Download Handout 5

PHCM 331 – Organic and Medicinal/Pharmaceutical Chemistry I
Handout # 5
Winter 2015 / 2016
The objectives of the 5th Handout are to know about:
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Cyclic Alkanes / Alkenes; Structure &Nomenclature
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Cycloalkanes and stereoisomerism.
•
Ring strains.
•
Substituted Cyclohexanes,
.
Comparison between boat and chair conformers
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Bicyclic / Polycyclic Alkanes
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Fused, Bridged, and spiro systems..
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Naming
Stereochemistry and higher polycyclic systems- Decaline
1.7 Cyclic Alkanes / Alkenes
1.7.1 Structure
A hydrocarbon that contains carbon atoms joined to form a ring is
called a cyclic hydrocarbon.
When all carbons of the ring are saturated (sp3 ), the hydrocarbon is called
cycloalkane.
When a double bond (sp2) is part of the ring, the hydrocarbon is called cycloalkene
1.7.2 Nomenclature
1)The naming members of this class is straightforward. Alkane/Alkene
names are preceded by the prefix cyclo-
2
2) If present in the ring, give double bond 1, 2 and substituents the lowest
possible numbers.
3) Count the number of carbon atoms in the ring and the number in the longest
substituent. If the number in the ring is equal to or greater than the number in the
substituent, the compound is named as an alkyl-substituted cycloalkane. For example:
Ethylcyclopentane
1-cyclopentylheptane
4) If there are two substituents per ring, they are sorted alphabetically for both,
numbering of ring carbons, and listing in the name.
1-Ethyl-2-methyl-cyclopentane
1-Methyl-2-propylcyclopentane
3
5) If there are three or more substituents, they are listed alphabetically in the name;
numbering of the substituted ring carbons is chosen so that the lowest possible sum
numbers results. Numbering in the ring may be clockwise or counterclockwise.
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1
5
2
4
3
4-Ethyl-2-methyl-1-propylcyclohexane
NOT
1………3………..4…………………..
NOT
5………1………..2…………………..
1.7.3 Cycloalkanes – stereoisomerism
As alkenes, rotation about C – C single bonds is not free in cyclic molecules.
For cycloalkanes, cis-trans isomers have to be considered:
Similar to alkenes, cis: both substituents (functional groups of interest) on the
same side of the ring
trans: substituents (functional groups of interest) on different sides of the ring
cis-1,2-Dimethylcyclopropane
trans-1,2-Dimethylcyclopropane
cis-1,2-Dimethylcyclopentane
trans-1,2-Dimethylcyclopentane
Br
Br
cis-1-Bromo-2-methylcyclohexane
1-Bromo-1methylcyclohexane
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1.7.4 Ring Strain and the Structure of Cycloalkanes
Some definitions:
Angle strain: it is the strain induced in a molecule when the
bond angles are different from the ideal tetrahedral bond
angle of 109.5o.
Torsional strain: it is caused by repulsion between the
bonding electrons of one substituent and the bonding
electrons of a nearby substituent.
Steric strain: it is caused by atoms or groups of atoms
approaching each other.
Total Strain Energies of Selected Cycloalkanes
Alkane
Strain Energy (kJ/mol)
Cyclopropane
114.2
Cyclobutane
Cyclopentane
Cyclohexane
110.9
25.9
0
5
Generally speaking, cyclic alkanes found in nature have five
or six-membered rings. On the other hand, compounds with
three and four-membered rings are found much less
frequently. This observation suggested that alkanes with
five- and six-membered rings must be more stable than
those with three- or four-membered rings. It was proposed
that such instability could be explained on the bases of
angle strain. Ideally, an sp3 hybridized carbon has bond
angles of 109.5. As a result, stability of a cycloalkane may
be predicted by determining how close the bond angle of a
planar cycloalkane is to 109.50. The angles of an equilateral
triangle are 60o. Therefore, the bond angles in a planar
cyclopropane are compressed from the ideal bond angle
of 109.5o to 60o, a 49.5o deviation causing angle strain.
