ORGANIC CHEMISTRY 307
CHAPTER 4 LECTURE NOTES
R. Boikess
CYCLOALKANES
Consider an imaginary process in which a homolytic cleavage of a C-H bond
at each end of a chain occurs and then the two ends come together to form a
new C-C bond. Such a compound has two fewer H’s (because there are no
ends), CnH2n. Its key feature is of course a ring. When there are no functional
groups the compound is called a cycloalkane and the chemistry is very like
that of alkanes with a very few exceptions that we will explain.
A. Representing Cycloalkanes.
1. Skeletal structures are almost universally used to show rings,
very important, we understand that there are enough H atoms to make
every C tetravalent. Regular polygons are used for the skeletal
structures of the cycloalkanes. Show examples and identify the
missing H’s. Except for the three carbon ring, none of the
cycloalkanes have the actual structure of the regular polygon. We will
see why soon.
B. Nomenclature
1. The presence of a ring is signaled by the prefix cyclo. The
number of atoms in the ring is designated in the usual way starting (no ring
with 2 atoms) with 3 = prop, 4 = but etc. The absence of any functional
groups is signaled by the suffix ane.
2. A substituted cycloalkane can be named in one of two ways.
(a) By identifying and locating the substituents on the
ring. Same basic method as for acyclic (define) alkanes, but if there is only
one substituent no number is needed because all positions are equivalent.
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Example, methylcycloheptane. If there is more than one substituent, then the
first one gets position 1and we follow the same rules as for chains. Consider
1, 2-diethylcyclopentane and number in the right direction. The rule of
numbering in the direction that gives the lower number at the first point of
difference. So 1,2,4- rather than 1,3,4- is chosen overriding the alphabetizing
of substituents. Alphabetizing only comes into play when there is a tie. So 1ethyl-2-methyl is preferred over 2-ethyl-1-methyl.
(b) By naming the ring as a cycloalkyl radical and
treating it like any other alkyl radical. Thus the name 3-cyclopropylhexane.
If the ring has additional substituents the point of attachment is always 1.
For example: (3-methylcyclobutyl) or (2.2-dimethylcyclopentyl).
(c) Which way do we do it? (a) or (b)? Use common
sense in situations where one name is much simpler than the other. Two
rules to follow in order
i. maximum number of substitutions on a single
unit of structure [1-ethyl-2-methylcyclohexane, not (2-methylcyclohexyl)
ethane. And dicyclobutylmethane, not (cyclobutylmethyl)cyclobutane
ii. smaller unit as a substituent on a larger [3cyclopropylpentane (3 on 5), not (1,1-diethylmethyl) cyclopropane (5 on 3)]
3. Free rotation around C-C bonds in cyclic structures is not
possible. (Because the ends are tied together). This restriction has a number
of consequences. One is that a ring has a top and bottom. Of course this
makes no difference (because you can just turn the ring over in space) except
when there are two or more substituents on two or more carbons of the ring.
Consider 1,2-dimethylcyclobutane. We could put both methyls
on the same side of the ring (the top or the bottom) or we could put the
methyls on opposite sides of the ring. Because there is no free rotation
around the C-C bonds of the ring, the only way these two arrangements can
be interconverted is by breaking a C-C bond, which does not occur
anywhere near room temperature. Therefore we are dealing with two
different compounds, two isomers, that do not differ in connectivity (what’s
attached to what) but only in the 3-D arrangement of the atoms. Notice the
difference becomes apparent only when you build a model or use a
projection drawing so that you represent the third dimension.. Such isomers
are called stereoisomers.
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We need a way to name different stereoisomers. In the case of
disubstituted cycloalkanes, we use the prefix cis to designate the isomer with
both groups on the same side of the ring and trans to designate the isomer
with the groups on the opposite side of the ring.
So the two isomers would be named cis-1,2-dimethylcyclobutane and
trans-1,2-dimethylcyclobutane.
C. Strain
1. The 3-D constraints that result from closing a ring of carbon atoms
have more ramifications than simply the creation of another kind of
isomerism. These ramifications come under the broad label of strain. Strain
is a destabilization of a species that results from unfavorable interactions due
to its 3-dimensional structure or shape.
2. Torsional Strain. We have already encountered one type of strain
called torsional strain that results from eclipsing of C-H bonds. As we shall
see, the restricted rotation in rings can lead to H-H eclipsing and therefore
torsional strain.
3. Angle Strain. The geometry of rings can cause a destabilization
called angle strain. An sp3 hybridized carbon atom prefers bond angles of
109.5°. But if you think of a ring as a regular polygon, its angles are not this
ideal value, but something different. The greater the difference between the
actual angle in the ring and 109.5°, the greater is the angle strain.
