bevdavid1 - 28-6-2006 at 15:38
I am suppose to explain how I can synthesize 2-chloropropane but I am not sure where to begin. So far I came up with making it into an alkene and
using an addition reaction (halogenation) in this case +HCl with the final product being 2-chloropropane. The starting materials are propane,
propene, and propyne. I am to consider each of these and decide on the best reaction. Give a brief introduction to the problem. Consider the pros
and cons of alternative procedures, including yield and ease of isolation of the final product. Show the reactions for each alternative. Cost is not
an issue. OK with all that said where should I start.
guy - 28-6-2006 at 18:18
The best choice is propene. The H+will attack the double bond as a electrophile, and the the Cl- will attach to the carbocation, and according to the
Markonikov rule, it will form 2-chloroprane.
Propane will not react. Propyne will form 2-chloropropene.
[Edited on 6/29/2006 by guy]
The_Davster - 28-6-2006 at 18:48
Isopropyl alcohol +thionyl chloride --> isopropyl chloride +SO2 +HCl
As for workup, neutralize mix with a weak base, like bicarb and distill off the 2-chloropropane
guy - 28-6-2006 at 19:54
Is it possibe to just use isopropyl alcohol and HCl to form isopropyl chloride by nucleophilic subsitution?
The_Davster - 28-6-2006 at 20:07
Yes it is, there is a thread detailing someones experiments with that, but I cannot for the life of me find it.
It is easier to get the reagents this way, I suggested the thionyl chloride method as it seems bevdavid has access.
(I want thionyl chloride
)
solo - 28-6-2006 at 20:26
Since you're dealing with a secondary alcohol you can try the Lucas test using ZnCl + HCl or you can use TCCA see thread on halogenation of primary
alcohols using the search engine...............solo
nk - 28-6-2006 at 21:13
Hmm... I think some of the posts above missed the point of your question.
I hope that this is a little closer to what you were looking for.
You're on the right track. If you are looking for 2-chloropropane it would most certainly be easiest to react the alkene with a HCl. [Note that this
isn't technically a halogenation. That would be reacting your alkene with X2 to yield a vicinal dihaloalkane (1,2-dichloropropane in this case).]
Starting from the alkyne isn't as nice and neat as starting from the alkene, but going from the alkyne to the alkene is easy enough. You simply need
to reduce the alkyne using catalytic hydrogenation. The trick is though that you don't want to over do it. You don't want to go all the way to the
Alkane. Just to the alkene. You do this using a "poisoned" Pd or Pt catalyst which is only reactive enough to reduce a triple bond as opposed to a
straight Pd or Pt catalyst which would reduce all the way to an alkane. (Warning: What I just said is somewhat oversimplified.) The most commonly used
one is called Lindlar's catalyst. Once you get the alkene, you simply proceed as above by treating with H-X.
Starting from the Alkane is the messiest way. Really the only thing you can do is light induced chlorination. The mechanism involves free radicals.
This will yield a mixture of 1-chloropropane and 2-chloropropane as products. You could then distill the 2-chloropropane. I believe that the reaction
yields 45% 1-chloropropane and 55% 2-chloropropane at 25C. Not the most efficient, but it works.
---
Guy: Everything you said is correct except that the nucleophile always attacks the electrophile. In this case, the double bond acts as the nucleophile
and it attacks the H in H-X. Only time that H attacks is hydride like in the case of NaBH4 or LAH.
Also, addition of HX to propyne will go all the way to 2,2-dichloropropane instead of just to 2-chloropropene.
The_Davster - 28-6-2006 at 22:32
Yeah, I did miss that.
Alkane is definatly the most mesy, with spearation of monochlorinated isomers as well as the inevitable polychlorinated ones.
There are a couple ways to go from alkyne to the 2-chloro
methylacetylene(propyne) --(oxymercuration/reduction)-->acetone
acetone --(reduductive ammination with ammonia)--> isopropylamine
isopropylamine--(1.Diazotize, 2. NaCl(also CuCl2)-->isopropyl chloride
or starting back at acetone if you do not like reductive ammination and diazotization
acetone --(NaBH4)-->isopropanol
isopropanol --(SOCl2)-->isopropyl chloride
propyne --(lindlar)--> propene
propene--(oxymercuration/reduction)-->isopropanol
isopropanol--(SOCl2)--isopropyl chloride
Now you have a couple more ways(can you tell H-X addition bores me
)
[Edited on 29-6-2006 by rogue chemist]