In each reaction box, place the best reagent and conditions from the list below: 1) Ph 2) CH3ONa in CH3OH 3) NaBH4 in EtOH 4) BzHs in THF 5) PCC in CH2Cl2 6) PhCO2H in CH2Cl2 7) O3 in CH2Cl2 8) PhLi in Et2O 9) H2O2/NaOH 10) H3PO4
In each reaction box, place the best reagent and conditions from the list below: 1) Ph 2) CH3ONa in CH3OH 3) NaBH4 in EtOH 4) BzHs in THF 5) PCC in CH2Cl2 6) PhCO2H in CH2Cl2 7) O3 in CH2Cl2 8) PhLi in Et2O 9) H2O2/NaOH 10) H3PO4
The Correct Answer and Explanation is:
Correct Reagents:
- PhLi in Et₂O
- H₃PO₄, Δ
Explanation:
This synthesis converts cyclohexanone into 1-phenylcyclohex-1-ene in a two-step process. The overall transformation involves the addition of a phenyl group and the formation of a carbon-carbon double bond.
Step 1: Nucleophilic Addition of a Phenyl Group
The first step is to add the phenyl (Ph) group to the cyclohexanone ring. The starting material is a ketone, which has an electrophilic carbonyl carbon. To form a new carbon-carbon bond at this position, a strong carbon-based nucleophile is required. From the provided list, phenyllithium (PhLi in Et₂O) is the ideal reagent. The highly polarized carbon-lithium bond in PhLi makes the phenyl group act as a powerful nucleophile. It attacks the carbonyl carbon of cyclohexanone, leading to the formation of a lithium alkoxide intermediate. A subsequent (implied) aqueous workup protonates the alkoxide to yield the tertiary alcohol, 1-phenylcyclohexan-1-ol.
Step 2: Dehydration of the Alcohol
The second step transforms the alcohol intermediate, 1-phenylcyclohexan-1-ol, into the final alkene product, 1-phenylcyclohex-1-ene. This is an elimination reaction, specifically a dehydration. Dehydration of alcohols is effectively carried out by heating them with a strong acid. The reagent H₃PO₄, Δ (phosphoric acid with heat) serves this purpose. The acid protonates the hydroxyl (-OH) group, converting it into an excellent leaving group (H₂O). A base (such as H₂O or the conjugate base H₂PO₄⁻) then abstracts a proton from an adjacent carbon, and the electrons from that C-H bond form a new π bond, expelling the water molecule. This reaction forms the more stable alkene, where the double bond is conjugated with the phenyl ring, resulting in the desired 1-phenylcyclohex-1-ene product.
