Show how will you synthesise: (i) 1-phenylethanol from a suitable alkene. (ii) cyclohexylmethanol using an alkyl halide by an Sₙ2 reaction. (iii) pentan-1-ol using a suitable alkyl halide?
1-phenylethanol can be synthesised from styrene (phenylethene) by acid-catalysed hydration. When styrene is treated with water in the presence of dilute sulphuric acid, it undergoes hydration to form 1-phenylethanol as the product.
C₆H₅-CH=CH₂ + H₂O → C₆H₅-CH(OH)-CH₃
Cyclohexylmethanol can be synthesised using cyclohexylmethyl bromide (or chloride) as the alkyl halide. When cyclohexylmethyl bromide is treated with aqueous NaOH, an Sₙ2 reaction occurs where the nucleophile (OH⁻) attacks the carbon atom, displacing the halide ion.
Cyclohexyl-CH₂Br + NaOH → Cyclohexyl-CH₂OH + NaBr
Pentan-1-ol can be synthesised using 1-bromopentane (or 1-chloropentane) as the suitable alkyl halide. When 1-bromopentane is treated with aqueous NaOH, an Sₙ2 substitution reaction takes place, yielding pentan-1-ol.
CH₃CH₂CH₂CH₂CH₂Br + NaOH → CH₃CH₂CH₂CH₂CH₂OH + NaBr
Explanation
The question tests understanding of alcohol synthesis methods. Part (i) uses alkene hydration - styrene gives a secondary carbocation intermediate that yields 1-phenylethanol. Parts (ii) and (iii) use Sₙ2 reactions where a primary alkyl halide reacts with hydroxide ion. Primary alkyl halides are preferred for Sₙ2 reactions as they offer less steric hindrance to the attacking nucleophile.