Transcript CHEM 210 PPT_Ch04x

4.1 Isomerism: A Relationship
• Two molecules are isomers of each other if they have the
same molecular formula, but they are different in some
way.
• Their atoms can be connected in different ways; they can
differ as a result of rotations about single bonds; or they
can be mirror images of each other.
4.2 Rotational Conformations, Newman
Projections, and Dihedral Angles
• A pair of conformational
isomers, also called
conformers, differ only by
rotations about single bonds.
• They are a type of
stereoisomers.
• Conformational isomers have
the same connectivity,
meaning that the same
atoms in both species are
bonded together by the same
types of bonds.
Rotational Conformations
• Rotational conformations describe molecules having different
angles of rotation about single bonds.
• Newman projections offer a convenient way to illustrate
rotational conformations.
Dihedral Angles
• Each angle of rotation defines a particular dihedral angle, Θ,
corresponding to the angle between the C-X and C-Y bonds in
the Newman projection.
4.3 Conformational Isomers: Energy
Changes and Conformational Analysis
• Conformational analysis helps us better understand the
nature of a rotation about a given bond.
• Conformational analysis is a plot of a molecule’s energy
as a function of that bond’s dihedral angle.
4.3 Conformational Isomers: Energy
Changes and Conformational Analysis
continued…
• Eclipsed conformations occur when the C-H (or C-R) bonds on
the front carbon atom cover, or “eclipse,” those on the rear
carbon atom in the Newman projections.
• Staggered conformations occur when C-H (or C-R) bonds on
the front carbon atom bisect those on the rear carbon atom.
• The forms at the right
from the left are:
staggered, eclipsed,
staggered, eclipsed,
staggered, eclipsed,
and finally staggered.
Torsional Strain
• The difference in energy between a staggered conformation
and an eclipsed conformation of H3C-CH3 is a result of
torsional strain.
• Torsional strain is an increase in energy (i.e., decrease in
stability) that arises in an eclipsed conformation.
• At room temperature,
staggered conformations
constantly interconvert
through rotation about
the C-C single bond.
Conformational Analysis of 1,2-Dibromoethane:
Steric Strain, Gauche Conformations,
and Anti Conformations
• Steric strain is an increase in energy that results from
electron repulsion between atoms or groups of atoms
not bonded together.
• If one H atom on each CH3 group in ethane (H3C-CH3) is
replaced by another substituent, the energies of the
three staggered conformations are no longer the same.
Gauche and Anti Conformation
Gauche and Anti Conformation
continued…
• Gauche
conformation: Bulky
groups are 60° apart
in a Newman
projection.
• Anti conformation:
Bulky groups are
180° apart in a
Newman projection.
Longer Molecules and
the Zig-Zag Conformation
• For longer molecules, such as
hexane, there are several
single bonds about which
rotation can occur.
• This gives rise to many
possible conformations.
• Each of these bonds can exist
in either a gauche or an anti
conformation.
• The most stable conformation
of hexane is the all-anti
conformation, sometimes
called the zig-zag
conformation.
4.4 Conformational Isomers:
Cyclic Alkanes and Ring Strain
• Conformational analyses can also be applied to ring
structures.
• Ring structures can also have their own conformations.
• The most common rings found in nature are five- and sixmembered rings (most stable cyclics).
Cyclic Alkanes and Ring Strain
continued…
• Chemists attribute much of the instability of other ring
sizes to ring strain, the increase in energy due to
geometric constraints on the ring.
• Ring strain can be quantified using heats of combustion,
the energy given off in the form of heat (DH°) during a
combustion reaction.
• Any difference in the heat of combustion per CH2 group
for two cyclic alkanes reflects a difference in ring strain
per CH2 group.
• The total ring strain is obtained by multiplying the strain
per CH2 group by the number of CH2 groups, n.
Cyclic Alkanes and Ring Strain
continued…
4.5 The Most Stable Conformations of Cyclohexane,
Cyclopentane, Cyclobutane, and Cyclopropane
• Cyclohexane has no ring strain.
• Cyclohexane is not a planar molecule.
• Its lowest energy conformation resembles a chair and is
therefore called a chair conformation.
4.5 The Most Stable Conformations of Cyclohexane,
Cyclopentane, Cyclobutane, and Cyclopropane
continued…
• In the chair conformation, all bond angles of the ring are
about 111°, which is close to the ideal tetrahedral angle
of 109.5°.
• Cyclohexane also has little to no torsional strain, because
all of the rotational conformations about the C-C bonds
are staggered.
Cyclopentane
• Cyclopentane is relatively stable in
the envelope conformation.