6
As described earlier, normal sigma bond between two carbon
atoms are formed by the overlap of two sp3 orbitals that
point directly at each other. In cyclopropane, overlapping
orbitals cannot point directly at each other.
Therefore, the orbital overlap is less effective than in a
normal C-C bond. Hence, the less effective orbital overlap
causes the C-C bond to be weaker and could be easily
broken i.e. reactive. For example, cyclopropane could be
readily hydrogenated to propane.
H2
Cyclopropane
Pt
Propane
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Because the C-C bonding orbitals in Cyclopropane cannot
point directly at each other, they have shapes that resemble
bananas and, consequently, are often called banana bonds.
In addition to angle strain, three-membered rings have
torsional and steric strains as a result of the fact that all
hydrogen atoms are eclipsed.
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Similarly, the bond angles in planar cyclobutane would have
to be compressed from 109.5o to 90o, the bond angle
associated with a planar square. Planar cyclobutane would
then be expected to have less angle strain than
cyclopropane because the bond angles in cyclobutane are
only 19.5o away from the ideal angle.
Considering angle strain as the only factor, it was predicted
that cyclopentane be the most stable of cycloalkanes
because its bond angles (108o) are closest to the ideal
tetrahedral one.
In addition, it may be predicted that cyclohexane, with bond
angles of 120o, would be less stable.
Alkane
Strain Energy
(kJ/mol)
Cyclopentane
25.9
Cyclohexane
0
9
Contrary to this prediction, it turned out that cyclohexane is more stable
than the five-membered ring! Why?
The assumption that all cyclic molecules are planar is not accurate.
Because three points define a plane, the carbons of cyclopropane
indeed lie in a plane as it cannot twist. As a result, cyclpropane is planar.
On the other hand, other cycloalkanes are not planar. They are capable of
twisting and bend in order to attain a structure that minimizes the three
different kind of strain (angle, torsional, and steric strains) that
destabilize a cyclic compound.
26 º
10
Although planar cyclobutane would have less angle strain
than cyclopropane, it could have more torsional strain
because it has eight pairs of eclipsed hydrogens, compared
with the six of cyclopropane. Hence, cyclobutane
is not planar molecule-it is bent molecule. Although this
increases the angle strain never the less, the increase is
more than compensated for by the decreased torsional strain.
Alkane
Strain Energy (kJ/mol)
Cyclopropane
114.2
Cyclobutane
110.9
11
Similarly, if cyclopentane was planar, it would have
essentially no angle strain. In this case however, its 10 pairs
of eclipsed hydrogens would be subject to considerable
torsional strain. Consequently, cyclopentane puckers,
allowing the hydrogens to become nearly staggered
although in doing so it acquires some angle strain.
Alkane
Strain Energy
(kJ/mol)
Cyclopentane 25.9
Cyclohexane
0
12
Such form is called the envelope conformation as the
shape resembles an envelope with the flap up.
In contrast to smaller rings, distortion from planarity in
cyclohexane relieves both the angle and torsional strain of
the planar structure. Once more, the internal angle in a
planar hexagon is 120o, larger, not smaller, than the ideal
sp3 angle. Deviation from planarity will decrease both this
angle and torsional strain from the six pairs of eclipsed
hydrogens in planar model.
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Remarkably, this relaxation produces a molecule in which
essentially all of the torsional and angle strain is gone. This
energy minimum cyclohexane is called the chair form. In the
chair conformer of cyclohexane, all bond angles are 111o
and all the adjacent bonds are staggered.
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Definitions
Equatorial carbon-hydrogen bonds are parallel to the ring
carbon-carbon bonds one bond away in chair cyclohexane.
