Section 2.7 Conformational Isomerism
• Stereoisomerism- isomer variations in spatial or 3-D
orientation of atoms.
• One type of stereoisomerism is conformational
isomerism.
• This is a more subtle form of isomerism than skeletal
and positional isomerism.
• Remember these differ in actual bonding arrangement
of atoms (the carbon skeleton or the position of
noncarbon atoms).
In conformational isomerism
• The bonding arrangement of atoms remains constant
• The relationship of atoms in space differs as a result of rotation
round carbon-carbon single bonds.
• This rotation occurs readily, with easy interconversion of
conformers.
• Conformational isomers are conformers.
• Example: ethane- CH3CH3
• The 2 C’s are connected by a single, sigma bond. Because sigma
molecular orbitals overlap in only one position the rotation of
the C’s around the single bond does not affect the degree of
overlap.
• This basically means the rotation is unrestricted.
• As the C’s in ethane rotate the H’s on the adjacent carbons
also change continually.
2 Extreme Forms
1. eclipsed- this is where the H’s on the adjacent C’s are lined
up with one another and are therefore as close together as
possible.
- this is the least stable form
- not very abundant.
See example drawn on the board.
2. Staggered- the H’s on adjacent C’s are as far apart as
possible.
- most stable form
See example on the board.
Conformational isomerism is represented in 2 ways.
1. Sawhorse diagram- stick drawings
2. Newman projections- uses end-on projection of a C-C bond.
See board for drawings and explanation.
2.8 Cycloalkanes- Conformational and
Geometric Isomerism
• Structure and Stability
• Saturated hydrocarbons possessing one or more rings
called cycloalkanes.
• Remember each corner represents a carbon with
enough H’s to satisfy valence.
• Cyclopropane (smallest) and cyclobutane are more
unstable then larger rings due to their structures.
• They are less stable because of the internal angles of the
ring.
• Remember each C is bonded to four things; sp3
hybridized and should be tetrahedral with 109.5 degree
angles.
• Because cyclopropane only has 3 carbons in the ring it
has the geometry of an equilateral triangle with internal
angles of 60o.
• Cyclobutane has 4 carbons but takes the geometry of a
square with only 90o angles.
• In both of these cases this causes a decreased orbital
overlap in the sigma bonds and internal angle strain.
• Cyclopentane is bent out of the plane and is
energetically very stable. The bond angles are at 108o
very close to 109.5o
• The larger cycloalkanes are large enough to have
flexibility through bond rotation to bond, twist, and
pucker until each carbon has the stable tetrahedral
angle.
Conformational Isomerism in Cyclohexane
• Cyclohexanes actually pucker to form the stable
compound with the bond angles of 109.5o. The two
puckering forms are called the boat and the chair.
• In both of these conformations the carbons are
tetrahedral and all bond angles are 109.5o. However,
the chair form is more stable and the predominant
conformer of the cyclohexane.
• You can compare the stability of the structures to see
why the chair form is more predominant.
• The carbons on the opposite ends (C-1 and C-4) are
pulled closer to each other causing steric interactions
between the hydrogen's.
• In the chair form these same two carbons are bent away
from each other not having this repulsion between the
hydrogen's.
• You can also see this by drawing Newman projections of
both the boat form and chair form.
• In the boat form the C2-C3 and C5-C6 carbons make the
eclipsed conformation, and are in the less stable
conformation.
• The chair formation puts the carbons in a staggered
conformation. This is the more stable conformation.
• See Page 58 Figure 2.7.
• There are two basic orientations of the hydrogen’s on
the chair form. They are the axial position and
equatorial position.
• Axial bonds mean that the hydrogen’s go up or down in a
straight line from the carbon it is bonded to. The axial
positions alternate up then down from carbon to carbon
starting with the hydrogen on carbon one being up.
• Equatorial bonds mean that the hydrogen’s are parallel
to the carbon it is bonding off of.
Drawing the Cyclohexane Chair
• You have to learn how to draw the chair form of
cyclohexane.
• You have to practice drawing in the axial and equatorial
positions.
• Starting left to right you draw an upward line for the
axial position and then alternate up and down for each
consecutive carbon.
• Next you add the equatorial in the perimeter positions.
• Look on page 58 to help guide you in your drawings.
• You will also have to be able to draw the cyclohexane
in the opposite manner after doing what is called a
ring flip.
• Please look at page 59 to see a ring flip.
Conformational Isomerism in Substituted
Cyclohexanes
• Axial positions in the chair formation are more
crowded than the equatorial positions.
• This is because the hydrogen's protrude directly above
or below the ring and are closer to each other than the
hydrogen’s in the equatorial positions.
• A substituent other than hydrogen would prefer to
bond in an equatorial position because they are more
stable.
• The compound is more stable if the substituent is
bonded in an equatorial position.
• There is an equilibrium occurring when a substituent
bonds in the axial position.
• This occurs because the cyclohexane ring is constantly
flipping between two conformations.
• This is because all axial positions become equatorial
positions during a ring flip (see page 60).
Geometric Isomerism in Cyclic Compounds-
• Geometric isomers (cis and trans isomers) is a
type of stereoisomerism in which atoms or
groups display orientation differences around a
double bond or ring.
Cis isomer is a geometric isomer in which groups
are on the same side of a ring or double bond.
• Trans isomer is a geometric isomer in which
groups are on the opposite sides of a ring or
double bond