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Topic: Combining alkynes into alkane chain  (Read 5431 times)

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Offline jayred

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Combining alkynes into alkane chain
« on: November 08, 2009, 01:23:06 AM »
So the problem I have to answer is how to synthesize (Z)-tricos-9-ene from 1-butyne, 1-pentyne and acetylene but only using 1-pentyne and acetylene once each. I know that the acetylene will form the alkyne triple bond in the middle of the final compound and that one alkane chain attached to the triple bond will come from 2x 1-butyne + 1-pentyne, and the other chain will come from just 2x 1-butyne.

What reactions can I do to combine these different alkynes into their respective chains that the acetylide ion can then form bonds with?

Offline azmanam

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Re: Combining alkynes into alkane chain
« Reply #1 on: November 08, 2009, 06:43:33 AM »
there's only one carbon-carbon bond forming reaction of alkynes that you've learned at this point.  do you remember what it is?

All the reactions you need are here:
http://www.cem.msu.edu/~reusch/VirtualText/addyne1.htm#add1
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Offline jayred

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Re: Combining alkynes into alkane chain
« Reply #2 on: November 08, 2009, 01:30:06 PM »
I should have stated that I know that you would have to use the terminal triple bond as an anion to attack a brominated species. So, I know that for example the chain composed of two 1-butynes, would be formed by brominating one mole of 1-butyne to get 1-bromobutane and combining that with 1 mol of 1-butyne anion. This would form n-octane. But how do you then get the acetylene to attack this normal alkane which is no longer a primary halide? If you need to convert it into a primary halide, how would you ensure that the bromination only occurs at the terminal end?

Offline orgopete

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Re: Combining alkynes into alkane chain
« Reply #3 on: November 08, 2009, 05:41:45 PM »
If you have an terminal alkene and make a radical addition of HBr, it adds to give the most stable radical intermediate. 
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