{"id":62,"date":"2018-11-19T20:16:58","date_gmt":"2018-11-19T20:16:58","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry2\/chapter\/10-5-simple-addition-to-alkynes\/"},"modified":"2018-11-19T20:16:58","modified_gmt":"2018-11-19T20:16:58","slug":"10-5-simple-addition-to-alkynes","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry2\/chapter\/10-5-simple-addition-to-alkynes\/","title":{"raw":"10.5. Simple addition to alkynes","rendered":"10.5. Simple addition to alkynes"},"content":{"raw":"\n

Addition of strong acids to alkynes is quite similar to the addition of strong acids to alkenes. The regiochemistry follows Markovnikov\u2019s Rule<\/em>, but the stereochemistry is often different. Addition to alkenes is usually not stereospecific, whereas alkynes usually undergo <\/span>anti <\/em>addition.<\/span>\n

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Now we will take a look at electrophilic addition reactions, particularly of alkynes. The reaction mechanisms, as you will notice, are quite similar to the electrophilic addition reactions of alkenes. The triple bonds of alkynes, because of its high electron density, are easily attacked by electrophiles, but are less reactive than alkenes due to the compact C-C electron cloud.As with electrophilic addition to unsymmetrical alkenes<\/a>, the Markovnikov rule is followed, adding the electrophile to the less substituted carbon.<\/p>\n

For simplicity, we will show the mechanism going via a vinyl carbocation, by analogy with the alkyl carbocation formed when alkenes react.  In fact, the vinyl carbocation is usually to unstable to be a real intermediate; the actual intermediate is more like a complex of the electrophile with the alkyne, which begins to place a partial positive charge on the nearby carbon.<\/p>\n

Here we will go through the following reactions listed below to learn the mechanisms behind these electrophilic additions of alkynes: (1) HX Addition to alkenes<\/a>, (2) Halogenation of alkynes<\/a> and (3) Hydration of alkynes.<\/p>\n

Figure
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\"Alkene_Alkyne<\/strong>\n

The addition of an electrophile to either an alkene or an alkyne will undergo the same steps listed below.<\/p>\n

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  1. Start with a reactant (either an alkene or an alkyne), which has $$\\pi$$ electrons. An electron pair moves from the $$\\pi$$ bond to the electrophilic proton to form a new covalent bond this is the electrophilic addition elementary step. A carbocation intermediate forms on the more stable carbon. As shown in the diagram above, the intermediate consists of hydrogen covalently bonded to the carbon and a positive charge on the other carbon.<\/li>\n
  2. The addition of the halide ion to the positive charged intermediate forms the second covalent bond giving you the haloalkane product (as seen on the left of the above diagram) and a haloalkene product (as seen on the right of the above diagram). The halide ion acts as a nucleophile, which is attracted to the positive charged carbon because it has electrons to donate or share. So when the nucleophilic halide (represented as X-<\/sup>) attacks, it aims for the positively charged carbon resulting in that second covalent bond as seen above.<\/li>\n<\/ol>\n

    Make Note: The way to determine where the addition of the proton and the halide takes place is based on Markovnikov\u2019s Rule, which states that the proton adds onto the carbon with the most hydrogens and the halogen prefers the most substituted carbon.<\/p>\n

    Now let's take a look at our first reaction, the addition of Hydrogen Halide.<\/p>\n<\/div>\n

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    Reaction 1:  Addition of hydrogen halide to an alkyne<\/h3>\n

    Summary:<\/strong>  Reactivity order of hydrogen halides<\/span>: HI > HBr > HCl > HF.<\/strong><\/strong>\n

    Follows Markovnikov\u2019s rule:<\/p>\n