Alkanes that are bonded into rings are known as
cycloalkanes. Cycloalkanes, less commonly known as saturated cyclic
hydrocarbons, are the basis for countless types of organic molecules, and are
constituted by many different conformations. Defining these cycloalkanes can be determined
by Some common ones are shown below:
Cyclopropane (3 Carbons) Cyclobutane
(4 Carbons)
Cyclopentane (5 Carbons) Cyclohexane
(6 Carbons)
Cycloheptane (7 Carbons) Cyclooctane
(8 Carbons)
Some cycloalkanes appear more readily in nature than others,
and the common cycloalkanes differ greatly in stability. This is due to strain,
which affects a molecule’s stability and ability to maintain a certain shape.
There are three types of strain:
1.)
Angle
Strain: When bonds in a cycloalkane ring are either compressed or expanded;
the ideal tetrahedral angle is 109.5 °, but if the angle is
forced to be greater or less, strain results.
2.)
Torsional
Strain: eclipsing bonds cause strain, and so if a ring has a great number
of bonds on adjacent atoms that are eclipsing one another, the torsional strain
is greatly increased.
3.)
Steric
Strain: atoms tend to repel each other, so if two atoms get too close to
one another, steric strain results.
All forms of strain increase the energy of a cycloalkane
ring. However, some cycloalkanes are much higher energy because of the way
strain manifests itself.
Below is an analysis of the source of strain for four of the
more common cycloalkanes.
Cyclopropane
Typical conformation
of cyclopropane
Cyclopropane is the highest energy cycloalkane, and also the
smallest, since it has the fewest carbons. Cyclopropane exhibits all forms of
strain.
Angle Strain: Angle strain in cyclopropane is incredibly high. This
is because the normal tetrahedral angles are compressed to 60°.
Torsional
Strain:
Torsional strain is also high in cyclopropane, as the bonds of all neighboring
hydrogens are eclipsing, which boosts the energy level.
Side view of
cyclopropane: eclipsing bonds
Steric
Strain:
There is very little steric strain in cyclopropane.
Cyclobutane
Typical conformation
of cyclobutane
Cyclobutane has less strain than cyclopropane, but again because
of angle strain, it is a rather unstable cycloalkane.
Angle Strain: Cyclobutane’s angles are at 90°, so angle strain is
still an issue due to the compression of the bond angles.
Torsional
Strain:
Just like cyclopropane, cyclobutane’s bonds with attached hydrogens are all
eclipsing, which creates a good deal of strain; there is actually more strain
than cyclopropane here.
Side view of
Cyclobutane: demonstrates eclipsing bonds
Steric
Strain:
Again, steric strain is not much of a factor for cyclobutane.
Cyclopentane
Flat
conformation of “Envelope”
conformation of
Cyclopentane Cyclopentane
Cyclopentane is the first ring in ascending order of size
that experiences
torsional strain as a result of conformation; as such, it
has another shape in addition to the traditional flat ring. This is called the
“envelope”, where one point of the pentagon is pointed up, so that there are
less eclipsing interactions. This form is the more common one in nature, since
it is more stable.
Angle
Strain: 108° is actually very close to the ideal tetrahedral
angle, so angle strain is greatly reduced in cyclopentane. However, it is
overtaken by torsional strain in regards to the shape of the most stable
molecule.
Torsional Strain: Because the envelope
conformation of cyclopentane allows a few bond interactions to become
staggered, it is more stable and therefore more common. The rest of the bond
interactions stay eclipsed however.
“Envelope” conformation of cyclopentane demonstrates staggered
bonds.
Steric
Strain: Steric strain is still not a problem for a single cyclopentane
ring, though like other cycloalkanes its substituents can cause steric strain
based on the relative positions to one another.
Cyclohexane
“Flat”
conformation of cyclohexane “Chair”
conformation of cyclohexane
“Boat” conformation of cyclohexane “Twist-boat” cyclohexane
Cyclohexane is a unique cycloalkane; it is known as the most
stable of all cycloalkanes. However, it is not the flat conformation of
cyclohexane that allows it most of its stability; it is the much more common
“chair” form that makes it so stable. “Chair” conformation cyclohexanes allow
for angles close to the tetrahedral angle, and they are configured in such a
way so that there are far less eclipsing interactions between attached
hydrogens. There are two other forms of cyclohexane, known as “boat” and
“twist-boat”. The “boat” form of cyclohexane is less common due to increased
strain, and “twist-boat” only exists briefly in specific circumstances.
Angle
Strain: In the “chair” conformation of cyclohexane, bond angles are at
111.5°, which is
very close to the ideal angle width.
Thus, “chair” cyclohexane is almost angle strain-free.
Torsional
Strain: “Chair” conformation cyclohexane also is free of torsional strain,
as the shape of the molecule allows all of the bond interactions to be
staggered, and therefore free of strain.
Side view of “chair” cyclohexane demonstrates
staggered bond formations and relaxed angle strain.
Steric
Strain: cyclohexane runs into steric strain because of the way its
carbon-hydrogen bonds are oriented. Half of them are known as axial, which run
along a vertical plane that is perpendicular to the ring itself, and the other
half are known as equatorial. Axial hydrogens repel each other, as do
equatorial hydrogens, even more so when substituents are attached to adjacent
axials or equatorials.
It is very much worth noting that medium sized rings (7-13
carbons) suffer from steric and torsional strain, but not from angle strain. 14
carbon cycloalkanes and anything larger do not have strain problems because
their shape allows them to configure in a way that has ideal angles and no
torsional strain.