Preparation of gem- and vic-Dihalides

gem-Dihalides: ethylidene dichloride is made either (i) by treating acetaldehyde with PCl5 (CH3CHO + PCl5 gives CH3CHCl2 + POCl3), or (ii) by adding HCl to acetylene via the intermediate vinyl chloride (HC#CH + HCl gives CH2=CHCl, which + HCl gives CH3CHCl2). vic-Dihalides: ethylene dichloride is made either (i) by adding chlorine to ethylene (CH2=CH2 + Cl2 gives ClCH2-CH2Cl), or (ii) by treating ethylene glycol wi …

Physical: both classes are sweet-smelling, colourless liquids with relatively high boiling points; ethylidene chloride boils lower than ethylene dichloride. Chemical -- three parallel reactions distinguish the two classes: (1) Hydrolysis with aqueous NaOH/KOH: a gem-dihalide gives an unstable geminal diol that collapses to an aldehyde or ketone (ethylidene chloride + 2 KOH(aq) gives an unstable CH3CH(OH)2, which loses H2O to give acetaldehyde + 2 KCl); a vic-dihalide instead gives a stable glycol directly (ethylene dichloride + 2 KOH(aq) gives ethylene glycol + 2 KCl) -- this hydrolysis difference (aldehyde/ketone vs a glycol) is the standard chemical test to tell the two classes apart. (2) Reaction with zinc (dehalogenation): both classes, treated with zinc dust in methanol, lose both halogens and give the alkene, e.g. ethylidene dichloride + Zn gives ethylene + ZnCl2, and ethylene dichloride + Zn likewise gives ethylene + ZnCl2. (3) Reaction with alcoholic KOH (dehydrohalogenation): both classes, treated with alcoholic KOH, l …

Preparation: (1) by reduction of chloroform, either with Zn + HCl (CHCl3 + 2[H] gives CH2Cl2 + HCl) or with H2/Ni; (2) by chlorination of methane (CH4 + Cl2/hv gives CH3Cl, which + Cl2/hv gives CH2Cl2, each step releasing HCl). Uses: methylene chloride serves as an aerosol-spray propellant, a solvent in paint remover, a process …