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 H2 n , where n equals any whole number of 3 or greater. Normally, these compounds are represented by geometric figures.

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.

Cyclohydrocarbons: Preparations

Cycloalkanes can be prepared by ring-cyclization reactions, such as a modified Wurtz reaction or a condensation reaction. Additionally, they can be prepared from cycloalkenes and cycloalkynes (Figure 1 ).

Figure 1

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 2 .

Figure 2

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.

BAYER STRAIN THEORY

According to the rule, compounds containing a ring will be strained due to increase in bond angle at the apexes.

If the bond angle is somewhere between 93 and 104 degree, then the ring loses its stability due to straining.

This straining is detected by Bayer and hence over straining causes the ring to de stabilize.