• Four of its five carbon atoms lie in
one plane, with the fifth carbon
outside of that plane.
• Bond angles range from 102° to
106° in the envelope conformation.
Therefore it has some angle strain.
• Strain is minimized by adopting a
nonplanar configuration.
Cyclopentane
continued…
Cyclobutane
• Most stable conformation is slightly puckered, with
interior angles of about 88°.
• Puckering of the cyclobutane ring relieves some torsional
strain.
Cyclopropane
• There is no alternative to
having all three carbon
atoms in the same plane,
because three points
define a plane.
• All three angles of the
ring are exactly 60° and
all three C-C bonds are
the same length, so the
ring forms an equilateral
triangle.
4.6 Conformational Isomers: Cyclopentane,
Cyclohexane, Pseudorotation, and Chair Flips
• All five possible envelope conformations of cyclopentane
interconvert via pseudorotation.
• Pseudorotation occurs by partial rotations about the C-C
bonds.
Cyclohexane’s Equivalent Carbon Atoms
• All six carbon atoms in the chair conformation of
cyclohexane are equivalent, making it impossible to
distinguish among them.
• Six hydrogen atoms occupy equatorial positions and six
occupy axial positions.
• Each carbon atom in cyclohexane is bonded to one of
each.
Cyclohexane’s Equatorial and Axial Positions
• Six hydrogen atoms occupy equatorial positions and six
occupy axial positions.
• Each carbon atom in cyclohexane is bonded to one of
each.
Chair Flipping
• A chair flip converts axial hydrogens into equatorial
hydrogens, and vice versa.
• Even though a chair flip interconverts axial and
equatorial positions on a cyclohexane ring, it does not
allow substituents to switch sides of the ring’s plane.
Energy Diagram for a Chair Flip
• Rather than occurring in a single step, a cyclohexane chair flip
involves multiple independent steps.
• Throughout such a chair flip, cyclohexane assumes conformations
known as the “half-chair,” the “twist-boat,” and the “boat.”
Cyclohexane Conformations
Higher in Energy
• The half-chair, twist-boat, and boat conformations in a
chair flip are higher in energy than the chair
conformation.
• This energy difference is due to added ring strain.
• Flagpole interactions between hydrogen atoms on an
opposing pair of carbon atoms exist in the boat
conformation.
• This is a form of steric strain that is absent in the chair
conformation.
Cyclohexane Conformations Higher in
Energy
continued…
4.7 Strategies for Success: Drawing Chair
Conformations of Cyclohexane
• Chemists have devised the shorthand notation for
drawing chair conformations.
4.7 Strategies for Success: Drawing Chair
Conformations of Cyclohexane
continued…
• The following provides the steps in order to successfully
draw a cyclohexane chair conformation with 6 axial
bonds and 6 equatorial bonds.
4.8 Conformational Isomers:
Monosubstituted Cyclohexane
• If one of the hydrogen atoms in cyclohexane is replaced by
another substituent, such as CH3, the results is a
monosubstituted cyclohexane.
• The two chair conformations of a monosubstituted
cyclohexane are not equivalent.
• Two nonequivalent chair forms are conformational isomers of
each other.
Monosubstitution and Sterics
• 1,3-Diaxial interactions are a form of steric strain.
• No such steric strain exists when the CH3 group is in the
equatorial position.
• A monosubstituted cyclohexane is more stable when the
substituent is found in an equatorial position.
• Bulky groups favor the equatorial position on a
cyclohexane ring.
Monosubstitution and Sterics
continued…
4.9 Disubstituted Cyclohexanes, Cis and
Trans Isomers, and Haworth Projections
• A chair flip does not switch a substituent from one side
of the plane to the other.
• The cis–trans relationship between any pair of
substituents on a cyclohexane ring is independent of the
particular chair conformation the species is in.
• Substituents that are cis to each other on a cyclohexane
ring remain cis after a chair flip; substituents that are
trans remain trans.
• Since the cis–trans relationship of any pair of
substituents is unaffected by a chair flip, it is often more
convenient to represent substituted cyclohexanes using
Haworth projections.
Haworth Projections
• The ring is depicted as being planar and substituents are
drawn perpendicular to that plane.
• Every substituent on a cyclohexane ring favors an
equatorial position rather than an axial position.
• The most stable chair conformation can usually be
predicted for a number of disubstituted cyclohexanes.
4.10 Strategies for Success:
Molecular Modeling Kits and Chair Flips
• Because chair flips affect the three-dimensional
arrangement of atoms in space and involve only rotations
about single bonds, molecular modeling kits can be
extremely helpful when comparing chair conformations.
