alkene

place syntheses below





Grignard reagents result from the reaction of an alkyl halide with magnesium metal. This reaction is always carried out in a dry (anhydrous) ether solvent, which is needed to solvate and stabilize the Grignard reagent as it forms. In order to get a tertiary alcohol, an ester must be added to a Grignard reagent, 2 equivalents of Grignard reagents to be precise. Addition of the first equivalent of the Grignard reagent produces an unstable intermediate that expels an alkoxide ion to give a ketone. The alkoxide ion is a suitable leaving group in this reaction. The ketone reacts with the second equivalent of the Grignard reagent forming the magnesium salt of a tertiary alkoxide. Protonation gives a tertiary alcohol with one of its alkyl groups derived from the ester and the other two derived from the Grignard reagent. Finally to make the alkene, I dehydrated the alcohol. Concentrated sulfuric acid is used as reagent for dehydration because the acid acts both as acidic catalysts and as dehydrating agents. The hydroxyl group of the alcohol is a poor leaving group but protonation by the acidic catalyst converts it to a good leaving group. In the second step, loss of water from the protonated alcohol gives a carbocation. The carbocation is a very strong acid: any weak base such as H₂O or HSO₄¯ can abstract the proton in the final step to give the alkene, keeping Zaitsev’s rule in mind.

by Moise Civil Reference: 1) L.G. Wade Jr. __Organic Chemistry__ 7th edition (Upper Saddle River New York Pearson/Prentice Hall 2010) pages 309, 444 2) http://www.organic-chemistry.org/synthesis/C2C/alkenes/terminalolefins.shtm

Moise : Some of these reactions make sense. The first reaction would work, but it would not create the products that you show. The second reaction would work, but I don’t know of any reaction that will couple propane with a germinal dihalide. Take a look at the target molecule, and disconnect it back thinking of esters and Grignard reagents. The final step involves an elimination reaction, but what would the regiochemistry of the E1 reaction that you propose be? Take a look at Zeitsef and Hofmann eliminations.

Yes This is the synthesis that I had in mind when I came up with this molecule, however, the E1 will produce the zeitsef procuct rather than the hofmann product.

Dr. Bondurant, so what would happen if i use as the reagents for the E1 reaction. You have to turn the hydroxyl into a good leaving group first. Then you do a hofmann e2. Ana Baird (extra-credit)

Ana : This is a nice use of the Wittig reaction, and an excellent synthetic strategy. I think that all of these reactions would work well. The only thing that I would recommend that you add is a reterosynthesis. // Dr. Bondurant, I have included the retrosynthesis on the paper. (A Baird) // // Ana: // // I see it This looks great. thanks. // // Bruce Bondurant //

This is Pierre latortue i have this information in this wed side [] also in the books Pierre: That is a good reference on Grignard reagents, which are central to this synthesis. Take a look at the symmetry of this molecule, and then disconnect it backwards to the starting material. I think you will see that what you have drawn above is somewhat diferent from the product you are trying to make. Bruce Bondurant.

OK now just put it together, and figure out a reagent that will give you a hofmann E2 elimination rather than a zeitsef E1.