Q30.
Question
Question: The cis ketone A is isomerized to a trans ketone B with aqueous NaOH. A similar isomerization does not occur with ketone C. (a) Draw the structure of B using a chair cyclohexane. (b) Label the substituents in C as cis or trans, and explain the difference in reactivity.
Step-by-Step Solution
VerifiedAnswer
(a) The structure of B using chair cyclohexane is shown below:
Trans isomer
(b) The substituents in C is labeled below:
The reactivity of the cis form is more than its trans counterpart.
In the presence of a base (NaOH), the cis isomer converts to a trans isomer because the acetyl group is present in the axial position; hence, it suffers from the syn-diaxial interaction. This is not the case in the trans isomer where it occupies the equatorial position and hence does not suffer from syn-diaxial interaction.
Representation of the interconversion from cis to trans in the presence of a base
The cis form is more reactive than the trans form because when the sp2 carbon of the acetyl group converts to sp3 carbon in the presence of a nucleophile, it does not suffer from syn-diaxial interaction as compared to its trans counterpart.
Representation of the nucleophilic attack on the cis and trans form
(a) The structure of B using chair cyclohexane is shown below:
Trans isomer
Representation of the trans isomer
(b) The substituents in C are labeled below:
Representation of the cis isomer
The reactivity of the cis form is more than its trans counterpart.