organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...

, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are the monocyclic

saturated Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds ** Saturated and unsaturated compounds **Degree of unsaturation ** Saturated fat or fatty ac ...

hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or e ...

. In other words, a cycloalkane consists only of

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

and

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

atoms arranged in a structure containing a single ring (possibly with

side chains In organic chemistry and biochemistry, a side chain is a chemical group that is attached to a core part of the molecule called the "main chain" or backbone. The side chain is a hydrocarbon branching element of a molecule that is attached to a ...

), and all of the carbon-carbon bonds are

single Single may refer to: Arts, entertainment, and media * Single (music), a song release Songs * "Single" (Natasha Bedingfield song), 2004 * "Single" (New Kids on the Block and Ne-Yo song), 2008 * "Single" (William Wei song), 2016 * "Single", by ...

. The larger cycloalkanes, with more than 20 carbon atoms are typically called ''cycloparaffins''. All cycloalkanes are isomers of alkenes. The cycloalkanes without side chains are classified as small ( cyclopropane and

cyclobutane Cyclobutane is a cycloalkane and organic compound with the formula (CH2)4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes. Cyclobutane itself is of no commercia ...

), common (

cyclopentane Cyclopentane (also called C pentane) is a highly flammable alicyclic hydrocarbon with chemical formula C5H10 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and below the plane. It occu ...

, cyclohexane, and

cycloheptane Cycloheptane is a cycloalkane with the molecular formula C7 H14. Cycloheptane is used as a nonpolar solvent for the chemical industry and as an intermediate in the manufacture of chemicals and pharmaceutical drugs. It may be derived by Clemmen ...

), medium (

cyclooctane Cyclooctane is a cycloalkane with the molecular formula (CH2)8. It is a simple colourless hydrocarbon, but it is often a reference compound for saturated eight-membered ring compounds in general. Cyclooctane has a camphoraceous odor. Conformat ...

through cyclotridecane), and large (all the rest). 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. In any case, the general form of the

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbol ...

for cycloalkanes is C_''n''H_{2(''n''+1−''r'')}, where ''n'' is the number of carbon atoms and ''r'' is the number of rings. The simpler form for cycloalkanes with only one ring is C_''n''H_2''n''.

Nomenclature

Cycloalkanes with one ring in their structure are named by prefixing the term "cyclo" to the name of linear alkane with the same number of

atoms in the main chain as a cycloalkane to be named has in its ring. For example, the name of cyclopropane (C₃H₆) containing a three-membered ring is derived from propane (C₃H₈) - an alkane having three

atoms in the main chain. The naming of polycyclic alkanes such as

bicyclic In chemistry, a bicyclic molecule () is a molecule that features two joined rings. Bicyclic structures occur widely, for example in many biologically important molecules like α-thujene and camphor. A bicyclic compound can be carbocyclic (all ...

alkanes and spiro alkanes is more complex, with the base name indicating the number of carbons in the ring system, a prefix indicating the number of rings (e.g., "bicyclo-"), and a numeric prefix before that indicating the number of carbons in each part of each ring, exclusive of junctions. For instance, a bicyclooctane that consists of a six-membered ring and a four-membered ring, which share two adjacent carbon atoms that form a shared edge, is .2.0bicyclooctane. That part of the six-membered ring, exclusive of the shared edge has 4 carbons. That part of the four-membered ring, exclusive of the shared edge, has 2 carbons. The edge itself, exclusive of the two vertices that define it, has 0 carbons. There is more than one convention (method or nomenclature) for the naming of compounds, which can be confusing for those who are just learning, and inconvenient for those who are well-rehearsed in the older ways. For beginners, it is best to learn IUPAC nomenclature from a source that is up to date, because this system is constantly being revised. In the above example .2.0bicyclooctane would be written bicyclo .2.0ctane 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 Terpineol is any of four isomeric monoterpenoids. Terpenoids are terpene that are modified by the addition of a functional group, in this case, an alcohol. Terpineols have been isolated from a variety of sources such as cardamom, cajuput oil ...

, 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. An example of the IUPAC method is given in the adjacent image. In this example the base name is listed first, which indicates the total number of carbons in both rings including the carbons making up the shared edge (e.g., "heptane", which means "hepta-" or 7 carbons, and "-ane", which indicates only single bonding between carbons). Then in front of the base name is the numerical prefix, which lists the number of carbons in each ring, excluding the carbons that are shared by each ring, plus the number of carbons on the bridge between the rings. In this case there are two rings with two carbons each and a single bridge with one carbon, excluding the carbons shared by it and the other two rings. There is a total of three numbers and they are listed in descending order separated by dots, thus: .2.1 Before the numerical prefix is another prefix indicating the number of rings (e.g., "bicyclo-"). Thus, the name is bicyclo .2.1eptane. The group of cycloalkanes are also known as naphthenes.

