Question

1)When optically active (R)-2- methyl cyclohexanone is treated with aqueous acid , its optical activity is lost, Explain the observation in detail.

2)Why is acid catalysed halogenation of ketones generally prefered over the base promoted halogenation?

3)Provide a detailed ,stepwise mechanism for the Robinson annulation reaction between 2- methyl cyclohexanone and methyl vinyl ketone (But-3-en-2-one

4) Give the correct structures for the compounds A to C in the following reaction schemes:
Acetophenone + pyrole (in acidic ) = A + 3 bromoprop-1-ne = B + (acidic , H2O)= C

Answer 1

1) Acid generates a planar enol species, and base generates a planar enolate species. Either way, the alpha carbon becomes sp2 hybridized during the course of that reaction, and therefore loses any stereochemical information.

2) Under acidic reaction conditions, selective monohalogenation at the a-postion is observed.The reaction can be catalyzed by addition of acid, but it is actually autocatalytic as the by-product is the hydrogen halide.

Under basic conditions, a-halogenation occurs.Typically the reaction can not be limited to monohalogenation and polyhalogenation occurs. This is because each electronegative halogen further enhances the stability of the enolate. The reaction proceeds via the very reactive enolate. so that acid catalyst preferd over basic condition.

3)