Cycloalkane

thumb|right|Ball-and-stick model of [[cyclobutane]]

In organic chemistry, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are the monocyclic saturated hydrocarbons. In other words, a cycloalkane consists only of hydrogen and carbon atoms arranged in a structure containing a single ring (possibly with side chains), and all of the carbon-carbon bonds are single. The larger cycloalkanes, with more than 20 <!--this could be less, now that I think about it the information in the paraffin article could be wrong, or not apply to the ring varieties --> carbon atoms are typically called cycloparaffins. All cycloalkanes are isomers of alkenes.

The cycloalkanes without side chains (also known as monocycloalkanes) are classified as small (cyclopropane and cyclobutane), common (cyclopentane, cyclohexane, and cycloheptane), medium (cyclooctane through cyclotridecane), and large (all the rest). <!--The need for the delineation between medium and large rings, is that ring strain is zero for all rings with 14 or more members, i.e. the large rings -->

Besides this standard definition by the International Union of Pure and Applied Chemistry (IUPAC), in some authors' usage the term cycloalkane includes also those saturated hydrocarbons that are polycyclic. because this system is constantly being revised. In the above example [4.2.0]-bicyclooctane would be written bicyclo[4.2.0]octane to fit the conventions for IUPAC naming. It then has room for an additional numerical prefix if there is the need to include details of other attachments to the molecule such as chlorine or a methyl group. Another convention for the naming of compounds is the common name, which is a shorter name and it gives less information about the compound. An example of a common name is terpineol, the name of which can tell us only that it is an alcohol (because the suffix "-ol" is in the name) and it should then have a hydroxyl group (–OH) attached to it.

The IUPAC naming system for organic compounds can be demonstrated using the example provided in the adjacent image. The base name of the compound, indicating the total number of carbons in both rings (including the shared edge), is listed first. For instance, "heptane" denotes "hepta-", which refers to the seven carbons, and "-ane", indicating single bonding between carbons. Next, the numerical prefix is added in front of the base name, representing the number of carbons in each ring (excluding the shared carbons) and the number of carbons present in the bridge between the rings. In this example, there are two rings with two carbons each and a single bridge with one carbon, excluding the carbons shared by both the rings. The prefix consists of three numbers that are arranged in descending order, separated by dots: [2.2.1]. Before the numerical prefix is another prefix indicating the number of rings (e.g., "bicyclo+"). Thus, the name is bicyclo[2.2.1]heptane.

Cycloalkanes as a group are also known as naphthenes, a term mainly used in the petroleum industry.

Properties

Containing only C–C and C–H bonds, cycloalkanes are similar to alkanes in their general properties. Cycloalkanes with high angle strain, such as cyclopropane, have weaker C–C bonds, promoting ring-opening reactions.

Cycloalkanes have higher boiling points, melting points, and densities than alkanes. This is due to stronger London forces because the ring shape allows for a larger area of contact.

Even-numbered cycloalkanes tend to have higher melting points than odd-numbered cycloalkanes. While variations in enthalpy and orientational entropy of the solid-phase crystal structure largely explain the odd-even alternation found in alkane melting points, conformational entropy of the solid and liquid phases has a large impact on cycloalkane melting points. For example, cycloundecane has a large number of accessible conformers near room temperature, giving it a low melting point,

|

|-

|Cyclobutane

|C₄H₈

| −90.7

| 151.2

| 0.840

|-

|Cyclononane

|C₉H₁₈

| 10–11

| 178.4

| 221

|-

|Cyclododecane

|C₁₂H₂₄

| 60.4

| 244.0

| 0.855 (extrapolated)

|-

|Cyclotridecane

|C₁₃H₂₆

| 24.5-->

|-

|Cyclohexadecane

|C₁₆H₃₂

| 60.6

| 337

Ring strain is highest for cyclopropane, in which the carbon atoms form a triangle and therefore have C–C–C bond angles. There are also three pairs of eclipsed hydrogens. The ring strain is calculated to be around 120&nbsp;kJ&nbsp;mol^&minus;1.

Cyclobutane has the carbon atoms in a puckered square with approximately 90° bond angles; "puckering" reduces the eclipsing interactions between hydrogen atoms. Its ring strain is therefore slightly less, at around 110&nbsp;kJ&nbsp;mol^&minus;1.