4. Steric Strain. Sometimes the geometry of the ring forces relatively
large groups of atoms (in this context even CH2 is relatively large) too close.
We have already seen this type of strain in gauche butane.The resulting
electronic repulsions cause a destabilization called steric strain.
5. Strain Energy. We can find the total destabilization that results
from these three kinds of strain in a given cycloalkane with thermochemical
measurements. As we have seen, the heat of combustion provides a
convenient way to measure relative stability of compounds. We can measure
the heat of combustion of a cycloalkane and compare it to a value we can
calculate. The measured heat of combustion will be more exothermic than
the calculated value. The difference between the measured and calculated
values is called the strain energy.
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One simple way that we can calculate the heat of combustion is by
using the measured value for the combustion of a CH2 group in alkanes,
where there is no strain. This value is 659 kJ/mol CH2. Thus the calculated
value for cyclopropane is 3 CH2 x 659 kJ/mol CH2 = 1977 kJ/mol. The
measured value is 2091 kJ/mol. So the strain energy of cyclopropane is 2091
– 1977 = 114 kJ/mol. We can repeat these calculations and measurements
for all the cycloalkanes. The results are listed in Table 4-2 of your text. You
should focus on the last two columns, which show the way in which strain
changes with ring size.
5. Let’s look at the structures of some of the cycloalkanes and try to
understand the source of their strain energy.
a. Cyclopropane. The three carbons of this molecule must lie
in a plane and the bond angles (as measured by the lines
connecting the nuclei must be 60°. So these bond angles are
very difficult for sp3 orbitals to achieve and instead the
overlap of the orbitals is not along the internuclear line and
therefore not as effective. This poor overlap forms a
relatively weak bond and is treated as angle strain. [Figure
4-2] In addition Cyclopropane has substantial torsional
strain because the flat structure forces all the H’s to be
eclipsed. A model will reveal this clearly. In fact any flat
structure will have complete H-H eclipsing.
b. Cyclobutane; two problems angle strain, geometry predicts
90° so not as bad as cyclopropane, but 4 pairs of C-H bonds
that are eclipsed leads to even more torsional strain. To
reduce torsional strain (at the expense of more angle strain)
cyclobutane puckers, reducing the CCC bond angle to 88°
(bad) but reduces the torsional strain gaining more than it
loses. [Figure 4-3] Notice that we use sawhorse projections
for rings. [Examine a Model] (Remember the molecule
always does what’s best) From Table 4-2 not as much strain
energy as cyclopropane.
c. Cyclopentane; less angle strain but more eclipsing than
cyclobutane. Geometry predicts bond angles of 180- 360/5 =
4
108°. Thus very little angle strain but 5 pairs of eclipsing
hydrogens. Ring puckers to 4 C’s in the plane and the 5th
bent up like an envelope flap. The CCC angles are about
105° and ring is dynamic. Each C bends up giving 5
conformations that are rapidly interconverting. Thus some
angle strain and still some torsional strain because we can’t
get all the H’s staggered in the envelope conformation.
[Figure 4-4]
d. Cyclohexane; has no strain and it is by far the most
important cycloalkane. Many of its derivatives are also very
important. Seems like it should have strain. The angle of a
regular hexagon is 180 – 360/6 = 120° and there are 6 pairs
of H-H eclipsings in a flat ring. So the ring is not flat and it
can assume nonplanar conformations in which the angles are
very close to 109.5°. These conformations have virtually no
angle strain. Remember conformations are related to each
other by partial rotation around single bonds. (the rotations
are not always so obvious). While there are a number of
conformations without angle strain, we are interested in
those conformations that have no H-H eclipsing either.
i. Chair. This conformation is called the chair and
there are two of them. It has no angle strain and no H-H
eclipsings. Build, play with, and study models of
chairs. [Figure 4.5] Understand why the two chairs are
different (although the same in energy for
unsubstituted). Emphasize axial and equatorial.
