Alcohols: identification of 1/2/3 alcohols, dehydration mechanism
- Which one of the following alcohols reacts instantaneously with Lucas reagent? Answer: 1. CH₃-C(CH₃)(CH₃)-OH (i.e. (CH₃)₃C-OH , drawn with CH₃ above and below the carbon) Key idea: Lucas reagent ( ZnCl₂ /conc. HCl) work... 1
- Major products A and B formed in the following reaction sequence, are[scheme: 2-methylcyclohexanol (OH and H_3C on adjacent ring carbons) PBr_3 A (major) [Δ]alc.\ KOH B (major), see figure] Answer: 2. A = [structure: 1-bromo-2-methylcyclohexane], B = [structure: 1-methylcyclohexene (double bond adjacent to methyl carbon)] Key id... 1
- Consider the following reaction and identify the product (P).CH_3-CH(CH_3)-CH(OH)-CH_3 (3 - Methylbutan-2-ol) HBr Product (P) Answer: 2. CH₃-C(Br)(CH₃)-CH₂-CH₃ Key idea: A secondary alcohol with HBr reacts through a carbocation ( S_N1 ), which can rearrange to a mor... 1
- Which amongst the following will be most readily dehydrated under acidic conditions ? Answer: 3. skeletal structure (3) bearing two OH, H ₃ C and H (see figure) Key idea: Acid-catalysed dehydration of an alcohol goes through a... 1
- Given below are two statements :Statement I :In Lucas test, primary, secondary and tertiary alcohols are distinguished on the basis of their reactivity with conc. HCl + ZnCl_2, known as Lucas Reagent.Statement II :Primary alcohols are most reactive a… Answer: 4. Statement I is correct but Statement II is incorrect. Key idea: Lucas reagent (conc. HCl + ZnCl₂ ) works via carbocation formatio... 1
- The compound A on treatment with Na gives B, and with PCl_5 gives C. B and C react together to give diethyl ether. A, B and C are in the order Answer: 4. C ₂ H ₅ OH, C ₂ H ₅ ONa, C ₂ H ₅ Cl A is ethanol. C₂H₅OH+Na→ C₂H₅ONa (B), and C₂H₅OH+PCl₅→ C₂H₅Cl (C). B and C then react by Will... 1
- Consider the reactions :X\ (C_2H_6O) Cu/573\ K A [Ag(NH_3)_2]^+,\ ^-OH,Δ Silver mirror observed; A ^-OH,Δ Y; A NH_2-NH-C(=O)-NH_2 ZIdentify A, X, Y and Z Answer: 2. A-Ethanal, X-Ethanol, Y-But-2-enal, Z-Semicarbazone. Key idea: C₂H₆O with Cu/573 K dehydrogenation means a primary alcohol. X = e... 1
- Which of the following reagents would distinguish cis-cyclopenta-1, 2-diol from the trans-isomer ? Answer: 1. Acetone Key idea: only the cis-1,2-diol has both -OH groups on the same side of the ring, so it can form a cyclic five-membered k... 1