Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

(i) Enolisation of 4-methylcyclohex-3-en-1-one at the $\mathrm{C}{\mathrm{H}}_2$ between $\mathrm{C}=\mathrm{O}$ and $\mathrm{C}=\mathrm{C}$ gives the conjugated dienol (a 3-hydroxy-1,3-cyclohexadiene system); ketonisation of this dienol at the far carbon returns a ketone in which the double bond has migrated into conjugation with the carbonyl - 3-methylcyclohex-2-en-1-one. Two enols are possible from the first compound : one (toward the other alpha $\mathrm{C}{\mathrm{H}}_2$) leads back only to starting material, while the dienol leads on to product; the whole system is in equilibrium favouring the enone with the conjugated alkene.

(ii) The stereocentre is the alpha carbon C-3. Enolisation removes the alpha C-H and flattens C-3 into an sp2 carbon of the enol (3-methyl-2-hydroxycyclohexene system), destroying the stereocentre. Reversal - protonation of the enol back at C-3 - can occur equally from either face of the planar double bond, regenerating R and S forms in equal amounts. Repeated enolization and ketonisation therefore converts an optically active sample into the racemate; no bond other than the alpha C-H is ever broken.