hmmmm...Organic Synthesis help, mayhaps?

[Lan(r)12]

Well, for our Organic I Final, we get a chance to answer a bonus question

Our prof gave us three questions and told us that one would be the bonus, and that we can figure out the right one and do it, or memorize all three.

But upon further research, the other two are Chemically Impossible.

 

So, if you Chemists out there could help me out that would be great!

 

The Q is:

"Propose a method to synthesize (E)-4-octene from coumpunds with three or fewer carbon atoms."

 

As it happens, I suck at the retro-thinking required for synthesis problem solving, so any info is greatly appreciated!

>>Thanks<<

 

Lan

[bob000555]

The question is so vague that the fallowing would technically be correct though I doubt it is what your professor wants:

 

Seal a mixture of simple hydrocarbons including methane, ethane and ethylene in an airtight piston absent any oxidizing agents. Compress the mixture several hundred fold, release the pressure. A small portion of the resulting compounds will be octane.

 

If your professor values ridiculously impractical thinking it may work.

[hermanntrude]

that method wouldnt do a damned thing. The question said E-4-octene, not octane. read the question! I feel like a loop-tape telling people to read the question.

 

Although you could take your octane and try to dehydrogenate it. But it'd be a crappy method... probably less than 1% yield. Not sure of a better method, though.

[bob000555]

Not sure of a better method, though.

Of course the yield would be ridiculously low, some fraction of a percent, but since you cant think of a better way stop complaining. The ethylene is there to encourage the formation of unsaturated compounds like e-4-octane; including acetylene in the mix would further favor unsaturated compounds. Often when they ask questions like this professors are looking for creativity over practicality.

Edited December 9, 2008 by bob000555

[hermanntrude]

not necessarily. I'd be prepared to bet there's a better way... probably witting reactions or something... or grignard

[Lan(r)12]

Well, any little bit helps...

[farmboy]

Perhaps look at a 2 carbon compound in the middle, and two three carbon compounds for the two sides.

[David M]

Hi Lan,

 

Let me see if I can help you. Farmboy was on the right track. First, the molecule (E)-4-octene is the hydrocarbon that is 8 carbons long and has a trans double bond between the C-4 and C-5 carbons. In other words, it's a symmetrical trans olefin with two n-propyl groups on each side of the double bond. The best way to synthesize a trans double bond is by performing a stereoselective reduction on the corresponding triple bonded compound, in this case, oct-4-yne. Two condtions that should work are the dissolving metal reaction conditons (Na metal / liq. NH3, -78 degrees C) or a lithium aluminium hydride reduction (LAH, THF). By the way, you could also try to get the olefin by doing a hydrogenation of the alkyne but it's very difficult to stop the reduction at the alkene state unless you use a poisoned catalyst. However, the problem with using a posinoned catalyst such as Lindlar catalyst is that the cis olefin is favored/formed.

 

The next part of the syntheis requires figuring out how to make oct-4-yne from compounds of 3 carbons or less. The best way is to do that is to react 1 equivalent of acetylene with 2 equivalents of propyl bromide in the presence of a strong base such as n-butyl lithium. If you add 2 eq. of BuLi to 1 eq of acetylene, you will get bis lithium acetylide [Li-C(triplebond)C-Li] which will undergo SN2 displacement of the alkyl halide to give you the desired alkyne...

 

Hope this helps!!!

Edited December 9, 2008 by David M