Halogenoalkanes, alcohols and spectra
Alcohols
Primary alcohols (RCH₂OH) are oxidised by acidified (orange → green) to aldehydes (distil off immediately) and then to carboxylic acids (heat under reflux). Secondary alcohols are oxidised to ketones. Tertiary alcohols are not oxidised (stays orange). Other reactions: combustion; dehydration with hot concentrated H₂SO₄ or Al₂O₃ → alkene; substitution with HX, PCl₅ or SOCl₂ → halogenoalkane; sodium → alkoxide + H₂; esterification with a carboxylic acid (conc. H₂SO₄ catalyst). The triiodomethane test (I₂ + NaOH, yellow precipitate) shows a group.
≈ 50 minHalogenoalkanes
The C–X bond is polar (C δ+), so halogenoalkanes are attacked by nucleophiles. Nucleophilic substitution: aqueous NaOH, heat under reflux → alcohol; KCN in ethanol → nitrile (chain lengthened by one carbon); excess NH₃ in ethanol, heat in a sealed tube → amine. SN2 (one step, transition state) for primary; SN1 (via carbocation) for tertiary. With ethanolic NaOH (hot), elimination gives an alkene. Reactivity: C–I > C–Br > C–Cl, because the C–I bond is weakest (bond enthalpy matters more than polarity). Hydrolysis rate can be compared with aqueous AgNO₃ in ethanol: AgI (yellow) forms fastest.
≈ 50 minInfrared and mass spectrometry
In infrared spectroscopy, bonds absorb IR at characteristic wavenumbers: O–H in alcohols (3200–3600 cm⁻¹, broad), O–H in acids (2500–3300, very broad), C=O (1680–1750, sharp, strong), N–H (3300–3500), C≡N (≈2250), C–H (2850–3100). The fingerprint region (below 1500 cm⁻¹) identifies a compound by matching. In mass spectrometry, the molecular ion (M⁺, highest significant ) gives . Fragments give structural clues (15 = CH₃⁺, 29 = C₂H₅⁺ or CHO⁺, 43 = CH₃CO⁺ or C₃H₇⁺). The M+1 peak (from ¹³C) gives the number of carbons: . M+2 peaks show Cl (3 : 1) or Br (1 : 1).
≈ 45 min