4.11 Constitutional Isomerism:
Identifying Constitutional Isomers
• Constitutional isomers, also called structural isomers,
share the same molecular formula but differ in their
connectivity.
4.12 Constitutional Isomers:
Index of Hydrogen Deficiency
(Degree of Unsaturation)
• Molecules that contain double bonds,
triple bonds, or rings are said to be
unsaturated because they have fewer
than the maximum number of hydrogen
atoms possible.
Index of Hydrogen
Deficiency (IHD)
• A molecule’s index of hydrogen deficiency (IHD), or
degree of unsaturation, is defined as half the number of
hydrogen atoms missing from that molecule compared to
an analogous, completely saturated molecule.
4.13 Strategies for Success: Drawing All
Constitutional Isomers of a Given Formula
• Being able to draw all constitutional isomers of a given
molecular formula can be useful.
• A systematic method to tackle these kinds of problems is
helpful.
Steps to Obtain Different Isomers
1. Determine the formula’s IHD.
This will tell you the possible combinations of double
bonds, triple bonds, and rings required in each isomer
you draw.
Steps to Obtain Different Isomers
continued...
2. Draw all possible structures that are unique in their
connectivity.
• Leave out any double bonds or triple bonds (they are
added later).
• Include rings. The number of rings must not exceed the
IHD computed from Step 1; each structure, however, may
contain fewer rings than the IHD.
Steps to Obtain Different Isomers
continued...
3. For each structure generated in Step 2, add a double
bond and/or a triple bond to achieve the total IHD
calculated in Step 1.
4. For each structure generated in Step 3, add halogen
atoms at various locations to generate as many unique
connectivities as possible.
Steps to Obtain Different Isomers of
C4H8F2
1. Determine the IHD
C4H8F2 has an IHD = 0.
C4H8F2
continued...
2. Draw isomers C4H8F2 that have no double bonds, triple
bonds, or halogens.
C4H8F2
continued...
3. No double or triple bonds can be added because IHD=0.
4. Add the two F atoms. Begin by adding one F atom to
various locations of the molecules.
C4H8F2
continued...
4. (cont’d) Add the second F atom to various locations.
C4H8F2 Isomers
4. (cont’d) Add the second F atom to various locations.
4.15 Constitutional Isomers and Biomolecules:
Amino Acids and Monosaccharides
• Leucine and isoleucine, two naturally occurring amino
acids, are constitutional isomers because they have the
same molecular formula, but differ in their connectivity.
• Monosaccharides provide many more examples of
constitutional isomers.
Monosaccharide Isomers
• Ribose and ribulose are
isomers that differ by the
location of the carbonyl
group (C=O).
• In ribose the carbonyl group
belongs to an aldehyde.
• In ribulose the carbonyl is
part of a ketone.
• Ribose is classified as an
aldose, whereas ribulose is a
ketose.
• Glucose is similarly classified
as an aldose, whereas
fructose is a ketose.
4.16 Saturation and
Unsaturation in Fats and Oils
• Oleic acid is a monounsaturated fatty acid, because it has
just one C=C double bond, whereas linolenic acid is a
polyunsaturated fatty acid, because it has three.
4.16 Saturation and
Unsaturation in Fats and Oils
• Linoleic and linolenic acids are further classified as
essential fatty acids because these are the only naturally
occurring fatty acids that cannot be synthesized in the
human body by any known chemical pathways.
Various Fatty Acids
Various Fatty Acids
continued…
Various Fatty Acids
continued…
• The number of double bonds affects the melting points
of these compounds because all C=C double bonds found
in naturally occurring fatty acids are cis.
• As a result, each double bond introduces a “kink” in the
carbon chain (i.e., oleic acid has a kink in its chain).
Summary and Conclusions
• Isomerism is a relationship between two or more
molecular species.
• Molecules are isomers of each other if they have the
same molecular formula, but are different in some way.
• Newman projections are used to show conformations
about single bonds.
• Torsional strain is the energy increase that appears in an
eclipsed conformation.
• Steric strain arises when groups not directly bonded
together are in close proximity.
Summary and Conclusions
continued…
• Heats of combustion provide insight into ring strain for
rings of various sizes.
• Cyclohexane has no ring strain because it adopts a chair
conformation, in which all angles are about 111°, and
torsional and steric strain are minimized.
• The two chair conformations of a monosubstituted
cyclohexane are not equivalent.
Summary and Conclusions
continued…
• The bulkier a substituent, the greater its tendency to
occupy an equatorial position.
• Disubstituted cyclohexanes introduce cis and trans
relationships relative to the plane of
the ring.
• The index of hydrogen deficiency is half the number of
hydrogen atoms missing from that molecule compared to
an analogous completely saturated molecule.