Cyclohexanes, conformation reactions

Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

Elimination Reactions and Cyclohexane Conformation The Deuterium Isotope Effect 420 The El Reaction 421... 

The mechanism in Scheme 19 is somewhat speculative, and relies heavily on the assumption that axial attack of solvent is more energetically favorable than equatorial attack of solvent on carbocations similar in structure to 64. However, the stereochemistry of acid-catalyzed hydrolysis of two diastereomeric hexa-hydrophenanthrene 9,10-epoxides (65 and 67) provides support for this proposal. These two epoxides contain transfused cyclohexane moieties that restrict the geometry of each epoxide to a single conformation. Reaction of 65 with H+ yields a single carbocation conformation 66, in which the hydroxyl group is forced to occupy an axial position (Scheme 20). 

Rate data and product studies for the solvolysis of the epimeric tosylates (134) and (135) have been interpreted in favour of the reaction of (134) through a chair cyclohexane conformation and (135) through a boat conformation, both leading to the unsymmetrical non-classical ion (136) via classical bicyclo[3,2,l]octan-2-yl cation Further data on the solvolysis of kauranoid esters have appeared. Whereas (137)... 

Hagemann s-ester a compound prepared by treating a mixture of formaldehyde and ethylacetoacetate with base and then with acid and heat, half-chair conformation the least stable conformation of cyclohexane, haloform reaction the reaction of a halogen and HO — with a methyl ketone, halogenation reaction with halogen (Br, Cl, Ij). 

Anti periplanar geometry for E2 reactions is particularly important in cyclohexane rings, where chair geometry forces a rigid relationship between the substituents on neighboring carbon atoms (Section 4.8). As pointed out by Derek Barton in a landmark 1950 paper, much of the chemical reactivity of substituted cyclohexanes is controlled by their conformation. Let s look at the E2 dehydro-halogenation of chlorocyclohexanes to see an example. 

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