For a theoretical planar cyclopentane the C–C–C bond angles would be 108°, very close to the measure of the tetrahedral angle. Actual cyclopentane molecules are puckered, but this changes only the bond angles slightly so that angle strain is relatively small. The eclipsing interactions are also reduced, leaving a ring strain of about 25&nbsp;kJ&nbsp;mol^&minus;1.

In cyclohexane the ring strain and eclipsing interactions are negligible because the puckering of the ring allows ideal tetrahedral bond angles to be achieved. In the most stable chair form of cyclohexane, axial hydrogens on adjacent carbon atoms are pointed in opposite directions, virtually eliminating eclipsing strain.

In medium-sized rings (7 to 13 carbon atoms) conformations in which the angle strain is minimised create transannular strain or Pitzer strain. At these ring sizes, one or more of these sources of strain must be present, resulting in an increase in strain energy, which peaks at 9 carbons (around 50&nbsp;kJ&nbsp;mol^&minus;1). After that, strain energy slowly decreases until 12 carbon atoms, where it drops significantly; at 14, another significant drop occurs and the strain is on a level comparable with 10&nbsp;kJ&nbsp;mol^&minus;1. At larger ring sizes there is little or no strain since there are many accessible conformations corresponding to a diamond lattice.

Synthesis reactions

Cycloalkanes, referred to as naphthenes, are a major substrate for the catalytic reforming process. In the presence of a catalyst and at temperatures of about 495 to 525&nbsp;°C, naphthenes undergo dehydrogenation to give aromatic derivatives:

300px|center|noice

The process provides a way to produce high octane gasoline.

In another major industrial process, cyclohexanol is produced by the oxidation of cyclohexane in air, typically using cobalt catalysts:

:2 C₆H₁₂ + O₂ → 2 C₆H₁₁OH

This process coforms cyclohexanone, and this mixture ("KA oil" for ketone-alcohol oil) is the main feedstock for the production of adipic acid, used to make nylon.

The small cycloalkanes – in particular, cyclopropane – have a lower stability due to Baeyer strain and ring strain. They react similarly to alkenes, though they do not react in electrophilic addition, but in nucleophilic aliphatic substitution. These reactions are ring-opening reactions or ring-cleavage reactions of alkyl cycloalkanes.

Many simple cycloalkanes are obtained from petroleum. They can be produced by hydrogenation of unsaturated, even aromatic precursors.

Numerous methods exist for preparing cycloalkanes by ring-closing reactions of difunctional precursors. For example, diesters are cyclized in the Dieckmann condensation:

:center|290px|The Dieckmann condensation

The acyloin condensation can be deployed similarly.

For larger rings (macrocyclizations) more elaborate methods are required since intramolecular ring closure competes with intermolecular reactions.

:center

The Diels-Alder reaction, a [4+2] cycloaddition, provides a route to cyclohexenes:

center|290px|The Dieckmann condensation

The corresponding [2+2] cycloaddition reactions, which usually require photochemical activation, result in cyclobutanes.

See also

• Prelog strain
• Conformational isomerism
• Alkane
• Cycloalkene
• Cycloalkyne

Notes

References

Nomenclature details:`

• Organic Chemistry IUPAC Nomenclature. Rule A-23. Hydrogenated Compounds from Fused Polycyclic Hydrocarbons http://www.acdlabs.com/iupac/nomenclature/79/r79_73.htm
• Organic Chemistry IUPAC Nomenclature.Rule A-31. Bridged Hydrocarbons: Bicyclic Systems. http://www.acdlabs.com/iupac/nomenclature/79/r79_163.htm
• Organic Chemistry IUPAC Nomenclature.Rules A-41, A-42: Spiro Hydrocarbons http://www.acdlabs.com/iupac/nomenclature/79/r79_196.htm
• Organic Chemistry IUPAC Nomenclature.Rules A-51, A-52, A-53, A-54:Hydrocarbon Ring Assemblies http://www.acdlabs.com/iupac/nomenclature/79/r79_158.htm

External links

• "Cycloalkanes" at the online Encyclopædia Britannica