[Figures 4.10 and 4.11]
ii. Understand ring flip and boat. Understand
flagpole and H-H eclipsing in boat. [Figure 4.7]
[Figure 4.9] [See animated mechanism on web site]
iii. Monosubstituted cyclohexanes. Back to axial
and equatorial Explain why eq is better than ax, two
equiv ways: gauche butane and 1,3-diaxial.[Figure 4.12]
Either way these interactions between atoms or groups of
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atoms that are too close are steric strain. Each gauche
butane interaction generates about 3.6 kJ mol-1 of strain.
e, Conformational analysis
Disubstituted cyclohexanes (dimethyl) discuss cis and
trans in cyclohexanes, which have a top and a bottom even
though they are not flat. Define configuration and contrast with
conformation. Then relate to e and a. Then do conformational
analysis on all (not all in book) three dimethylcyclohexanes
including ring flips. [Table 4.4] If two conformations differ
only in the number of gauche butane interactions we can
calculate the approximate energy difference between them
based on the observation that each gauche butane interaction
generates about 3.6 kJ mol-1 of strain.
i. Discuss and show the two 1,4-diMe’s.
is e,a and has two gauche butane interactions = 7.1 kJ mol-1 and no
change on ring flip
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has no gauche butane interactions in the di-equatorial(e,e) and 4 in the
di-axial(a,a)
ii. cis-1,2 is e,a and has 3 gauche butanes, two of the
axial with the ring and one Me-Me. No change on ring flip.
(draw and discuss) Build a model.
trans-1,2 is e,e and has only one gauche butane Me-Me.
Ring flip converts to a,a which has 4 gauche butanes with the
ring, but the methyls are anti. (draw and discuss) Build a model.
iii. cis-1,3 is e,e and has no gauche butanes (the Me’s too
far). Ring flip converts to a,a very bad, four gauche butanes
plus a 1,3-diaxial. (draw and discuss) Build a model.
trans-1,3 is e,a and ring flip doesn’t change it. Two
gauche butanes from the axial but the Me’s are too far apart to
interact. (draw and discuss) Build a model.
Note the more eq and fewer ax the more stable.
iv. Then talk about t-butyl, which almost freezes the ring
flip. (K>5000). Note that the data in Table 4.3 in the text is
obtained by studying the equilibrium between the two
configurations (isomers) of a given 4-substituted tbutylcyclohexane. You should look at those values, because
they are a very good guide to relative sizes of groups and
atoms. Some of the results are surprising. For example, why
does I appear to be smaller than Br?
5. Larger Rings. Ring sizes from 7 to 12 are called medium
rings and they all have strain (Table 4-2). The strain is primarily steric strain
because all of these rings pucker to relieve angle strain and torsional strain
from eclipsed hydrogens. But atoms on opposite sides of the ring approach
each other too closely as a result.
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6. Polycyclic Ring Systems
Many of the most interesting organic compounds are those with more
than one ring. While you might guess that such compounds simply have two
or more rings attached to a carbon backbone, actually most polycyclic
compounds have two or more rings that have some atoms in common. Such
rings are said to be fused or bridged depending on the way in which they
share atoms.
In fused rings, two adjacent carbon atoms are shared between two
rings. In bridged rings the carbon atoms that are shared are nonadjacent.
Your text discusses a number of such compounds of special interest
including the steroids, which have a basic structure that includes four fused
rings. Many well known compounds have bridged rings. Examples are
cocaine, camphor, codeine, morphine, and strychnine.
Let’s look at a few relatively simple, but important polycyclic
ring systems
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a.
Decalin (common name), A fused ring system, two six
membered rings sharing two adjacent carbons. [Figure
4.13]. Note the 10 carbons and the common name.
Many important compounds have a decalin as part of
their ring systems (steroids) or structure. Note that for
this system the rings are large enough so that ring
fusion can be cis or trans. Will not always be the case.
If the ends aren’t close, the cis is more likely because
the ends won’t reach each other in the trans. Note that
the trans isomer of decalin is more stable than the cis
because all the C-C bonds between rings are equatorial
in the trans.
b.
Norbornane (common name) is a typical bridged rather
than fused system. Related to terpenes. Note the atoms
that are shared by the rings. They are not adjacent.
Note the 1 carbon bridge and how it must be joined cis.
c.
Naming ring systems in which two rings share two
atoms. (not in text)
i.
ii.
iii.
Two rings are signaled by prefix “bicyclo”
Number of carbon atoms in the ring system
signaled in the usual way. Decalin is a
bicylcodecane and norbornane is a
bicycloheptane.
Identify the shared atoms and the three paths
to go between them. Count the number of C
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iv.
v.
vi.
atoms in each path. It is 2, 2, and 1 for
norbornane and 4, 4, and 0 for decalin.
Put these numbers in [] between bicyclo and
alkane, separate by periods.
Names are bicyclo[2.2.1]heptane and
bicyclo[4.4.0]decane
Show two more examples:
bicyclo[1.1.0]butane and
bicyclo[3.2.1]octane
bicyclo[3.2.1]octane
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