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.