Alkynes: acidic character and addition reactions
- Identify the final product [D] obtained in the following sequence of reactions.CH_3CHO [ii)\ H_3O^+]i)\ LiAlH_4 [A] [Δ]H_2SO_4 [B] HBr [C], then [C] + bromobenzene Na/dry\ ether [D] Answer: 2. ethylbenzene (structure) Key idea: Follow the sequence, ending with a Wurtz-Fittig reaction. CH₃CHO with LiAlH₄ gives ethanol [A]... 1
- The most suitable reagent for the following conversion, is :H_3C-C C-CH_3 → cis-2-butene Answer: 2. H ₂ , Pd/C, quinoline Partial reduction of an alkyne to a cis-alkene needs syn addition of H₂ on a poisoned catalyst, i.e. Lindla... 1
- Predict the correct intermediate and product in the following reaction :H_3C-C CH [HgSO_4]H_2O,\ H_2SO_4 intermediate (A) → product (B) Answer: 3. A : H₃C-C(OH)=CH₂ B : H₃C-C(=O)-CH₃ Key idea: hydration of an alkyne (Kucherov reaction) first gives an enol, which tautomerises... 1
- Which one is the correct order of acidity ? Answer: 1. CHequiv CH > CH₃-Cequiv CH > CH₂=CH₂ > CH₃-CH₃ Key idea: acidity of a C-H bond increases with the s-character of carbon. Ethyne (... 1
- In the reaction H-C CH [(2)\ CH_3CH_2Br](1)\ NaNH_2/liq.NH_3 X [(2)\ CH_3CH_2Br](1)\ NaNH_2/liq.NH_3 Y, X and Y are : Answer: 1. X = 1-Butyne ; Y = 3-Hexyne Key idea: the terminal alkyne H is acidic, so NaNH₂ removes it and the acetylide ion attacks the alky... 1