Hydrocarbon Chemistry

Hydrocarbon Chemistry CHE 225 Reactions of Alkynes 1.

Addition of hydrogen halide (HX): a. Example

b.

Mechanism

This reaction is very similar to alkene hydrohalogenation. With one equivalent of hydrogen hali de, this reaction can be stopped at t he production of the vinylic halide. If an excess of hydrogen halide is used, then the geminal dihalide is produced.

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2.

Addition of halide (X2):

a.

Example

b.

Mechanism

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3.

Hydration: a. Example

b.

Mechanism

Mercury (II) ion is the elctrophile in this reaction. It interacts with the l ess highly substituted carbon atom of the triple bond. The resulting vinylic carbocation is attacked by water, which i s subsequently deprotonated. The mercury ion is replaced by acidic hydrogen to form an enol intermediate. The enol tautomerizes to t he more stable ketone.

Note:

Hydration of terminal alkynes always leads to the production of methyl ketones. Symmetrical alkynes lead to only one ketone product, whereas unsymmetrical internal alkynes lead to mixtures of ketones.

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4.

Hydroboration: a. Example

b.

Mechanism

Hydroboration of an alkyne involves the react ion of an alkyne with borane. The boron atom has an empty p-orbital which acts as an electrophile toward the triple bond. Hydride transfer from borane to the other carbon atom of t he triple bond occurs simultaneously with bond formation to boron. A four-atom transition state closely describes the bond-breaking and bond-forming interactions. Because borane has three hydrogen atoms attached, each borane molecule reacts with three alkynes to f orm a trialkenyl borane. Hydrogen peroxide oxidizes the Nurul Uyun Binti Ahmad

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boron atom, and the carbon-boron bonds are replaced with carbon-oxygen bonds. You are not responsible for this portion of the mechanism. The resulting enol tautomerizes to the more stable ketone.

Note:

Hydration of terminal alkynes always leads to the production of methyl ketones. Symmetrical alkynes lead to only one ketone product, whereas unsymmetrical internal alkynes lead to mixtures of ketones.

5.

Oxidative cleavage with potassium permanganate:

a.

Example

b.

Mechanism No mechanism is required for this reaction. Potassium permanganate oxidative cleavage of an alkyne is a very complicated chemical reaction. However, you must be aware of the result: each carbon of the carbon-carbon triple bond is o xidized up to the carboxylic acid. Additionally, all acetylenic hydrogen atoms are replaced with -OH groups.

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Note:

In the case of a terminal alkyne, the t erminal carbon atom is removed as carbon dio xide. This is a chain-shortening reaction (see top reaction above). This reaction works best for symmetrical alkynes, because, in this case, only one product is produced (see middle example above). For unsymmetrical alkynes mixtures are form ed (see bottom example above).

6.

Hydrogenation to alkane: a. Example

b.

Mechanism

Hydrogen gas adsorbs onto the surface of the catalyst. The hydrogen adds to the same face of the alkyne reducing it toan alkene. The alkene can continue to react with more hydrogen to reduce the alkene to the alkane. 7.

Hydrogenation to (Z )-alkene: a. Example

b.

Mechanism

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Hydrogen gas adsorbs onto the surface of the catalyst. The hydrogen adds to the same face of the alkynereducing it to an alkene. The catalyst has been deactivated by treatment with lead acetate and quinone, and is thus unreactive toward alkenes. The reactionstops at the production of the (Z )-alkene 8.

Hydrogenation to ( E )-alkene: a. Example

b.

Mechanism

This reaction is often referred to as a "solvated electron" reaction. Lithium metal donates an el ectron to the alkyne forming a radical anion that is protonated by the solvent. Lit hium metal again donates an electron to the vinylic radical to form a new anion which is again protonated by solvent.

Note: The trans-stereochemistry is determined when the more stable trans- vinylic anion is formed. Equilibrium is not established before protonation occurs, therefore, the trans- product is isolated. Alkenes are much less reactive under these conditions, so the alkene product is isolable.

Return to: Reactions of Alkanes Page Last Updated: 07/19/2007 02:01:51 © Neil Glagovich 2011 Created and maintained by Neil Glagovich

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