Laboratory Preparation of 2-chloropropane (isopropyl chloride)
(An early version of this post was first published on Science Madness)
Mono haloalkanes like chloropropane can be prepared by halogenation of alcohols in a typical substitution reaction:
ROH + HX → RX + H2O
… where X is a halogen and R an alkyl group. In the case of X = Cl the reaction is catalysed by considerable amounts of zinc chloride (Lucas reagent). Usually a reagent mix of 1 mol alcohol, 2 mol HCl (as 37 w% solution) and 2 mol ZnCl2 (anh.) is called for. For the case of the chlorination of 2-propanol (isopropanol) references to specific methodology appeared difficult to find. Here is presented a preparation adapted from Organic Syntheses, Coll. Vol. 5. p.27 (1925), for n-butyl chloride (1-chlorobutane).
58 ml of isopropanol (1.2 mol), 200 ml of HCl 37 w% (2.4 mol) and 332 mol of ZnCl2 (anh.) (2.4 mol) were loaded into a 500 ml round bottom flask (RBF) and connected to the set up below (bad photo, sorry!):
Left: sand bath with electrical hot plate, 500 ml RBF, Vigreux column 500 mm active length, still head with thermocouple.
Middle: Liebig condenser active length 400 mm with cooling circuit (about 10 C water temperature).
Right: 100 ml measuring cylinder in iced water, as distillate receiver.
This ‘reactive fractionation’ went surprisingly smoothly and was over in less than 30 minutes. Distillate started coming over at 33.3 C and stayed stable for quite a while, with distillate coming over too fast to count drops. All the while HCl fumes came over too (see Note 1).
The reaction equation presented above is in reality an equilibrium reaction and removing one of the reaction products (here by distillation) causes the equilibrium to shift to the right.
About 20 minutes in head temperature started dropping slowly, distilling rate slowed down and eventually the flasks content more or less stopped boiling altogether.
45 ml of distillate had been collected by then. This was worked up more or less (to scale) as OrgSynth’s preparation for n-butyl chloride but minus the H2SO4 treatment (because I’m not sure what its purpose is). Two washes with 25 ml cold water, here’s what it looked like after the first one:
Clear separation between an organic phase and water can be seen. The wash water stank of chlorinated hydrocarbon, very similar to dry cleaning fluid (perchloroethylene). Then I cooled the separation funnel again and washed with 6 ml of cold 10 % Na2CO3: a small amount of CO2 bubbles evolved.
The organic phase has now been placed in a Simax reagent bottle with 5 g of CaCl2 (anh.) in the refrigerator.
I have to decide now whether or not to distil this product or not. It’s intended for a Friedel Crafts alkylation of benzene (which I’ve yet to prepare!) Any advice on further work up is welcome.
Note 1: the Org. Synth. preparation called for a device to capture the HCl fumes. I didn’t bother with that and found good ventilation to be sufficient. But considerable scale up would probably necessitate such a device.