SolutionsThe reverse step
Exercise 1
Write the reverse step and the corresponding real reaction for: a) a primary alcohol from an ester; b) a primary amine from an amide; c) a cis alkene from an alkyne.
a) RCH2OH ⇒ RCO2R′; reaction: LiAlH4 in ether, then H2O. b) RCH2NH2 ⇒ RCONH2; reaction: LiAlH4 (or the Hofmann degradation, which gives RNH2 with one carbon fewer). c) Z alkene ⇒ alkyne; reaction: H2 with Lindlar's catalyst.
Exercise 2
A student proposes the reverse step chlorobenzene ⇒ benzene + Cl− “by SN2 of chloride on the ring”. What is wrong? What is the correct reverse step?
The chlorine synthon has the wrong polarity and the reaction does not exist: an aromatic ring does not undergo SN2, and benzene has no leaving group. Chlorine is introduced as an electrophile, Cl+, by electrophilic aromatic substitution: chlorobenzene ⇒ benzene + Cl+, whose equivalent is Cl2 with FeCl3.