2301 Chemistry Workshop 17 Solutions: More Multistep Organic Synthesis
Friday, March 27
Cl
A challenge in both of these problems is to use the few reactions you know to make what you want. (You might imagine this issue could be resolved for professional chemists by knowing lots of reactions, but in the end, even they get stuckthere are never enough reactions to make everything.) Here, the only thing you know to do with a hydrocarbon is to halogenate it, and you only know how to do mono-halogenation and multi-halogenation, always at the more substituted/more stabilized position. Here, that is at the benzylic carbon. Di-halogenation here generates something that can be eliminated to yield the product.
Cl
excess
Cl KOtBu
Cl
Cl2 h
CN
This problem is a bit harder, because chemistry clearly has to happena CN has to be addedat a position thats not the most substituted. So, although we have to start with halogenation again (we dont know anything else to do with propane), its clear were going to have to do some trickery to translate halogenation at the most substituted position to substitution at the least substituted position. One thing we cant do is just say Ill halogenate to get the material I want:
Cl2
Cl
Cl
h
Yes, if you were to expose propane to chlorine gas for a while, you would get this but molecule, youd get hundreds of other molecules too. (Monochlorinated, other dichlorinated, with chlorines all over the place.) A central goal of these synthesis problems is to propose ways of making the products selectively. An obvious way to approach these problems is to go forward from starting materials after all, thats what a chemist would physically do in the labbut another way to think about synthesis is backwards from the end. Given a product, from what materials might we start? Our product has two functional parts to it, an alkene and a cyano (CN) group. How might we get to those functional groups? The CN could be introduced via SN2 by cyanide (CN-), or the alkene could be made by elimination:
Br CN N C
KOt Bu (for E2) N Br C and not C Br Br N or N C
The bottom, elimin...