Skip to main content. Organic Chemistry. Search for:. Learning Objective Identify the general properties of cycloalkanes. Key Points Cycloalkanes with one ring in their carbon backbone have the chemical formula C n H 2n. The ring structure prevents cycloalkanes from achieving a tetrahedral configuration around their carbon atoms, which produces ring strain.
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Cycloalkanes are also saturated, meaning that all of the carbons atoms that make up the ring are single bonded to other atoms no double or triple bonds. There are also polycyclic alkanes, which are molecules that contain two or more cycloalkanes that are joined, forming multiple rings. Many organic compounds found in nature or created in a laboratory contain rings of carbon atoms with distinguishing chemical properties; these compounds are known as cycloalkanes.
Cycloalkanes only contain carbon-hydrogen bonds and carbon-carbon single bonds, but in cycloalkanes, the carbon atoms are joined in a ring. The smallest cycloalkane is cyclopropane.
By joining the carbon atoms in a ring,two hydrogen atoms have been lost. Cyclic compounds are not all flat molecules. All of the cycloalkanes, from cyclopentane upwards, exist as "puckered rings".
Cyclohexane, for example, has a ring structure that looks like this:. Figure 4. Note: The cyclohexane molecule is constantly changing, with the atom on the left, which is currently pointing down, flipping up, and the atom on the right flipping down. During this process, another slightly less stable form of cyclohexane is formed known as the "boat" form. In this arrangement, both of these atoms are either pointing up or down at the same time.
In addition to being saturated cyclic hydrocarbons, cycloalkanes may have multiple substituents or functional groups that further determine their unique chemical properties. The most common and useful cycloalkanes in organic chemistry are cyclopentane and cyclohexane, although other cycloalkanes varying in the number of carbons can be synthesized.
Understanding cycloalkanes and their properties are crucial in that many of the biological processes that occur in most living things have cycloalkane-like structures. Although polycyclic compounds are important, they are highly complex and typically have common names accepted by IUPAC.
The naming of cycloalkanes follows a simple set of rules that are built upon the same basic steps in naming alkanes. Cyclic hydrocarbons have the prefix "cyclo-". For simplicity, cycloalkane molecules can be drawn in the form of skeletal structures in which each intersection between two lines is assumed to have a carbon atom with its corresponding number of hydrogens. The longest straight chain contains 10 carbons, compared with cyclopropane, which only contains 3 carbons. Because cyclopropane is a substituent, it would be named a cyclopropyl-substituted alkane.
A carbon with multiple substituents should have a lower number than a carbon with only one substituent or functional group. One way to make sure that the lowest number possible is assigned is to number the carbons so that when the numbers corresponding to the substituents are added, their sum is the lowest possible.
A dash"-" must be placed between the numbers and the name of the substituent. After the carbon number and the dash, the name of the substituent can follow. When there is only one substituent on the parent chain, indicating the number of the carbon atoms with the substituent is not necessary. The numbers must be separated by commas, and the name of the functional group that follows must be separated by a dash. When there are two of the same functional group, the name must have the prefix "di".
When there are three of the same functional group, the name must have the prefix "tri". When there are four of the same functional group, the name must have the prefix "tetra". However, these prefixes cannot be used when determining the alphabetical priorities. There must always be commas between the numbers and the dashes that are between the numbers and the names. A suffix or other element s designating functional groups that may be present in the compound. As an introduction to the IUPAC nomenclature system, we shall first consider the alkanes and cycloalkanes, since these compounds provide the foundation on which the nomenclature of functional groups is built.
To use this site Click Here. A common "ane" suffix identifies these compounds as alkanes. Longer chain alkanes are well known, and their names may be found in many reference and text books. The names methane through decane should be memorized, since they constitute the root of many IUPAC names.
Fortunately, common numerical prefixes are used in naming chains of five or more carbon atoms. Some important behavior trends and terminologies: i. The formulas and structures of these alkanes increase uniformally by a CH 2 increment. A uniform variation of this kind in a series of compounds is called homologous.
Beginning with butane C 4 H 10 , and becoming more numerous with larger alkanes, we note the existence of alkane isomers.
