STRUCTURAL FORMULAS OF CYCLOHYDROCARBONS

Cyclohydrocarbons: Structural Formulas

Cyclohydrocarbons are the cyclic, or ring, analogues of the alkanes, alkenes, and alkynes. These hydrocarbons are often referred to as alicyclic compounds, and the simplest class is made up of the cycloalkanes. Their general molecular formula is C _n H _2 n , where n equals any whole number of 3 or greater. Normally, these compounds are represented by geometric figures.

REACTIONS OF CYCLOHYDROCARBONS

Cyclohydrocarbons: Reactions

Due to angle strain, the bonds in three‐ and four‐membered carbon rings are weak. Because of these weak bonds, cyclopropane and cyclobutane undergo reactions that are atypical of alkanes. For example, cyclopropane reacts with halogens dissolved in carbon tetra‐chloride to form dihaloalkanes.

Under similar conditions, straight‐chain propane does not react.

In general, cycloalkanes undergo the normal reactions of the aliphatic alkanes (the straight‐chain and branched‐chain alkanes). Thus, cyclopentane will react with halogens in ultraviolet light to form halosubstituted cycloalkanes.

Cycloalkenes and cycloalkynes undergo the ordinary addition reactions of alkenes and alkynes. Cyclopropene, cyclopropyne, cyclobutene, and cyclobutyne also undergo ring‐opening reactions.

PREPARATION OF CYCLOHYDROCARBONS

Cyclohydrocarbons: Preparations

Cycloalkenes and cycloalkynes are normally prepared from cycloalkanes by ordinary alkene‐forming reactions, such as dehydration, dehalogenation, and dehydrohalogenation. Typical preparations for cyclohexene and cyclohexyne are illustrated in Figure .


     


               Figure 

REACTIVITY AND STRESSES OF CYCLOHYDROCARBONS

Cyclohydrocarbons: Reactivity, Stresses of Small Rings

All cycloalkane ring carbon atoms are sp ³ hybridized, requiring bond angles that must be tetrahedral, or approximately 110°. However, three‐ and four‐membered carbon rings are planar, so their bonding angles are 60° and 90°, respectively. The small size of these bond angles compared to the tetrahedral angle means that the orbital overlap region cannot exist directly between two carbon atoms. Rather, the two carbons are located at a slight angle to the overlap region, an arrangement that creates a weaker, more reactive bond. This type of bonding strain is called angle strain. Five‐membered rings have a bond angle of 108°, which is very close to the tetrahedral angle. As a result, this ring system possesses little angle strain. Rings of six carbons or more bend and thus maintain the stable tetrahedral bonding angle.

In both the chair and boat forms of cyclohexane, there is no angle strain; however, the boat form has another type of ring strain called torsional strain. Torsional strain is caused by the interaction of hydrogen atoms or substituents that are bonded to either adjacent or nonadjacent carbon atoms and situated in an eclipsed fashion. The boat form of cyclohexane has two forms of torsional strain. The first type is caused by the interaction of atoms or groups that are eclipsed on adjacent carbons. It occurs between the four lower hydrogen atoms on the four carbons at the bottom of the boat form of cyclohexane. The second type is caused by eclipsed atoms or groups on nonadjacent carbons. This occurs between the eclipsed hydrogen atoms of the two upper carbons of the boat form. These two types of torsional strain account for the higher energy states of the eclipsed cycloalkanes compared to the cycloalkanes with staggered arrangements. Because the chair form of cyclohexane does not have torsional strain, it is more stable and has a lower energy state than the boat form.

STEREOCHEMISTRY OF CYCLOHYDROCARBONS

Cyclohydrocarbons: Stereochemistry

Because ring compounds have restricted rotation around their carbon‐carbon single bonds, cycloalkanes with two or more substituents have cis and trans stereoisomers. For example, 1,2‐dibromobutane can exist as

Cycloalkane rings of five or fewer carbon atoms form planar or very nearly planar rings. For such molecules, cis and trans isomerism refers to the location of substituents as either on the same side of the ring plane ( cis) or on opposite sides of the ring plane ( trans). Cycloalkanes that contain six or more carbons bend out of plane, or “pucker.” For example, cyclohexane exists in three forms.

The two end structures are called chair forms, while the middle structure is called the boat form. The chair form of cyclohexane is more stable than the boat form.

Because the carbon atoms of cyclohexane form single bonds with each other, the bonds must be sp ³ hybridized. This type of hybridization creates a tetrahedral shape at each atom. Thus, the two bonds projecting from each carbon atom in the ring must form an angle of approximately 110°. This arrangement leads to the following structure.

The hydrogen atoms designated as H _a occupy axial positions. Their bonds roughly parallel an imaginary axis through the center of the carbon ring. The hydrogen atoms designated as H _e occupy equatorial positions. Each of these bonds is roughly 110° away from the axial bond on that carbon. Cis and trans substituent positions can be defined as those positions either on the same side ( cis) or opposite sides ( trans) of a plane that bisects the molecule through a designated pair of carbon atoms. For example, in 1,2‐disubstitutions (the substituents are represented in the diagram below as A and B), the cis‐1,2‐isomer has one substituent in the axial position and one substituent in the equatorial position.
 

In both cases, the A and B groups are located on the same side of the plane. Thus, equatorial‐axial substitutions lead to the cis isomer.

The trans isomer has both substituents in either axial or equatorial positions.

In both cases, the A and B groups are on opposite sides of the plane bisecting the adjacent carbons, creating trans isomers.

In 1,3‐disubstitution, axial‐axial or equatorial‐equatorial arrangement generates cis‐isomers, while axial‐equatorial substitutions lead to trans isomers.

Finally, in 1,4‐disubstitutions, axial‐axial or equatorial‐equatorial substitutions lead to the trans‐isomer, while axial‐equatorial substitutions generate the cis‐isomer.

NOMENCLATURE OF CYCLOHYDROCARBONS

Cyclohydrocarbons: Nomenclature

Substituted cycloalkanes are named in a manner similar to the openchain, or aliphatic, alkanes. The following rules summarize the International Union of Pure and Applied Chemistry (IUPAC) nomenclature for substituted cycloalkanes.

1. Determine the number of carbon atoms in the ring and in the largest substituent. If the ring has more carbons than the substituent, the compound is an alkyl‐substituted cycloalkane. If the substituent possesses more carbons than the ring, the compound is a cycloalkyl alkane.
   
   
   
   
2. If an alkyl‐substituted cycloalkane has more than one substituent, the ring is numbered so the substituents have the lowest sum of numbers.
   
   
   
   
3. If the molecule possesses two or more different substituent groups, the number one position is determined by alphabetical priority.