Axial carbon-hydrogen bonds are parallel and pointing either
straight up or down in chair cyclohexane
Equatorial carbon-hydrogen bond
Axial carbon-hydrogen bond
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Cyclohexane rapidly interconverts between two stable chair
conformations because of the ease of rotation about its C-C
bonds. Such process is called ring flip. When the chair
conformers interconvert, bonds that equatorial in one chair
conformer become axial in the other chair conformer and
vice versa.
1.7.5
Substituted Cyclohexanes
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Cyclohexane can also exist in a “boat conformation”
Similar to the chair conformer, the boat conformer is free of
angle strain. However, the boat conformer is not as stable
because some of its bonds are eclipsed, giving torsional
strain to the molecule. In addition, the boat conformer is
further destabilized by the close proximity of the
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“flagpole hydrogens” which causes steric strain.
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1.7.6 Conclusion: Comparison between the boat and chair conformers
H
H
1
CH2
H
H In the boat form, the bonded atoms are in
the less stable eclipsed conformation,
whereas in the chair form, they are staggered
H
H
CH2
4
H
H
Newman projection of the chair conformer
1
4
In the boat form, carbons 1, 4 are pulled
toward each other causing steric interactions
between the “flagpole” hydrogens.
In the chair form, these same carbons are
bent away from each other, and thus are
not subject to mutual repulsion.
It should be noted that while cyclohexane interconverts from
one chair conformer to the other, it can assume other
conformations namely, half-chair and twist-boat. As
expected,because the chair conformers are the most stable
conformers, at any instant more molecules of cyclohexane
are in chair conformations than in any other conformation.
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Interesting to note that it has been calculated that, for
every thousand molecules of cyclohexane in a chair
conformation, no more than two molecules are in the next
most stable conformations-the twist-boat. Cis trans in
cycloalkanes
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1.7.7 Bicyclic and Polycyclic Alkanes
Many of molecules that are encountered in organic chemistry contain more than
one ring.
Compounds that contain two fused or bridged rings are named bicycloalkanes.
1.7.8 Fused, Bridged, and spiro ring systems.
If two rings linked by one common bond (i.e two atoms), they are named ‘fused’.
Cyclic hydrocarbons containing one (or more) pairs of carbon atoms common to
two (or more) rings, are named ‘bridged’.
If two rings are only linked by one atom, this is a ‘spiro’ compound.
Note that with a terahedral carbon the two rings are perpendicular to each other.
Fused rings
bridged rings
spiro compound
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1.7.8.1 Naming:
1. The name of the alkane corresponding to the total number of carbon
atoms in the rings is considered as the parent name.
For example the following compound contains seven carbon atoms is,
therefore, a bicycloheptane.
2. The carbon atoms common to both rings are called bridgeheads. Each
bond, or chain of atoms connecting the bridgehead atom, is called a bridge.
3.
Interpose in the name an expression in brackets that denotes the number
of carbon atoms in each bridge (in order of decreasing length).
For example,
4. If substituents are present, number the bridged ring system beginning at
one bridgehead proceeding first along the longest bridge to the other
bridgehead, then along the next longest bridge back to the first
bridgehead. The shortest bridge is numbered last.
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5.
In case of Spiro compounds, use the prefix ‘spiro’, stem name and
count of ring atoms are done as described above.
spiro[5.4]decane
•Problems
•Give names for each of the following bicyclic alkanes:
Write the structure of a bicyclic compound that is an isomer of
bicycle[2.1.1]hexanes and give its name.
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1.7.9 Stereochemistry and higher polycyclic systems
Decaline
One of the most important bicyclic systems is bicyclo[4.4.0] decaline a compound
that is usually called by the common name decaline. It has the following structure:
Decaline shows cis-trans isomerism:
In cis-decaline the two hydrogen atoms attached to the bridged atoms lie
on the same side of the ring; in trans-decaline they are on the opposite
sides. To indicate this their structures are usually drawn as follows:
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More examples
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