For example, there are five C 6 H 14 isomers, shown below as abbreviated line formulas A through E :. Although these distinct compounds all have the same molecular formula, only one A can be called hexane.
How then are we to name the others? The IUPAC system requires first that we have names for simple unbranched chains, as noted above, and second that we have names for simple alkyl groups that may be attached to the chains. Examples of some common alkyl groups are given in the following table.
Note that the "ane" suffix is replaced by " yl " in naming groups. The symbol R is used to designate a generic unspecified alkyl group. Find and name the longest continuous carbon chain. Identify and name groups attached to this chain. Number the chain consecutively, starting at the end nearest a substituent group.
Designate the location of each substituent group by an appropriate number and name. Assemble the name, listing groups in alphabetical order. The prefixes di, tri, tetra etc. Halogen substituents are easily accomodated, using the names: fluoro F- , chloro Cl- , bromo Br- and iodo I-.
If the halogen is bonded to a simple alkyl group an alternative "alkyl halide" name may be used. Thus, C 2 H 5 Cl may be named chloroethane no locator number is needed for a two carbon chain or ethyl chloride. For additional examples of how these rules are used in naming branched alkanes, and for some sub-rules of nomenclature Click Here.
Cycloalkanes have one or more rings of carbon atoms. The simplest examples of this class consist of a single, unsubstituted carbon ring, and these form a homologous series similar to the unbranched alkanes. The last yellow shaded column gives the general formula for a cycloalkane of any size.
If a simple unbranched alkane is converted to a cycloalkane two hydrogen atoms, one from each end of the chain, must be lost. Hence the general formula for a cycloalkane composed of n carbons is C n H 2n.
Substituted cycloalkanes are named in a fashion very similar to that used for naming branched alkanes. The chief difference in the rules and procedures occurs in the numbering system. Since all the carbons of a ring are equivalent a ring has no ends like a chain does , the numbering starts at a substituted ring atom.
For a monosubstituted cycloalkane the ring supplies the root name table above and the substituent group is named as usual. A location number is unnecessary. If two different substituents are present on the ring, they are listed in alphabetical order, and the first cited substituent is assigned to carbon 1. The numbering of ring carbons then continues in a direction clockwise or counter-clockwise that affords the second substituent the lower possible location number. If several substituents are present on the ring, they are listed in alphabetical order.
Location numbers are assigned to the substituents so that one of them is at carbon 1 and the other locations have the lowest possible numbers, counting in either a clockwise or counter-clockwise direction. The name is assembled, listing groups in alphabetical order and giving each group if there are two or more a location number. For examples of how these rules are used in naming substituted cycloalkanes Click Here. Small rings, such as three and four membered rings, have significant angle strain resulting from the distortion of the sp 3 carbon bond angles from the ideal This angle strain often enhances the chemical reactivity of such compounds, leading to ring cleavage products.
It is also important to recognize that, with the exception of cyclopropane, cycloalkyl rings are not planar flat. The three dimensional shapes assumed by the common rings especially cyclohexane and larger rings are described and discussed in the Conformational Analysis Section. Hydrocarbons having more than one ring are common, and are referred to as bicyclic two rings , tricyclic three rings and in general, polycyclic compounds.
The structural relationship of rings in a polycyclic compound can vary. They may be separate and independent, or they may share one or two common atoms. Some examples of these possible arrangements are shown in the following table. Hydrocarbons incorporating double or triple carbon-carbon bonds are called unsaturated because hydrogen can be added to the multiple bond, converting the compound to an alkane or cycloalkane.
Such compounds are called alkenes and alkynes respectively. Because they are isomeric with cycloalkanes or bicycloalkanes, their names must clearly convey the presence of functional unsaturation. This is done by changing the ane suffix in the name of an alkane to ene for a double bond, or yne for a triple bond, as illustrated by the following examples. The location of a multiple bond in a chain is designated by a number, just as is done for substituents on a chain.
A complete treatment of alkene and alkyne nomenclature is presented below. Alkenes and alkynes are hydrocarbons which respectively have carbon-carbon double bond and carbon-carbon triple bond functional groups. The molecular formulas of these unsaturated hydrocarbons reflect the multiple bonding of the functional groups:.
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