Cycloalkanes

 

Table of Content

Preparation of Cycloalkanes Physical Properties of Cycloalkanes Chemical Properties of Cycloalkanes

This class of hydrocarbons are carbocyclic hydrocarbons in which carbon atoms are joined by single covalent bonds to form a closed ring of carbon atoms. They are alicyclic in nature and have the general formula C_nH_2n. Following are some homologous members of this category.

           

                

The nomenclature of cycloalkanes has already reported in chapter-1.Lower members of cyclo alkanes are usually reported by simple geometrical drawing as  etc. i.e. a triangle for cyclopropane, a square for cyclobutane, a pentagon for cyclopentane and a hexagon for cyclohexane.The removal of one H atom from cycloalkanes give rise to cycloalkyl radicals e.g.

 

Refer to this video for naming of cycloalkanes: 

 

Refer to this video for naming of bicyclic alkanes: 

• Preparation of Cycloalkanes

Following methods are commonly used for the preparation of cycloalkanes.

(a) From dihalogen compounds : α-ω elimination from dihalides having halogen atoms on two ends of carbon chain (α-ω dihalides) with Na or Zn dust gives rise to the formation of cycloalkanes. The method can be regarded as intramolecular Wurtz reaction and is called Freund reaction. This can be used to prepare cycloalkanes from three to six carbon atoms.

           

(b) By Clemmensen reduction : The reduction of cyclic ketones by Zn-Hg/HCI gives cycloalkanes.

  

(c) From alkenes : Alkenes on treating with CH₂I₂ in presence of Zn-Cu couple or by diazomethane (CH₂N₂) in presence of U.V. light gives derivatives of cycloalkanes.

(d) By Dieckmann cylisation : Esters of dicarboxylic acids on heating in presence of sodium ethoxide undergo intramolecular Claisen condensation to gives cyclic β-keto esters. These β-keto esters on hydrolysis and subsequent heating gives cyclic ketones which are reduced by Zn-Hg/HCI to give cycloalkanes.

  

 

(e) From aromatic compounds: Six membered cyclo compounds can be easily obtained by the catalytic reduction of benzene and its derivatives.

           

• Physical Properties of Cycloalkanes

1. Cyclopropane and cyclo butane are gases, next three members are liquid and higher members are solid.
2. These are insoluble in water but soluble in alcohol or acetone.
3. Their density increases with increase in molecular weight. Lower members are lighter than water and floats over it. It is therefore cyclohexane floats over water.
4. The boiling points increase with increase in molecular weight. The boiling point are also higher than their corresponding alkenes and alkanes.

• Chemical Properties of Cycloalkanes

Cyclopropane and cyclobutane are comparatively less stable due to more strain in ring (Follow Baeyer's strain theory) and thus tend to breakup and open the ring to act like a double bond species to give addition reactions. The cycloalkanes from cyclopentane and onwards show remarkable similaries with alkanes due to their stability. The stability of higher cycloalkanes is due to less strained ring as well as each carbon atom being sp³ hybridized like alkanes. It is therefore higher cycloalkanes are not attacked by acids, alkalies and common oxidizing under normal conditions. Following are some important reaction of cycloalkanes.

(a) Free radical substitution : Cycloalkanes are halogenated in presence of sun light or UV light like alkane.

(b) Addition reactions : Cyclopropane being the most strained ring and more reactive undergoes addition reactions like alkenes.

Cyclobutane is relatively less reactive as it does not undergo addition reactions under normal conditions. However it undergoes these reactions under special conditions.

 

(c) Oxidation : Cycloalkanes are oxidized by alkaline potassium permanganate to dicarboxylic acids involving ring fusion.

 

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