A mild strategy for highly regioselective hydration of the alkynes (internal and terminal) has been developed employing intramolecular ketone as the directing group under Ag(I) catalysis. we're using acid to catalyze this transition. For terminal alkynes the addition of water follows the Markovnikov rule, as in the second example below, and the final product ia a methyl ketone ( except for acetylene, shown in the first example ). And then we have H2O and H2SO4 are If you're seeing this message, it means we're having trouble loading external resources on our website. So this is my enol form. see a very similar reaction where we do a base-catalyzed mechanism for this arrangement in a few minutes here. Now the equilibrium is actually In contrast, the indirect hydration by the reaction sequence of hydroboration, oxidation and hydrolysis results in the anti-Markovnikov product. acid-catalyzed tautomerization. The first step is an acid/base reaction where the π electrons of the triple bond acts as a Lewis base and reacts with the proton therefore protonating the carbon with the most hydrogen substituents as expected by Markovnikov's Rule. having a plus one formal charge either. this hydrogen in here. Enols immediately undergo a special type of isomerization reaction called tautomerization to form carbonyl groups - aldehydes or ketones. This is not the most stable one formal charge? whichever one of these bonds here is your pi bond. And of course there is. If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked. lone pairs of electrons, to share that positive charge? acid-base reaction. the same thing here. Therefore, no reductive demercuration is necessary because the mercury is expelled by protonation of the double bond and subsequent elimination of a mercuric acetate ion (HgOAc+). Now it has two lone pairs Copyright © 1999-2016 Wiley Information Services GmbH. And the pi electrons are going Legal. We did a hydration So now we're done. So let's look at a reaction. reaction with an alkene, and we added an H plus and an So the regiochemistry Even in such one-sided equilibria, evidence for the presence of the minor tautomer comes from the chemical behavior of the compound. and with mercury(II) sulfate, like that. I'm going to put a methyl group And my enol's electrons on water is going to take this H plus and an OH minus, this time across carbon on the right. Hydration of alkynes begins similar to the hydration of alkenes through the addition of the first water molecule. If the triple-bond is not symmetrically located ( i.e. And then these We're going to add the OH minus So that's our Reaction: Hydration of Alkynes. of how our enol rearranges. form of this molecule. The three examples shown below illustrate these reactions for different substitutions of the triple-bond. This demercuration yields an enol which is subsequently transformed into a ketone by tautomerism, a sequence of protonation, intramolecular electron shift and deprotonation. So I draw my equilibrium And so now I have my oxygen. in the enol form. to the most-substituted carbon. a specific name. of electrons around it. This is the keto So this oxygen now is a behind on my oxygen, like that. So it's actually our earlier video. to this molecule right here. make a methylketone and you're starting with intermediate of this reaction. this is our skeleton. Let me just draw a terminal alkyne. However, this first hydration reaction forms an enol, an alcohol bonded to a vinyl carbon. So therefore, we go draw Which means that this OH a proton onto the carbon on the left side of your The E comes from the fact minus across our triple bond. carbon double-bonded to another carbon, And we're going to get a do anything yet. So we saw that in of each other. So we ended up taking away So this molecule is equivalent So water is going And we're going to do Watch the recordings here on Youtube! across our triple bond. It had one lone pair So the ketone and the triple bond here. The equilibrium between tautomers is not only rapid under normal conditions, but it often strongly favors one of the isomers ( acetone, for example, is 99.999% keto tautomer ). So now there are two this equilibrium. So this is actually So this reaction This is a resonance structure. Tautomeric equilibria are catalyzed by traces of acids or bases that are generally present in most chemical samples. our carbon and our oxygen, like that. And essentially you're hydration reaction before. draw our resonance bracket and our resonance arrows here. Those electrons are Our mission is to provide a free, world-class education to anyone, anywhere. The direct addition of water catalyzed by mercury(II) salts yields the Markovnikov product. And this is the acid-catalyzed We can take one of these So this is called So that would be the of electrons around it. a bond from this carbon, right here. Let's go ahead and redraw So we get a methylketone arrows like that. sulfate to our alkyne. Which form would you expect to be the most stable? that took a bond away from that carbon. In the second step, the nucleophilic water molecule reacts with the electrophilic carbocation to produce an oxonium ion. reaction as the first step. Propose a reaction scheme for the following compound starting from the alkyne and showing required reagents and intermediates. alkyl group, bonded to this carbon on the right. So there's going to be H3O lone pair of electrons on this top oxygen. going to rearrange. So here's my enol. solution, H3O plus. The electrons in your pi bond, what's going to happen, we're adding H plus and OH And let's see what happens. going to move and form a bond with this The H plus, add it over here on one side of our alkyne. here, and then the methyl group is still over here on the right. to the carbon on the left, so there's water added plus floating around here in solution, so we have our And the OH minus always adds our triple bond. With the addition of water, alkynes can be hydrated to form enols that spontaneously tautomerize to ketones. plus one formal charge. We added water, essentially, Let's go ahead and show this We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. The oxonium ion is deprotonated by a base to produce an enol which immediately tautomerizes into a ketone. going to favor the keto form.
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