Cycloalkanes

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.