoms or groups of atoms approaching each
other too closely.

Conformational Isomers: Cycloalkanes
Cycloalkanes: Ring Strain
1. Cyclopropane
• Large ring strain due to angle compression
• Very reactive, weak bonds
• Torsional strain because of eclipsed hydrogens

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2. Cyclobutane
– puckering from planar cyclobutane reduces torsional strain
but increases angle strain
– the conformation of minimum energy is a puckered
“butterfly” conformation
– strain energy is about 110 kJ (26.3 kcal)/mol

3. Cyclopentane
– puckering from planar cyclopentane reduces torsional
strain, but increases angle stain
– the conformation of minimum energy is a puckered
“envelope” conformation
– strain energy is about 42 kJ (6.5 kcal)/mol

2. Cyclohexane
• Chair conformation: the most stable puckered conformation
of a cyclohexane ring
– all bond C-C-C bond angles are 110.9°
– all bonds on adjacent carbons are staggered

2. Cyclohexane
 Axial and equatorial

 ring flip

2. Cyclohexane
• Boat conformation: carbons 1 and 4 are bent toward each
other
– there are four sets of eclipsed C-H interactions and one flagpole interaction
– a boat conformation is less stable than a chair conformation by 27 kJ (6.5
kcal)/mol

2. Cyclohexane
• Twist-boat conformation
– approximately 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation
– approximately 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation

2. Cyclohexane
• Half-chair conformation

Conformational Energy

Chapter 3 35
=>

II. Amine Inversion

Amine Inversion
• The lone-pair electrons on nitrogen allow an amine to turn
“inside out” rapidly at room temperature.

• The lone pair is required for inversion: Quaternary ammonium
ions—ions with four bonds to nitrogen and hence no lone
pair—do not invert.
• amine inversion takes place through a transition state in which
the sp3 nitrogen becomes an sp2 nitrogen.

Configurational Isomers
I. Cis-Trans Isomers

 Alkenes and cyclic alkanes

 Alkenes

cis isomer
the isomer with the hydrogens on the same side of the double bond
trans isomer
the isomer with the hydrogens on opposite sides of the double bond


Exercise

“If the hydrogens are on the same side of the double bond, it is the cis isomer;
if they are on opposite sides of the double bond, it is the trans isomer.”

The E,Z System of Nomenclature
The Z isomer has the high-priority groups on the SAME side.
The E isomer has the high-priority groups on the OPPOSITE side.

Rule 1. The relative priorities of the two groups depend on the
atomic numbers of the atoms that are bonded directly to the sp2
carbon. The greater the atomic number, the higher is the priority.

Rule 2. If the two substituents bonded to an carbon start with the
same atom (there is a tie), you must move outward from the point
of attachment and consider the atomic numbers of the atoms that
are attached to the “tied” atoms.

Rule 3. If an atom is doubly bonded to another atom, the priority
system treats it as if it were singly bonded to two of those atoms. If
an atom is triply bonded to another atom, the priority system
treats it as if it were singly bonded to three of those atoms.

Rule 4. In the case of isotopes (atoms with the same atomic
number, but different mass numbers), the mass number is used to
determine the relative priorities.

Exercise:
Draw and label the E and Z isomers for each of the following
compounds:

 cyclic alkanes

The cis isomer has its substituents on the same side of the ring.
The trans isomer has its substituents on opposite sides of the ring.

 cyclic alkanes

Exercise:
Determine whether each of the following compounds is a cis isomer
or a trans isomer:

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