Sciencemadness Discussion Board

2,3-dichloropropene synthesis?

Assured Fish - 16-6-2018 at 23:44

Ok so my target molecule is 2,3-dicholoropropene and i have slightly convoluted way to get there.
Im kinda looking for not only a route to my target that does't require 1,2,3 trichloropropane but also id like to figure out if my reaction could work in the manner i propose.

My route involves reacting hydroxy acetone with 2 equivalents of thionyl chloride, the first mole would react with the alpha hydroxy group forming the nasty chloroacetone in situ and a mole of SO2 and HCl, the HCl im hoping could protonate the ketone forming an enol, specifically 1-chloro-2-hydroxypropene.
This i was hoping would then go on to react with another mole of thionyl chloride to prepare 1,2-dichloropropene, which could then be cracked (isomerised) to form the desirable 2,3-dichloropropene.

It doesnt work.png - 2kB

What i want to happen.png - 3kB

That is what i want to happen however im unsure as to whether it would work as i dont know if that Cl- anion must attack the carbon first followed by elimination of the leaving group in an intermediate step, or if the leaving group would leave first allowing the Cl- anion to pick up that electron thats left behind.

From there (assuming it worked) i would like to isomerise that double bond to the more saturated carbon like so.

isomerization of dichloropropene.png - 1kB

Unfortunately ive no idea how to calculate the enthalpy for such a reaction.

Metacelsus - 17-6-2018 at 05:33

I don't think your planned isomerization is viable. Since you're planning to use thionyl chloride anyway, why not react it with glycerol to form 1,2,3-trichloropropane, and then do an elimination?

From wikipedia:

1,2,3-Trichloropropane can be produced via the chlorination of propylene. Other reported methods for producing 1,2,3-trichloropropane include the addition of chlorine to allyl chloride, reaction of thionyl chloride with glycerol, and the reaction of phosphorus pentachloride with either 1,3- or 2,3-dichloropropanol. TCP also may be produced as a byproduct of processes primarily used to produce chemicals such as dichloropropene (a soil fumigant), propylene chlorohydrin, propylene oxide, dichlorohydrin, and glycerol.[3]

[Edited on 6-17-2018 by Metacelsus]

clearly_not_atara - 17-6-2018 at 07:30

That isomerization breaks Zaitsev's rule I think. The fact that you want the anti-Zaitsev compound makes this one tricky.

One way to get around that is to start with 3-chloropropyne and add HCl. It might be possible to make this from propargyl alcohol which IIRC can be made from acetylene + formaldehyde, although you need a different chlorinating agent than SOCl2 I'm pretty sure.

DJF90 - 17-6-2018 at 09:02

If the bromide is not suitable, then I suspect the chloride would behave similarly.

Assured Fish - 17-6-2018 at 21:23

Yea it seems I applied zaitsevs rule to the enolation without thinking to apply it to the isomerization.
I was hoping to avoid 1,2,3 trihalopropane due to its carcinogenicity, however i suppose a fume hood and respirator would easily mitigate any dangers, perhaps my fear is unnecessary.

Anyway i shouldn't complain given that the elimination route is significantly more OTC that my method, allyl bromide could be halogenated with bromine to form the desirable 1,2,3-tribromopropane.

Thanks for the replies guys.

UC235 - 18-6-2018 at 16:11

I'd be shocked if the target product is somehow not carcinogenic as well. Probably quite the lachrymator too. A fume hood is in order.