[beheada]

We've moved onto organic synthesis, which is kind of exciting, but also quite complicated. Here's the setup for this... I have an exam Tuesday and these are some practice problems we were attempting to solve in class on Thursday, but he didn't give us the answers, and we don't have another class before the Exam. I've completed the last two synthesis but just want verification that my answers are correct. So here goes...I don't have a program to draw the structures, so I'm just going to list the reagents and my line of thought for the synth.

3) Outline the synthesis of acetone (2-propanone) from 2,3-dibromo-2,3-dimethylbutane.

ANSWER:

From the first compound, dehydrohalogenation using a strong base (a la KOH/NaOH), turning the 1,2 Carbons into a double bond. From here, reduce using H₂/catalyst, to yield the same starting molecule, minus the 2 Bromine. Once again dehydrohalogenation using KOH to yield 2,3-dimethyl-2-butene. Next, I hydroxylated the alkene using 1.OsO₄ 2. NaHSO₃, H₂O and to the product resultant diol I used HIO₄/H₂O to create 2 of the desired ketone.... acetone. PHEW! ((Keep in mind we only have a certain number of reactions we know how to use, just the ones in our alkane/alkene/alkyne chapters))

4) Outline synthesis of trans-5-methyl-2-hexene from 4-methyl-1-pentane...

ANSWER:
Strategy - I noticed that this is the same molecule except there's an extra carbon. Tragically, the only way we know how to add carbons so far is from an alkyne, which means I need to get the alkene to an alkyne somehow, add a carbon, and then reduce.

Synthesis: From the starting compound I used Cl2/CH2Cl2 to add two halogens. Next, a dehydrohalogenation of vicinal dihalides using 2 KOH/ethanol to yield the alkyne. Next, I ripped off the proton using NaNH2/NH3, then added a carbon with an alkyl halide CH3Br. Next was a reduction using H2/Lindlar catalyst to drop the alkyne back down to an alkene and yield the product, trans-5-methyl-2-hexene.



Are both of these proposed synthesis under the given framework?

[Yggdrasil]

3) I'm not sure whether your dehydrohalogenations will proceed as you say.  I think the internal alkene (2-bromo-2,3-dimethyl-2-butene) would be prefered to the product you say (3-bromo-2,3-dimethyl-1-butene).  However, your procedure of dehydrohalogenate, reduce, then dehydrohalogenate should still produce your desired alkene  (but, I haven't studied organic chemisty for a while, so maybe get a second opinion on this).  Also, you could produce acetone directly from your 2,3-dimethyl-2-butene by ozonolysis and save a few steps.

4)  I thought the Lindlar catalyst selectively made cis-alkenes.  You want the trans alkene, so you're going to have to find another way of reducing your alkyne.

[beheada]

3) The ozonolysis works too, and yields 2 2-propanone(s)... which is something I completely overlooked. But I'm almost positive (90%) both of the synths work.

4) RIGHT! I jacked up there. Since I wanted trans instead of cis, I had to go with a reduction using lithium in ammonia instead of the lindlar catalyst.


rayfe