Properties

Table of cycloalkanes

Cycloalkanes are similar to alkanes in their general physical properties, but they have higher boiling points,

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...

s, and

densities Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek language, Greek letter Rho (letter), rho), although the Latin letter ''D'' ca ...

than alkanes. This is due to stronger London forces because the ring shape allows for a larger area of contact. Containing only C–C and C–H bonds, unreactivity of cycloalkanes with little or no ring strain (see below) are comparable to non-cyclic alkanes.

Conformations and ring strain

In cycloalkanes, the carbon atoms are ''sp''³ hybridized, which would imply an ideal tetrahedral bond angle of 109° 28′ whenever possible. Owing to evident geometrical reasons, rings with 3, 4, and (to a small extent) also 5 atoms can only afford narrower angles; the consequent deviation from the ideal tetrahedral bond angles causes an increase in potential energy and an overall destabilizing effect. Eclipsing of hydrogen atoms is an important destabilizing effect, as well. The

strain energy In physics, the elastic potential energy gained by a wire during elongation with a tensile (stretching) force is called strain energy. For linearly elastic materials, strain energy is: : U = \frac 1 2 V \sigma \epsilon = \frac 1 2 V E \epsilon ...

of a cycloalkane is the increase in energy caused by the compound's geometry, and is calculated by comparing the experimental standard enthalpy change of combustion of the cycloalkane with the value calculated using average bond energies. Molecular mechanics calculations are well suited to identify the many conformations occurring particularly in medium rings. Ring strain is highest for cyclopropane, in which the carbon atoms form a triangle and therefore have 60 °C–C–C bond angles. There are also three pairs of eclipsed hydrogens. The ring strain is calculated to be around 120 kJ mol⁻¹.

Cyclobutane Cyclobutane is a cycloalkane and organic compound with the formula (CH2)4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes. Cyclobutane itself is of no commercia ...

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 kJ mol⁻¹. For a theoretical planar

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 kJ mol⁻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 In organic chemistry, transannular strain (also called Prelog strain after chemist Vladimir Prelog) is the unfavorable interactions of ring substituents on non-adjacent carbons. These interactions, called transannular interactions, arise from a ...

Pitzer strain In chemistry, a molecule experiences strain when its chemical structure undergoes some stress which raises its internal energy in comparison to a strain-free reference compound. The internal energy of a molecule consists of all the energy stored ...

. 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 kJ mol⁻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 kJ mol⁻¹. At larger ring sizes there is little or no strain since there are many accessible conformations corresponding to a diamond lattice. Ring strain can be considerably higher in bicyclic systems. For example,

bicyclobutane Bicyclobutane is an organic compound with the formula C4H6. It is a bicyclic molecule consisting of two ''cis''-fused cyclopropane rings, and is a colorless and easily condensed gas. Bicyclobutane is noted for being one of the most strained co ...

, C₄H₆, is noted for being one of the most strained compounds that is isolatable on a large scale; its strain energy is estimated at 267 kJ mol⁻¹.

Reactions

The simple and the bigger cycloalkanes are very stable, like

alkanes In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in whi ...

, and their reactions, for example, radical chain reactions, are like alkanes. The small cycloalkanes – in particular, cyclopropane – have a lower stability due to

Baeyer strain In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles are s ...

and ring strain. They react similarly to

alkenes In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

, though they do not react in

electrophilic addition In organic chemistry, an electrophilic addition reaction is an addition reaction where a chemical compound containing a double or triple bond has a π bond broken, with the formation of two new σ bonds.March, Jerry; (1985). Advanced Organic Che ...

, but in

nucleophilic aliphatic substitution In chemistry, a nucleophilic substitution is a class of chemical reactions in which an electron-rich chemical species (known as a nucleophile) replaces a functional group within another electron-deficient molecule (known as the electrophile). Th ...

. These reactions are ring-opening reactions or ring-cleavage reactions of

alkyl cycloalkane Alkyl cycloalkanes are chemical compounds with an alkyl group with a single ring of carbons to which hydrogens are attached according to the formula :CnH2n. They are named analogously to their normal alkane counterpart of the same carbon count: ...

s. Cycloalkanes can be formed in a Diels–Alder reaction followed by a

catalytic hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic ...

. Medium rings exhibit larger rates in for example nucleophilic substitution reactions but smaller ones in ketone reduction. This is due to conversion from ''sp³''- to ''sp²''-state, or vice versa, and the preference for ''sp²''-states in medium rings, where some of the unfavourable torsional strain in saturated rings is relieved. Molecular mechanics calculations of strain energy differences SI between ''sp³''- and ''sp²''-states show linear correlations with rates ( as logk ) of many redox or substitution reactions.Schneider, H.-J.; Schmidt, G.; Thomas F. J. Am. Chem. Soc., 1983, 105, 3556.https://pubs.acs.org/doi/pdf/10.1021/ja00349a031

References

* *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

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